[feature] request blocking by http headers (#2409)

31 files changed, 2254 insertions, 76 deletions

diff --git a/internal/api/client/admin/admin.go b/internal/api/client/admin/admin.go
index 16c5fa8f8..6173218e0 100644
--- a/internal/api/client/admin/admin.go
+++ b/internal/api/client/admin/admin.go
@@ -35,6 +35,10 @@ const (
 	DomainAllowsPath        = BasePath + "/domain_allows"
 	DomainAllowsPathWithID  = DomainAllowsPath + "/:" + IDKey
 	DomainKeysExpirePath    = BasePath + "/domain_keys_expire"
+	HeaderAllowsPath        = BasePath + "/header_allows"
+	HeaderAllowsPathWithID  = HeaderAllowsPath + "/:" + IDKey
+	HeaderBlocksPath        = BasePath + "/header_blocks"
+	HeaderBlocksPathWithID  = HeaderAllowsPath + "/:" + IDKey
 	AccountsPath            = BasePath + "/accounts"
 	AccountsPathWithID      = AccountsPath + "/:" + IDKey
 	AccountsActionPath      = AccountsPathWithID + "/action"
@@ -95,6 +99,16 @@ func (m *Module) Route(attachHandler func(method string, path string, f ...gin.H
 	attachHandler(http.MethodGet, DomainAllowsPathWithID, m.DomainAllowGETHandler)
 	attachHandler(http.MethodDelete, DomainAllowsPathWithID, m.DomainAllowDELETEHandler)
 
+	// header filtering administration routes
+	attachHandler(http.MethodGet, HeaderAllowsPathWithID, m.HeaderFilterAllowGET)
+	attachHandler(http.MethodGet, HeaderBlocksPathWithID, m.HeaderFilterBlockGET)
+	attachHandler(http.MethodGet, HeaderAllowsPath, m.HeaderFilterAllowsGET)
+	attachHandler(http.MethodGet, HeaderBlocksPath, m.HeaderFilterBlocksGET)
+	attachHandler(http.MethodPost, HeaderAllowsPath, m.HeaderFilterAllowPOST)
+	attachHandler(http.MethodPost, HeaderBlocksPath, m.HeaderFilterBlockPOST)
+	attachHandler(http.MethodDelete, HeaderAllowsPathWithID, m.HeaderFilterAllowDELETE)
+	attachHandler(http.MethodDelete, HeaderBlocksPathWithID, m.HeaderFilterBlockDELETE)
+
 	// domain maintenance stuff
 	attachHandler(http.MethodPost, DomainKeysExpirePath, m.DomainKeysExpirePOSTHandler)
 
diff --git a/internal/api/client/admin/headerfilter.go b/internal/api/client/admin/headerfilter.go
new file mode 100644
index 000000000..7b1a85c86
--- /dev/null
+++ b/internal/api/client/admin/headerfilter.go
@@ -0,0 +1,173 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package admin
+
+import (
+	"context"
+	"errors"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/oauth"
+)
+
+// getHeaderFilter is a gin handler function that returns details of an HTTP header filter with provided ID, using given get function.
+func (m *Module) getHeaderFilter(c *gin.Context, get func(context.Context, string) (*apimodel.HeaderFilter, gtserror.WithCode)) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		errWithCode := gtserror.NewErrorUnauthorized(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if !*authed.User.Admin {
+		const text = "user not an admin"
+		errWithCode := gtserror.NewErrorForbidden(errors.New(text), text)
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if _, err := apiutil.NegotiateAccept(c, apiutil.JSONAcceptHeaders...); err != nil {
+		errWithCode := gtserror.NewErrorNotAcceptable(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	filterID, errWithCode := apiutil.ParseID(c.Param("ID"))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	filter, errWithCode := get(c.Request.Context(), filterID)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, filter)
+}
+
+// getHeaderFilters is a gin handler function that returns details of all HTTP header filters using given get function.
+func (m *Module) getHeaderFilters(c *gin.Context, get func(context.Context) ([]*apimodel.HeaderFilter, gtserror.WithCode)) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		errWithCode := gtserror.NewErrorUnauthorized(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if !*authed.User.Admin {
+		const text = "user not an admin"
+		errWithCode := gtserror.NewErrorForbidden(errors.New(text), text)
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if _, err := apiutil.NegotiateAccept(c, apiutil.JSONAcceptHeaders...); err != nil {
+		errWithCode := gtserror.NewErrorNotAcceptable(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	filters, errWithCode := get(c.Request.Context())
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, filters)
+}
+
+// createHeaderFilter is a gin handler function that creates a HTTP header filter entry using provided form data, passing to given create function.
+func (m *Module) createHeaderFilter(c *gin.Context, create func(context.Context, *gtsmodel.Account, *apimodel.HeaderFilterRequest) (*apimodel.HeaderFilter, gtserror.WithCode)) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		errWithCode := gtserror.NewErrorUnauthorized(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if !*authed.User.Admin {
+		const text = "user not an admin"
+		errWithCode := gtserror.NewErrorForbidden(errors.New(text), text)
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if _, err := apiutil.NegotiateAccept(c, apiutil.JSONAcceptHeaders...); err != nil {
+		errWithCode := gtserror.NewErrorNotAcceptable(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	var form apimodel.HeaderFilterRequest
+
+	if err := c.ShouldBind(&form); err != nil {
+		errWithCode := gtserror.NewErrorBadRequest(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	filter, errWithCode := create(
+		c.Request.Context(),
+		authed.Account,
+		&form,
+	)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	apiutil.JSON(c, http.StatusOK, filter)
+}
+
+// deleteHeaderFilter is a gin handler function that deletes an HTTP header filter with provided ID, using given delete function.
+func (m *Module) deleteHeaderFilter(c *gin.Context, delete func(context.Context, string) gtserror.WithCode) {
+	authed, err := oauth.Authed(c, true, true, true, true)
+	if err != nil {
+		errWithCode := gtserror.NewErrorUnauthorized(err, err.Error())
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	if !*authed.User.Admin {
+		const text = "user not an admin"
+		errWithCode := gtserror.NewErrorForbidden(errors.New(text), text)
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	filterID, errWithCode := apiutil.ParseID(c.Param("ID"))
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	errWithCode = delete(c.Request.Context(), filterID)
+	if errWithCode != nil {
+		apiutil.ErrorHandler(c, errWithCode, m.processor.InstanceGetV1)
+		return
+	}
+
+	c.Status(http.StatusAccepted)
+}
diff --git a/internal/api/client/admin/headerfilter_create.go b/internal/api/client/admin/headerfilter_create.go
new file mode 100644
index 000000000..d74dc5e15
--- /dev/null
+++ b/internal/api/client/admin/headerfilter_create.go
@@ -0,0 +1,102 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package admin
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// HeaderFilterAllowPOST swagger:operation POST /api/v1/admin/header_allows headerFilterAllowCreate
+//
+// Create new "allow" HTTP request header filter.
+//
+// The parameters can also be given in the body of the request, as JSON, if the content-type is set to 'application/json'.
+// The parameters can also be given in the body of the request, as XML, if the content-type is set to 'application/xml'.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	consumes:
+//	- application/json
+//	- application/xml
+//	- application/x-www-form-urlencoded
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'200':
+//			description: The newly created "allow" header filter.
+//			schema:
+//				"$ref": "#/definitions/headerFilter"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterAllowPOST(c *gin.Context) {
+	m.createHeaderFilter(c, m.processor.Admin().CreateAllowHeaderFilter)
+}
+
+// HeaderFilterBlockPOST swagger:operation POST /api/v1/admin/header_blocks headerFilterBlockCreate
+//
+// Create new "block" HTTP request header filter.
+//
+// The parameters can also be given in the body of the request, as JSON, if the content-type is set to 'application/json'.
+// The parameters can also be given in the body of the request, as XML, if the content-type is set to 'application/xml'.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	consumes:
+//	- application/json
+//	- application/xml
+//	- application/x-www-form-urlencoded
+//
+//	produces:
+//	- application/json
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'200':
+//			description: The newly created "block" header filter.
+//			schema:
+//				"$ref": "#/definitions/headerFilter"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterBlockPOST(c *gin.Context) {
+	m.createHeaderFilter(c, m.processor.Admin().CreateBlockHeaderFilter)
+}
diff --git a/internal/api/client/admin/headerfilter_delete.go b/internal/api/client/admin/headerfilter_delete.go
new file mode 100644
index 000000000..806e62a04
--- /dev/null
+++ b/internal/api/client/admin/headerfilter_delete.go
@@ -0,0 +1,96 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package admin
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// HeaderFilterAllowDELETE swagger:operation DELETE /api/v1/admin/header_allows/{id} headerFilterAllowDelete
+//
+// Delete the "allow" header filter with the given ID.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	parameters:
+//	-
+//		name: id
+//		type: string
+//		description: Target header filter ID.
+//		in: path
+//		required: true
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'202':
+//			description: Accepted
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: not found
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterAllowDELETE(c *gin.Context) {
+	m.deleteHeaderFilter(c, m.processor.Admin().DeleteAllowHeaderFilter)
+}
+
+// HeaderFilterBlockDELETE swagger:operation DELETE /api/v1/admin/header_blocks/{id} headerFilterBlockDelete
+//
+// Delete the "block" header filter with the given ID.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	parameters:
+//	-
+//		name: id
+//		type: string
+//		description: Target header filter ID.
+//		in: path
+//		required: true
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'202':
+//			description: Accepted
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: not found
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterBlockDELETE(c *gin.Context) {
+	m.deleteHeaderFilter(c, m.processor.Admin().DeleteAllowHeaderFilter)
+}
diff --git a/internal/api/client/admin/headerfilter_get.go b/internal/api/client/admin/headerfilter_get.go
new file mode 100644
index 000000000..5bca6d18d
--- /dev/null
+++ b/internal/api/client/admin/headerfilter_get.go
@@ -0,0 +1,164 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package admin
+
+import "github.com/gin-gonic/gin"
+
+// HeaderFilterAllowGET swagger:operation GET /api/v1/admin/header_allows/{id} headerFilterAllowGet
+//
+// Get "allow" header filter with the given ID.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	parameters:
+//	-
+//		name: id
+//		type: string
+//		description: Target header filter ID.
+//		in: path
+//		required: true
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'200':
+//			description: The requested "allow" header filter.
+//			schema:
+//				"$ref": "#/definitions/headerFilter"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: not found
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterAllowGET(c *gin.Context) {
+	m.getHeaderFilter(c, m.processor.Admin().GetAllowHeaderFilter)
+}
+
+// HeaderFilterBlockGET swagger:operation GET /api/v1/admin/header_blocks/{id} headerFilterBlockGet
+//
+// Get "block" header filter with the given ID.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	parameters:
+//	-
+//		name: id
+//		type: string
+//		description: Target header filter ID.
+//		in: path
+//		required: true
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'200':
+//			description: The requested "block" header filter.
+//			schema:
+//				"$ref": "#/definitions/headerFilter"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: not found
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterBlockGET(c *gin.Context) {
+	m.getHeaderFilter(c, m.processor.Admin().GetBlockHeaderFilter)
+}
+
+// HeaderFilterAllowsGET swagger:operation GET /api/v1/admin/header_allows headerFilterAllowsGet
+//
+// Get all "allow" header filters currently in place.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'200':
+//			description: All "allow" header filters currently in place.
+//			schema:
+//				type: array
+//				items:
+//					"$ref": "#/definitions/headerFilter"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: not found
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterAllowsGET(c *gin.Context) {
+	m.getHeaderFilters(c, m.processor.Admin().GetAllowHeaderFilters)
+}
+
+// HeaderFilterBlocksGET swagger:operation GET /api/v1/admin/header_blocks headerFilterBlocksGet
+//
+// Get all "allow" header filters currently in place.
+//
+//	---
+//	tags:
+//	- admin
+//
+//	security:
+//	- OAuth2 Bearer:
+//		- admin
+//
+//	responses:
+//		'200':
+//			description: All "block" header filters currently in place.
+//			schema:
+//				type: array
+//				items:
+//					"$ref": "#/definitions/headerFilter"
+//		'400':
+//			description: bad request
+//		'401':
+//			description: unauthorized
+//		'403':
+//			description: forbidden
+//		'404':
+//			description: not found
+//		'500':
+//			description: internal server error
+func (m *Module) HeaderFilterBlocksGET(c *gin.Context) {
+	m.getHeaderFilters(c, m.processor.Admin().GetBlockHeaderFilters)
+}
diff --git a/internal/api/model/headerfilter.go b/internal/api/model/headerfilter.go
new file mode 100644
index 000000000..96ba819f5
--- /dev/null
+++ b/internal/api/model/headerfilter.go
@@ -0,0 +1,55 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package model
+
+// HeaderFilter represents a regex value filter applied to one particular HTTP header (allow / block).
+type HeaderFilter struct {
+	// The ID of the header filter.
+	// example: 01FBW21XJA09XYX51KV5JVBW0F
+	// readonly: true
+	ID string `json:"id"`
+
+	// The HTTP header to match against.
+	// example: User-Agent
+	Header string `json:"header"`
+
+	// The header value matching regular expression.
+	// example: .*Firefox.*
+	Regex string `json:"regex"`
+
+	// The ID of the admin account that created this header filter.
+	// example: 01FBW2758ZB6PBR200YPDDJK4C
+	// readonly: true
+	CreatedBy string `json:"created_by"`
+
+	// Time at which the header filter was created (ISO 8601 Datetime).
+	// example: 2021-07-30T09:20:25+00:00
+	// readonly: true
+	CreatedAt string `json:"created_at"`
+}
+
+// HeaderFilterRequest is the form submitted as a POST to create a new header filter entry (allow / block).
+//
+// swagger:model headerFilterCreateRequest
+type HeaderFilterRequest struct {
+	// The HTTP header to match against (e.g. User-Agent).
+	Header string `form:"header" json:"header" xml:"header"`
+
+	// The header value matching regular expression.
+	Regex string `form:"regex" json:"regex" xml:"regex"`
+}
diff --git a/internal/api/util/response.go b/internal/api/util/response.go
index 150d2ac2e..753eaefb8 100644
--- a/internal/api/util/response.go
+++ b/internal/api/util/response.go
@@ -39,14 +39,17 @@ var (
 	StatusAcceptedJSON = mustJSON(map[string]string{
 		"status": http.StatusText(http.StatusAccepted),
 	})
+	StatusForbiddenJSON = mustJSON(map[string]string{
+		"status": http.StatusText(http.StatusForbidden),
+	})
 	StatusInternalServerErrorJSON = mustJSON(map[string]string{
 		"status": http.StatusText(http.StatusInternalServerError),
 	})
 	ErrorCapacityExceeded = mustJSON(map[string]string{
-		"error": "server capacity exceeded!",
+		"error": "server capacity exceeded",
 	})
-	ErrorRateLimitReached = mustJSON(map[string]string{
-		"error": "rate limit reached!",
+	ErrorRateLimited = mustJSON(map[string]string{
+		"error": "rate limit reached",
 	})
 	EmptyJSONObject = mustJSON("{}")
 	EmptyJSONArray  = mustJSON("[]")
diff --git a/internal/cache/cache.go b/internal/cache/cache.go
index dabf151ff..73e3ad6f0 100644
--- a/internal/cache/cache.go
+++ b/internal/cache/cache.go
@@ -18,21 +18,26 @@
 package cache
 
 import (
+	"github.com/superseriousbusiness/gotosocial/internal/cache/headerfilter"
 	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
 	"github.com/superseriousbusiness/gotosocial/internal/log"
 )
 
 type Caches struct {
-	// GTS provides access to the collection of gtsmodel object caches.
-	// (used by the database).
+	// GTS provides access to the collection of
+	// gtsmodel object caches. (used by the database).
 	GTS GTSCaches
 
-	// AP provides access to the collection of ActivityPub object caches.
-	// (planned to be used by the typeconverter).
-	AP APCaches
+	// AllowHeaderFilters provides access to
+	// the allow []headerfilter.Filter cache.
+	AllowHeaderFilters headerfilter.Cache
 
-	// Visibility provides access to the item visibility cache.
-	// (used by the visibility filter).
+	// BlockHeaderFilters provides access to
+	// the block []headerfilter.Filter cache.
+	BlockHeaderFilters headerfilter.Cache
+
+	// Visibility provides access to the item visibility
+	// cache. (used by the visibility filter).
 	Visibility VisibilityCache
 
 	// prevent pass-by-value.
@@ -45,7 +50,6 @@ func (c *Caches) Init() {
 	log.Infof(nil, "init: %p", c)
 
 	c.GTS.Init()
-	c.AP.Init()
 	c.Visibility.Init()
 
 	// Setup cache invalidate hooks.
@@ -58,7 +62,6 @@ func (c *Caches) Start() {
 	log.Infof(nil, "start: %p", c)
 
 	c.GTS.Start()
-	c.AP.Start()
 	c.Visibility.Start()
 }
 
@@ -67,7 +70,6 @@ func (c *Caches) Stop() {
 	log.Infof(nil, "stop: %p", c)
 
 	c.GTS.Stop()
-	c.AP.Stop()
 	c.Visibility.Stop()
 }
 
diff --git a/internal/cache/domain/domain.go b/internal/cache/domain/domain.go
index 051ec5c1b..1b836ed28 100644
--- a/internal/cache/domain/domain.go
+++ b/internal/cache/domain/domain.go
@@ -21,7 +21,6 @@ import (
 	"fmt"
 	"strings"
 	"sync/atomic"
-	"unsafe"
 
 	"golang.org/x/exp/slices"
 )
@@ -37,17 +36,17 @@ import (
 // The .Clear() function can be used to invalidate the cache,
 // e.g. when an entry is added / deleted from the database.
 type Cache struct {
-	// atomically updated ptr value to the
 	// current domain cache radix trie.
-	rootptr unsafe.Pointer
+	rootptr atomic.Pointer[root]
 }
 
 // Matches checks whether domain matches an entry in the cache.
 // If the cache is not currently loaded, then the provided load
 // function is used to hydrate it.
 func (c *Cache) Matches(domain string, load func() ([]string, error)) (bool, error) {
-	// Load the current root pointer value.
-	ptr := atomic.LoadPointer(&c.rootptr)
+	// Load the current
+	// root pointer value.
+	ptr := c.rootptr.Load()
 
 	if ptr == nil {
 		// Cache is not hydrated.
@@ -60,35 +59,32 @@ func (c *Cache) Matches(domain string, load func() ([]string, error)) (bool, err
 
 		// Allocate new radix trie
 		// node to store matches.
-		root := new(root)
+		ptr = new(root)
 
 		// Add each domain to the trie.
 		for _, domain := range domains {
-			root.Add(domain)
+			ptr.Add(domain)
 		}
 
 		// Sort the trie.
-		root.Sort()
+		ptr.Sort()
 
-		// Store the new node ptr.
-		ptr = unsafe.Pointer(root)
-		atomic.StorePointer(&c.rootptr, ptr)
+		// Store new node ptr.
+		c.rootptr.Store(ptr)
 	}
 
-	// Look for a match in the trie node.
-	return (*root)(ptr).Match(domain), nil
+	// Look for match in trie node.
+	return ptr.Match(domain), nil
 }
 
 // Clear will drop the currently loaded domain list,
 // triggering a reload on next call to .Matches().
-func (c *Cache) Clear() {
-	atomic.StorePointer(&c.rootptr, nil)
-}
+func (c *Cache) Clear() { c.rootptr.Store(nil) }
 
 // String returns a string representation of stored domains in cache.
 func (c *Cache) String() string {
-	if ptr := atomic.LoadPointer(&c.rootptr); ptr != nil {
-		return (*root)(ptr).String()
+	if ptr := c.rootptr.Load(); ptr != nil {
+		return ptr.String()
 	}
 	return "<empty>"
 }
diff --git a/internal/cache/headerfilter/filter.go b/internal/cache/headerfilter/filter.go
new file mode 100644
index 000000000..96b6e757f
--- /dev/null
+++ b/internal/cache/headerfilter/filter.go
@@ -0,0 +1,105 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package headerfilter
+
+import (
+	"fmt"
+	"net/http"
+	"sync/atomic"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/headerfilter"
+)
+
+// Cache provides a means of caching headerfilter.Filters in
+// memory to reduce load on an underlying storage mechanism.
+type Cache struct {
+	// current cached header filters slice.
+	ptr atomic.Pointer[headerfilter.Filters]
+}
+
+// RegularMatch performs .RegularMatch() on cached headerfilter.Filters, loading using callback if necessary.
+func (c *Cache) RegularMatch(h http.Header, load func() ([]*gtsmodel.HeaderFilter, error)) (string, string, error) {
+	// Load ptr value.
+	ptr := c.ptr.Load()
+
+	if ptr == nil {
+		// Cache is not hydrated.
+		// Load filters from callback.
+		filters, err := loadFilters(load)
+		if err != nil {
+			return "", "", err
+		}
+
+		// Store the new
+		// header filters.
+		ptr = &filters
+		c.ptr.Store(ptr)
+	}
+
+	// Deref and perform match.
+	return ptr.RegularMatch(h)
+}
+
+// InverseMatch performs .InverseMatch() on cached headerfilter.Filters, loading using callback if necessary.
+func (c *Cache) InverseMatch(h http.Header, load func() ([]*gtsmodel.HeaderFilter, error)) (string, string, error) {
+	// Load ptr value.
+	ptr := c.ptr.Load()
+
+	if ptr == nil {
+		// Cache is not hydrated.
+		// Load filters from callback.
+		filters, err := loadFilters(load)
+		if err != nil {
+			return "", "", err
+		}
+
+		// Store the new
+		// header filters.
+		ptr = &filters
+		c.ptr.Store(ptr)
+	}
+
+	// Deref and perform match.
+	return ptr.InverseMatch(h)
+}
+
+// Clear will drop the currently loaded filters,
+// triggering a reload on next call to ._Match().
+func (c *Cache) Clear() { c.ptr.Store(nil) }
+
+// loadFilters will load filters from given load callback, creating and parsing raw filters.
+func loadFilters(load func() ([]*gtsmodel.HeaderFilter, error)) (headerfilter.Filters, error) {
+	// Load filters from callback.
+	hdrFilters, err := load()
+	if err != nil {
+		return nil, fmt.Errorf("error reloading cache: %w", err)
+	}
+
+	// Allocate new header filter slice to store expressions.
+	filters := make(headerfilter.Filters, 0, len(hdrFilters))
+
+	// Add all raw expression to filter slice.
+	for _, filter := range hdrFilters {
+		if err := filters.Append(filter.Header, filter.Regex); err != nil {
+			return nil, fmt.Errorf("error appending exprs: %w", err)
+		}
+	}
+
+	return filters, nil
+}
diff --git a/internal/config/config.go b/internal/config/config.go
index 173999b53..68c065852 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -163,6 +163,7 @@ type Configuration struct {
 	AdvancedThrottlingRetryAfter time.Duration `name:"advanced-throttling-retry-after" usage:"Retry-After duration response to send for throttled requests."`
 	AdvancedSenderMultiplier     int           `name:"advanced-sender-multiplier" usage:"Multiplier to use per cpu for batching outgoing fedi messages. 0 or less turns batching off (not recommended)."`
 	AdvancedCSPExtraURIs         []string      `name:"advanced-csp-extra-uris" usage:"Additional URIs to allow when building content-security-policy for media + images."`
+	AdvancedHeaderFilterMode     string        `name:"advanced-header-filter-mode" usage:"Set incoming request header filtering mode."`
 
 	// HTTPClient configuration vars.
 	HTTPClient HTTPClientConfiguration `name:"http-client"`
diff --git a/internal/config/const.go b/internal/config/const.go
index 29e4b14e8..48087c4ce 100644
--- a/internal/config/const.go
+++ b/internal/config/const.go
@@ -17,10 +17,16 @@
 
 package config
 
-// Instance federation mode determines how this
-// instance federates with others (if at all).
 const (
+	// Instance federation mode determines how this
+	// instance federates with others (if at all).
 	InstanceFederationModeBlocklist = "blocklist"
 	InstanceFederationModeAllowlist = "allowlist"
 	InstanceFederationModeDefault   = InstanceFederationModeBlocklist
+
+	// Request header filter mode determines how
+	// this instance will perform request filtering.
+	RequestHeaderFilterModeAllow    = "allow"
+	RequestHeaderFilterModeBlock    = "block"
+	RequestHeaderFilterModeDisabled = ""
 )
diff --git a/internal/config/defaults.go b/internal/config/defaults.go
index 5aba6c689..3996b133f 100644
--- a/internal/config/defaults.go
+++ b/internal/config/defaults.go
@@ -135,6 +135,7 @@ var Defaults = Configuration{
 	AdvancedThrottlingRetryAfter: time.Second * 30,
 	AdvancedSenderMultiplier:     2, // 2 senders per CPU
 	AdvancedCSPExtraURIs:         []string{},
+	AdvancedHeaderFilterMode:     RequestHeaderFilterModeDisabled,
 
 	Cache: CacheConfiguration{
 		// Rough memory target that the total
diff --git a/internal/config/flags.go b/internal/config/flags.go
index 45ba70f9e..350f56635 100644
--- a/internal/config/flags.go
+++ b/internal/config/flags.go
@@ -158,6 +158,7 @@ func (s *ConfigState) AddServerFlags(cmd *cobra.Command) {
 		cmd.Flags().Duration(AdvancedThrottlingRetryAfterFlag(), cfg.AdvancedThrottlingRetryAfter, fieldtag("AdvancedThrottlingRetryAfter", "usage"))
 		cmd.Flags().Int(AdvancedSenderMultiplierFlag(), cfg.AdvancedSenderMultiplier, fieldtag("AdvancedSenderMultiplier", "usage"))
 		cmd.Flags().StringSlice(AdvancedCSPExtraURIsFlag(), cfg.AdvancedCSPExtraURIs, fieldtag("AdvancedCSPExtraURIs", "usage"))
+		cmd.Flags().String(AdvancedHeaderFilterModeFlag(), cfg.AdvancedHeaderFilterMode, fieldtag("AdvancedHeaderFilterMode", "usage"))
 
 		cmd.Flags().String(RequestIDHeaderFlag(), cfg.RequestIDHeader, fieldtag("RequestIDHeader", "usage"))
 	})
diff --git a/internal/config/helpers.gen.go b/internal/config/helpers.gen.go
index 393a1b1e9..72b2a7fd9 100644
--- a/internal/config/helpers.gen.go
+++ b/internal/config/helpers.gen.go
@@ -2600,6 +2600,31 @@ func GetAdvancedCSPExtraURIs() []string { return global.GetAdvancedCSPExtraURIs(
 // SetAdvancedCSPExtraURIs safely sets the value for global configuration 'AdvancedCSPExtraURIs' field
 func SetAdvancedCSPExtraURIs(v []string) { global.SetAdvancedCSPExtraURIs(v) }
 
+// GetAdvancedHeaderFilterMode safely fetches the Configuration value for state's 'AdvancedHeaderFilterMode' field
+func (st *ConfigState) GetAdvancedHeaderFilterMode() (v string) {
+	st.mutex.RLock()
+	v = st.config.AdvancedHeaderFilterMode
+	st.mutex.RUnlock()
+	return
+}
+
+// SetAdvancedHeaderFilterMode safely sets the Configuration value for state's 'AdvancedHeaderFilterMode' field
+func (st *ConfigState) SetAdvancedHeaderFilterMode(v string) {
+	st.mutex.Lock()
+	defer st.mutex.Unlock()
+	st.config.AdvancedHeaderFilterMode = v
+	st.reloadToViper()
+}
+
+// AdvancedHeaderFilterModeFlag returns the flag name for the 'AdvancedHeaderFilterMode' field
+func AdvancedHeaderFilterModeFlag() string { return "advanced-header-filter-mode" }
+
+// GetAdvancedHeaderFilterMode safely fetches the value for global configuration 'AdvancedHeaderFilterMode' field
+func GetAdvancedHeaderFilterMode() string { return global.GetAdvancedHeaderFilterMode() }
+
+// SetAdvancedHeaderFilterMode safely sets the value for global configuration 'AdvancedHeaderFilterMode' field
+func SetAdvancedHeaderFilterMode(v string) { global.SetAdvancedHeaderFilterMode(v) }
+
 // GetHTTPClientAllowIPs safely fetches the Configuration value for state's 'HTTPClient.AllowIPs' field
 func (st *ConfigState) GetHTTPClientAllowIPs() (v []string) {
 	st.mutex.RLock()
diff --git a/internal/db/basic.go b/internal/db/basic.go
index 7cd690aef..3a8e2af8d 100644
--- a/internal/db/basic.go
+++ b/internal/db/basic.go
@@ -25,10 +25,6 @@ type Basic interface {
 	// For implementations that don't use tables, this can just return nil.
 	CreateTable(ctx context.Context, i interface{}) error
 
-	// CreateAllTables creates *all* tables necessary for the running of GoToSocial.
-	// Because it uses the 'if not exists' parameter it is safe to run against a GtS that's already been initialized.
-	CreateAllTables(ctx context.Context) error
-
 	// DropTable drops the table for the given interface.
 	// For implementations that don't use tables, this can just return nil.
 	DropTable(ctx context.Context, i interface{}) error
diff --git a/internal/db/bundb/basic.go b/internal/db/bundb/basic.go
index e68903efa..488f59ad5 100644
--- a/internal/db/bundb/basic.go
+++ b/internal/db/bundb/basic.go
@@ -22,7 +22,6 @@ import (
 	"errors"
 
 	"github.com/superseriousbusiness/gotosocial/internal/db"
-	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
 	"github.com/superseriousbusiness/gotosocial/internal/log"
 	"github.com/uptrace/bun"
 )
@@ -120,39 +119,6 @@ func (b *basicDB) CreateTable(ctx context.Context, i interface{}) error {
 	return err
 }
 
-func (b *basicDB) CreateAllTables(ctx context.Context) error {
-	models := []interface{}{
-		&gtsmodel.Account{},
-		&gtsmodel.Application{},
-		&gtsmodel.Block{},
-		&gtsmodel.DomainBlock{},
-		&gtsmodel.EmailDomainBlock{},
-		&gtsmodel.Follow{},
-		&gtsmodel.FollowRequest{},
-		&gtsmodel.MediaAttachment{},
-		&gtsmodel.Mention{},
-		&gtsmodel.Status{},
-		&gtsmodel.StatusToEmoji{},
-		&gtsmodel.StatusFave{},
-		&gtsmodel.StatusBookmark{},
-		&gtsmodel.ThreadMute{},
-		&gtsmodel.Tag{},
-		&gtsmodel.User{},
-		&gtsmodel.Emoji{},
-		&gtsmodel.Instance{},
-		&gtsmodel.Notification{},
-		&gtsmodel.RouterSession{},
-		&gtsmodel.Token{},
-		&gtsmodel.Client{},
-	}
-	for _, i := range models {
-		if err := b.CreateTable(ctx, i); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
 func (b *basicDB) DropTable(ctx context.Context, i interface{}) error {
 	_, err := b.db.NewDropTable().Model(i).IfExists().Exec(ctx)
 	return err
diff --git a/internal/db/bundb/bundb.go b/internal/db/bundb/bundb.go
index f7417cfeb..d9415eff4 100644
--- a/internal/db/bundb/bundb.go
+++ b/internal/db/bundb/bundb.go
@@ -67,6 +67,7 @@ type DBService struct {
 	db.Basic
 	db.Domain
 	db.Emoji
+	db.HeaderFilter
 	db.Instance
 	db.List
 	db.Marker
@@ -193,6 +194,10 @@ func NewBunDBService(ctx context.Context, state *state.State) (db.DB, error) {
 			db:    db,
 			state: state,
 		},
+		HeaderFilter: &headerFilterDB{
+			db:    db,
+			state: state,
+		},
 		Instance: &instanceDB{
 			db:    db,
 			state: state,
diff --git a/internal/db/bundb/headerfilter.go b/internal/db/bundb/headerfilter.go
new file mode 100644
index 000000000..087b65c82
--- /dev/null
+++ b/internal/db/bundb/headerfilter.go
@@ -0,0 +1,207 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package bundb
+
+import (
+	"context"
+	"net/http"
+	"time"
+	"unsafe"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+	"github.com/uptrace/bun"
+)
+
+type headerFilterDB struct {
+	db    *DB
+	state *state.State
+}
+
+func (h *headerFilterDB) AllowHeaderRegularMatch(ctx context.Context, hdr http.Header) (string, string, error) {
+	return h.state.Caches.AllowHeaderFilters.RegularMatch(hdr, func() ([]*gtsmodel.HeaderFilter, error) {
+		return h.GetAllowHeaderFilters(ctx)
+	})
+}
+
+func (h *headerFilterDB) AllowHeaderInverseMatch(ctx context.Context, hdr http.Header) (string, string, error) {
+	return h.state.Caches.AllowHeaderFilters.InverseMatch(hdr, func() ([]*gtsmodel.HeaderFilter, error) {
+		return h.GetAllowHeaderFilters(ctx)
+	})
+}
+
+func (h *headerFilterDB) BlockHeaderRegularMatch(ctx context.Context, hdr http.Header) (string, string, error) {
+	return h.state.Caches.BlockHeaderFilters.RegularMatch(hdr, func() ([]*gtsmodel.HeaderFilter, error) {
+		return h.GetBlockHeaderFilters(ctx)
+	})
+}
+
+func (h *headerFilterDB) BlockHeaderInverseMatch(ctx context.Context, hdr http.Header) (string, string, error) {
+	return h.state.Caches.BlockHeaderFilters.InverseMatch(hdr, func() ([]*gtsmodel.HeaderFilter, error) {
+		return h.GetBlockHeaderFilters(ctx)
+	})
+}
+
+func (h *headerFilterDB) GetAllowHeaderFilter(ctx context.Context, id string) (*gtsmodel.HeaderFilter, error) {
+	filter := new(gtsmodel.HeaderFilterAllow)
+	if err := h.db.NewSelect().
+		Model(filter).
+		Where("? = ?", bun.Ident("id"), id).
+		Scan(ctx); err != nil {
+		return nil, err
+	}
+	return fromAllowFilter(filter), nil
+}
+
+func (h *headerFilterDB) GetBlockHeaderFilter(ctx context.Context, id string) (*gtsmodel.HeaderFilter, error) {
+	filter := new(gtsmodel.HeaderFilterBlock)
+	if err := h.db.NewSelect().
+		Model(filter).
+		Where("? = ?", bun.Ident("id"), id).
+		Scan(ctx); err != nil {
+		return nil, err
+	}
+	return fromBlockFilter(filter), nil
+}
+
+func (h *headerFilterDB) GetAllowHeaderFilters(ctx context.Context) ([]*gtsmodel.HeaderFilter, error) {
+	var filters []*gtsmodel.HeaderFilterAllow
+	err := h.db.NewSelect().
+		Model(&filters).
+		Scan(ctx, &filters)
+	return fromAllowFilters(filters), err
+}
+
+func (h *headerFilterDB) GetBlockHeaderFilters(ctx context.Context) ([]*gtsmodel.HeaderFilter, error) {
+	var filters []*gtsmodel.HeaderFilterBlock
+	err := h.db.NewSelect().
+		Model(&filters).
+		Scan(ctx, &filters)
+	return fromBlockFilters(filters), err
+}
+
+func (h *headerFilterDB) PutAllowHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter) error {
+	if _, err := h.db.NewInsert().
+		Model(toAllowFilter(filter)).
+		Exec(ctx); err != nil {
+		return err
+	}
+	h.state.Caches.AllowHeaderFilters.Clear()
+	return nil
+}
+
+func (h *headerFilterDB) PutBlockHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter) error {
+	if _, err := h.db.NewInsert().
+		Model(toBlockFilter(filter)).
+		Exec(ctx); err != nil {
+		return err
+	}
+	h.state.Caches.BlockHeaderFilters.Clear()
+	return nil
+}
+
+func (h *headerFilterDB) UpdateAllowHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter, cols ...string) error {
+	filter.UpdatedAt = time.Now()
+	if len(cols) > 0 {
+		// If we're updating by column,
+		// ensure "updated_at" is included.
+		cols = append(cols, "updated_at")
+	}
+	if _, err := h.db.NewUpdate().
+		Model(toAllowFilter(filter)).
+		Column(cols...).
+		Where("? = ?", bun.Ident("id"), filter.ID).
+		Exec(ctx); err != nil {
+		return err
+	}
+	h.state.Caches.AllowHeaderFilters.Clear()
+	return nil
+}
+
+func (h *headerFilterDB) UpdateBlockHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter, cols ...string) error {
+	filter.UpdatedAt = time.Now()
+	if len(cols) > 0 {
+		// If we're updating by column,
+		// ensure "updated_at" is included.
+		cols = append(cols, "updated_at")
+	}
+	if _, err := h.db.NewUpdate().
+		Model(toBlockFilter(filter)).
+		Column(cols...).
+		Where("? = ?", bun.Ident("id"), filter.ID).
+		Exec(ctx); err != nil {
+		return err
+	}
+	h.state.Caches.BlockHeaderFilters.Clear()
+	return nil
+}
+
+func (h *headerFilterDB) DeleteAllowHeaderFilter(ctx context.Context, id string) error {
+	if _, err := h.db.NewDelete().
+		Table("header_filter_allows").
+		Where("? = ?", bun.Ident("id"), id).
+		Exec(ctx); err != nil {
+		return err
+	}
+	h.state.Caches.AllowHeaderFilters.Clear()
+	return nil
+}
+
+func (h *headerFilterDB) DeleteBlockHeaderFilter(ctx context.Context, id string) error {
+	if _, err := h.db.NewDelete().
+		Table("header_filter_blocks").
+		Where("? = ?", bun.Ident("id"), id).
+		Exec(ctx); err != nil {
+		return err
+	}
+	h.state.Caches.BlockHeaderFilters.Clear()
+	return nil
+}
+
+// NOTE:
+// all of the below unsafe cast functions
+// are only possible because HeaderFilterAllow{},
+// HeaderFilterBlock{}, HeaderFilter{} while
+// different types in source, have exactly the
+// same size and layout in memory. the unsafe
+// cast simply changes the type associated with
+// that block of memory.
+
+func toAllowFilter(filter *gtsmodel.HeaderFilter) *gtsmodel.HeaderFilterAllow {
+	return (*gtsmodel.HeaderFilterAllow)(unsafe.Pointer(filter))
+}
+
+func toBlockFilter(filter *gtsmodel.HeaderFilter) *gtsmodel.HeaderFilterBlock {
+	return (*gtsmodel.HeaderFilterBlock)(unsafe.Pointer(filter))
+}
+
+func fromAllowFilter(filter *gtsmodel.HeaderFilterAllow) *gtsmodel.HeaderFilter {
+	return (*gtsmodel.HeaderFilter)(unsafe.Pointer(filter))
+}
+
+func fromBlockFilter(filter *gtsmodel.HeaderFilterBlock) *gtsmodel.HeaderFilter {
+	return (*gtsmodel.HeaderFilter)(unsafe.Pointer(filter))
+}
+
+func fromAllowFilters(filters []*gtsmodel.HeaderFilterAllow) []*gtsmodel.HeaderFilter {
+	return *(*[]*gtsmodel.HeaderFilter)(unsafe.Pointer(&filters))
+}
+
+func fromBlockFilters(filters []*gtsmodel.HeaderFilterBlock) []*gtsmodel.HeaderFilter {
+	return *(*[]*gtsmodel.HeaderFilter)(unsafe.Pointer(&filters))
+}
diff --git a/internal/db/bundb/headerfilter_test.go b/internal/db/bundb/headerfilter_test.go
new file mode 100644
index 000000000..d7e2b26ee
--- /dev/null
+++ b/internal/db/bundb/headerfilter_test.go
@@ -0,0 +1,125 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package bundb_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/suite"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+)
+
+type HeaderFilterTestSuite struct {
+	BunDBStandardTestSuite
+}
+
+func (suite *HeaderFilterTestSuite) TestAllowHeaderFilterGetPutUpdateDelete() {
+	suite.testHeaderFilterGetPutUpdateDelete(
+		suite.db.GetAllowHeaderFilter,
+		suite.db.GetAllowHeaderFilters,
+		suite.db.PutAllowHeaderFilter,
+		suite.db.UpdateAllowHeaderFilter,
+		suite.db.DeleteAllowHeaderFilter,
+	)
+}
+
+func (suite *HeaderFilterTestSuite) TestBlockHeaderFilterGetPutUpdateDelete() {
+	suite.testHeaderFilterGetPutUpdateDelete(
+		suite.db.GetBlockHeaderFilter,
+		suite.db.GetBlockHeaderFilters,
+		suite.db.PutBlockHeaderFilter,
+		suite.db.UpdateBlockHeaderFilter,
+		suite.db.DeleteBlockHeaderFilter,
+	)
+}
+
+func (suite *HeaderFilterTestSuite) testHeaderFilterGetPutUpdateDelete(
+	get func(context.Context, string) (*gtsmodel.HeaderFilter, error),
+	getAll func(context.Context) ([]*gtsmodel.HeaderFilter, error),
+	put func(context.Context, *gtsmodel.HeaderFilter) error,
+	update func(context.Context, *gtsmodel.HeaderFilter, ...string) error,
+	delete func(context.Context, string) error,
+) {
+	t := suite.T()
+
+	// Create new example header filter.
+	filter := gtsmodel.HeaderFilter{
+		ID:       "some unique id",
+		Header:   "Http-Header-Key",
+		Regex:    ".*",
+		AuthorID: "some unique author id",
+	}
+
+	// Create new cancellable test context.
+	ctx := context.Background()
+	ctx, cncl := context.WithCancel(ctx)
+	defer cncl()
+
+	// Insert the example header filter into db.
+	if err := put(ctx, &filter); err != nil {
+		t.Fatalf("error inserting header filter: %v", err)
+	}
+
+	// Now fetch newly created filter.
+	check, err := get(ctx, filter.ID)
+	if err != nil {
+		t.Fatalf("error fetching header filter: %v", err)
+	}
+
+	// Check all expected fields match.
+	suite.Equal(filter.ID, check.ID)
+	suite.Equal(filter.Header, check.Header)
+	suite.Equal(filter.Regex, check.Regex)
+	suite.Equal(filter.AuthorID, check.AuthorID)
+
+	// Fetch all header filters.
+	all, err := getAll(ctx)
+	if err != nil {
+		t.Fatalf("error fetching header filters: %v", err)
+	}
+
+	// Ensure contains example.
+	suite.Equal(len(all), 1)
+	suite.Equal(all[0].ID, filter.ID)
+
+	// Update the header filter regex value.
+	check.Regex = "new regex value"
+	if err := update(ctx, check); err != nil {
+		t.Fatalf("error updating header filter: %v", err)
+	}
+
+	// Ensure 'updated_at' was updated on check model.
+	suite.True(check.UpdatedAt.After(filter.UpdatedAt))
+
+	// Now delete the header filter from db.
+	if err := delete(ctx, filter.ID); err != nil {
+		t.Fatalf("error deleting header filter: %v", err)
+	}
+
+	// Ensure we can't refetch it.
+	_, err = get(ctx, filter.ID)
+	if err != db.ErrNoEntries {
+		t.Fatalf("deleted header filter returned unexpected error: %v", err)
+	}
+}
+
+func TestHeaderFilterTestSuite(t *testing.T) {
+	suite.Run(t, new(HeaderFilterTestSuite))
+}
diff --git a/internal/db/bundb/migrations/20231212144715_add_header_filters.go b/internal/db/bundb/migrations/20231212144715_add_header_filters.go
new file mode 100644
index 000000000..2d671bf46
--- /dev/null
+++ b/internal/db/bundb/migrations/20231212144715_add_header_filters.go
@@ -0,0 +1,54 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package migrations
+
+import (
+	"context"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/uptrace/bun"
+)
+
+func init() {
+	up := func(ctx context.Context, db *bun.DB) error {
+		for _, model := range []any{
+			&gtsmodel.HeaderFilterAllow{},
+			&gtsmodel.HeaderFilterBlock{},
+		} {
+			_, err := db.NewCreateTable().
+				IfNotExists().
+				Model(model).
+				Exec(ctx)
+			if err != nil {
+				return err
+			}
+		}
+
+		return nil
+	}
+
+	down := func(ctx context.Context, db *bun.DB) error {
+		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
+			return nil
+		})
+	}
+
+	if err := Migrations.Register(up, down); err != nil {
+		panic(err)
+	}
+}
diff --git a/internal/db/db.go b/internal/db/db.go
index 2914d9b59..361687e94 100644
--- a/internal/db/db.go
+++ b/internal/db/db.go
@@ -30,6 +30,7 @@ type DB interface {
 	Basic
 	Domain
 	Emoji
+	HeaderFilter
 	Instance
 	List
 	Marker
diff --git a/internal/db/headerfilter.go b/internal/db/headerfilter.go
new file mode 100644
index 000000000..5fe8a5b17
--- /dev/null
+++ b/internal/db/headerfilter.go
@@ -0,0 +1,73 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package db
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+)
+
+type HeaderFilter interface {
+	// AllowHeaderRegularMatch performs an headerfilter.Filter.RegularMatch() on cached allow header filters.
+	// (Note: the actual matching code can be found under ./internal/headerfilter/ ).
+	AllowHeaderRegularMatch(ctx context.Context, hdr http.Header) (string, string, error)
+
+	// AllowHeaderInverseMatch performs an headerfilter.Filter.InverseMatch() on cached allow header filters.
+	// (Note: the actual matching code can be found under ./internal/headerfilter/ ).
+	AllowHeaderInverseMatch(ctx context.Context, hdr http.Header) (string, string, error)
+
+	// BlockHeaderRegularMatch performs an headerfilter.Filter.RegularMatch() on cached block header filters.
+	// (Note: the actual matching code can be found under ./internal/headerfilter/ ).
+	BlockHeaderRegularMatch(ctx context.Context, hdr http.Header) (string, string, error)
+
+	// BlockHeaderInverseMatch performs an headerfilter.Filter.InverseMatch() on cached block header filters.
+	// (Note: the actual matching code can be found under ./internal/headerfilter/ ).
+	BlockHeaderInverseMatch(ctx context.Context, hdr http.Header) (string, string, error)
+
+	// GetAllowHeaderFilter fetches the allow header filter with ID from the database.
+	GetAllowHeaderFilter(ctx context.Context, id string) (*gtsmodel.HeaderFilter, error)
+
+	// GetBlockHeaderFilter fetches the block header filter with ID from the database.
+	GetBlockHeaderFilter(ctx context.Context, id string) (*gtsmodel.HeaderFilter, error)
+
+	// GetAllowHeaderFilters fetches all allow header filters from the database.
+	GetAllowHeaderFilters(ctx context.Context) ([]*gtsmodel.HeaderFilter, error)
+
+	// GetBlockHeaderFilters fetches all block header filters from the database.
+	GetBlockHeaderFilters(ctx context.Context) ([]*gtsmodel.HeaderFilter, error)
+
+	// PutAllowHeaderFilter inserts the given allow header filter into the database.
+	PutAllowHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter) error
+
+	// PutBlockHeaderFilter inserts the given block header filter into the database.
+	PutBlockHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter) error
+
+	// UpdateAllowHeaderFilter updates the given allow header filter in the database, only updating given columns if provided.
+	UpdateAllowHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter, cols ...string) error
+
+	// UpdateBlockHeaderFilter updates the given block header filter in the database, only updating given columns if provided.
+	UpdateBlockHeaderFilter(ctx context.Context, filter *gtsmodel.HeaderFilter, cols ...string) error
+
+	// DeleteAllowHeaderFilter deletes the allow header filter with ID from the database.
+	DeleteAllowHeaderFilter(ctx context.Context, id string) error
+
+	// DeleteBlockHeaderFilter deletes the block header filter with ID from the database.
+	DeleteBlockHeaderFilter(ctx context.Context, id string) error
+}
diff --git a/internal/gtsmodel/headerfilter.go b/internal/gtsmodel/headerfilter.go
new file mode 100644
index 000000000..d1fcb146e
--- /dev/null
+++ b/internal/gtsmodel/headerfilter.go
@@ -0,0 +1,54 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package gtsmodel
+
+import (
+	"time"
+	"unsafe"
+)
+
+func init() {
+	// Note that since all of the below calculations are
+	// constant, these should be optimized out of builds.
+	const filterSz = unsafe.Sizeof(HeaderFilter{})
+	if unsafe.Sizeof(HeaderFilterAllow{}) != filterSz {
+		panic("HeaderFilterAllow{} needs to have the same in-memory size / layout as HeaderFilter{}")
+	}
+	if unsafe.Sizeof(HeaderFilterBlock{}) != filterSz {
+		panic("HeaderFilterBlock{} needs to have the same in-memory size / layout as HeaderFilter{}")
+	}
+}
+
+// HeaderFilterAllow represents an allow HTTP header filter in the database.
+type HeaderFilterAllow struct{ HeaderFilter }
+
+// HeaderFilterBlock represents a block HTTP header filter in the database.
+type HeaderFilterBlock struct{ HeaderFilter }
+
+// HeaderFilter represents an HTTP request filter in
+// the database, with a header to match against, value
+// matching regex, and details about its creation.
+type HeaderFilter struct {
+	ID        string    `bun:"type:CHAR(26),pk,nullzero,notnull,unique"`                    // ID of this item in the database
+	Header    string    `bun:",nullzero,notnull,unique:header_regex"`                       // Request header this filter pertains to
+	Regex     string    `bun:",nullzero,notnull,unique:header_regex"`                       // Request header value matching regular expression
+	AuthorID  string    `bun:"type:CHAR(26),nullzero,notnull"`                              // Account ID of the creator of this filter
+	Author    *Account  `bun:"-"`                                                           // Account corresponding to AuthorID
+	CreatedAt time.Time `bun:"type:timestamptz,nullzero,notnull,default:current_timestamp"` // when was item created
+	UpdatedAt time.Time `bun:"type:timestamptz,nullzero,notnull,default:current_timestamp"` // when was item last updated
+}
diff --git a/internal/headerfilter/filter.go b/internal/headerfilter/filter.go
new file mode 100644
index 000000000..ab2aa914c
--- /dev/null
+++ b/internal/headerfilter/filter.go
@@ -0,0 +1,136 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package headerfilter
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+	"net/textproto"
+	"regexp"
+)
+
+// Maximum header value size before we return
+// an instant negative match. They shouldn't
+// go beyond this size in most cases anywho.
+const MaxHeaderValue = 1024
+
+// ErrLargeHeaderValue is returned on attempting to match on a value > MaxHeaderValue.
+var ErrLargeHeaderValue = errors.New("header value too large")
+
+// Filters represents a set of http.Header regular
+// expression filters built-in statistic tracking.
+type Filters []headerfilter
+
+type headerfilter struct {
+	// key is the header key to match against
+	// in canonical textproto mime header format.
+	key string
+
+	// exprs contains regular expressions to
+	// match values against for this header key.
+	exprs []*regexp.Regexp
+}
+
+// Append will add new header filter expression under given header key.
+func (fs *Filters) Append(key string, expr string) error {
+	var filter *headerfilter
+
+	// Ensure in canonical mime header format.
+	key = textproto.CanonicalMIMEHeaderKey(key)
+
+	// Look for existing filter
+	// with key in filter slice.
+	for i := range *fs {
+		if (*fs)[i].key == key {
+			filter = &((*fs)[i])
+			break
+		}
+	}
+
+	if filter == nil {
+		// No existing filter found, create new.
+
+		// Append new header filter to slice.
+		(*fs) = append((*fs), headerfilter{})
+
+		// Then take ptr to this new filter
+		// at the last index in the slice.
+		filter = &((*fs)[len((*fs))-1])
+
+		// Setup new key.
+		filter.key = key
+	}
+
+	// Compile regular expression.
+	reg, err := regexp.Compile(expr)
+	if err != nil {
+		return fmt.Errorf("error compiling regexp %q: %w", expr, err)
+	}
+
+	// Append regular expression to filter.
+	filter.exprs = append(filter.exprs, reg)
+
+	return nil
+}
+
+// RegularMatch returns whether any values in http header
+// matches any of the receiving filter regular expressions.
+// This returns the matched header key, and matching regexp.
+func (fs Filters) RegularMatch(h http.Header) (string, string, error) {
+	for _, filter := range fs {
+		for _, value := range h[filter.key] {
+			// Don't perform match on large values
+			// to mitigate denial of service attacks.
+			if len(value) > MaxHeaderValue {
+				return "", "", ErrLargeHeaderValue
+			}
+
+			// Compare against regular exprs.
+			for _, expr := range filter.exprs {
+				if expr.MatchString(value) {
+					return filter.key, expr.String(), nil
+				}
+			}
+		}
+	}
+	return "", "", nil
+}
+
+// InverseMatch returns whether any values in http header do
+// NOT match any of the receiving filter regular expressions.
+// This returns the matched header key, and matching regexp.
+func (fs Filters) InverseMatch(h http.Header) (string, string, error) {
+	for _, filter := range fs {
+		for _, value := range h[filter.key] {
+			// Don't perform match on large values
+			// to mitigate denial of service attacks.
+			if len(value) > MaxHeaderValue {
+				return "", "", ErrLargeHeaderValue
+			}
+
+			// Compare against regular exprs.
+			for _, expr := range filter.exprs {
+				if !expr.MatchString(value) {
+					return filter.key, expr.String(), nil
+				}
+			}
+		}
+	}
+	return "", "", nil
+}
diff --git a/internal/middleware/headerfilter.go b/internal/middleware/headerfilter.go
new file mode 100644
index 000000000..18c9d1e67
--- /dev/null
+++ b/internal/middleware/headerfilter.go
@@ -0,0 +1,251 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package middleware
+
+import (
+	"sync"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/headerfilter"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+)
+
+var (
+	allowMatches = matchstats{m: make(map[string]uint64)}
+	blockMatches = matchstats{m: make(map[string]uint64)}
+)
+
+// matchstats is a simple statistics
+// counter for header filter matches.
+// TODO: replace with otel.
+type matchstats struct {
+	m map[string]uint64
+	l sync.Mutex
+}
+
+func (m *matchstats) Add(hdr, regex string) {
+	m.l.Lock()
+	key := hdr + ":" + regex
+	m.m[key]++
+	m.l.Unlock()
+}
+
+// HeaderFilter returns a gin middleware handler that provides HTTP
+// request blocking (filtering) based on database allow / block filters.
+func HeaderFilter(state *state.State) gin.HandlerFunc {
+	switch mode := config.GetAdvancedHeaderFilterMode(); mode {
+	case config.RequestHeaderFilterModeDisabled:
+		return func(ctx *gin.Context) {}
+
+	case config.RequestHeaderFilterModeAllow:
+		return headerFilterAllowMode(state)
+
+	case config.RequestHeaderFilterModeBlock:
+		return headerFilterBlockMode(state)
+
+	default:
+		panic("unrecognized filter mode: " + mode)
+	}
+}
+
+func headerFilterAllowMode(state *state.State) func(c *gin.Context) {
+	_ = *state //nolint
+	// Allowlist mode: explicit block takes
+	// precedence over explicit allow.
+	//
+	// Headers that have neither block
+	// or allow entries are blocked.
+	return func(c *gin.Context) {
+
+		// Check if header is explicitly blocked.
+		block, err := isHeaderBlocked(state, c)
+		if err != nil {
+			respondInternalServerError(c, err)
+			return
+		}
+
+		if block {
+			respondBlocked(c)
+			return
+		}
+
+		// Check if header is missing explicit allow.
+		notAllow, err := isHeaderNotAllowed(state, c)
+		if err != nil {
+			respondInternalServerError(c, err)
+			return
+		}
+
+		if notAllow {
+			respondBlocked(c)
+			return
+		}
+
+		// Allowed!
+		c.Next()
+	}
+}
+
+func headerFilterBlockMode(state *state.State) func(c *gin.Context) {
+	_ = *state //nolint
+	// Blocklist/default mode: explicit allow
+	// takes precedence over explicit block.
+	//
+	// Headers that have neither block
+	// or allow entries are allowed.
+	return func(c *gin.Context) {
+
+		// Check if header is explicitly allowed.
+		allow, err := isHeaderAllowed(state, c)
+		if err != nil {
+			respondInternalServerError(c, err)
+			return
+		}
+
+		if !allow {
+			// Check if header is explicitly blocked.
+			block, err := isHeaderBlocked(state, c)
+			if err != nil {
+				respondInternalServerError(c, err)
+				return
+			}
+
+			if block {
+				respondBlocked(c)
+				return
+			}
+		}
+
+		// Allowed!
+		c.Next()
+	}
+}
+
+func isHeaderBlocked(state *state.State, c *gin.Context) (bool, error) {
+	var (
+		ctx = c.Request.Context()
+		hdr = c.Request.Header
+	)
+
+	// Perform an explicit is-blocked check on request header.
+	key, expr, err := state.DB.BlockHeaderRegularMatch(ctx, hdr)
+	switch err {
+	case nil:
+		break
+
+	case headerfilter.ErrLargeHeaderValue:
+		log.Warn(ctx, "large header value")
+		key = "*" // block large headers
+
+	default:
+		err := gtserror.Newf("error checking header: %w", err)
+		return false, err
+	}
+
+	if key != "" {
+		if expr != "" {
+			// Increment block matches stat.
+			// TODO: replace expvar with build
+			// taggable metrics types in State{}.
+			blockMatches.Add(key, expr)
+		}
+
+		// A header was matched against!
+		// i.e. this request is blocked.
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func isHeaderAllowed(state *state.State, c *gin.Context) (bool, error) {
+	var (
+		ctx = c.Request.Context()
+		hdr = c.Request.Header
+	)
+
+	// Perform an explicit is-allowed check on request header.
+	key, expr, err := state.DB.AllowHeaderRegularMatch(ctx, hdr)
+	switch err {
+	case nil:
+		break
+
+	case headerfilter.ErrLargeHeaderValue:
+		log.Warn(ctx, "large header value")
+		key = "" // block large headers
+
+	default:
+		err := gtserror.Newf("error checking header: %w", err)
+		return false, err
+	}
+
+	if key != "" {
+		if expr != "" {
+			// Increment allow matches stat.
+			// TODO: replace expvar with build
+			// taggable metrics types in State{}.
+			allowMatches.Add(key, expr)
+		}
+
+		// A header was matched against!
+		// i.e. this request is allowed.
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func isHeaderNotAllowed(state *state.State, c *gin.Context) (bool, error) {
+	var (
+		ctx = c.Request.Context()
+		hdr = c.Request.Header
+	)
+
+	// Perform an explicit is-NOT-allowed check on request header.
+	key, expr, err := state.DB.AllowHeaderInverseMatch(ctx, hdr)
+	switch err {
+	case nil:
+		break
+
+	case headerfilter.ErrLargeHeaderValue:
+		log.Warn(ctx, "large header value")
+		key = "*" // block large headers
+
+	default:
+		err := gtserror.Newf("error checking header: %w", err)
+		return false, err
+	}
+
+	if key != "" {
+		if expr != "" {
+			// Increment allow matches stat.
+			// TODO: replace expvar with build
+			// taggable metrics types in State{}.
+			allowMatches.Add(key, expr)
+		}
+
+		// A header was matched against!
+		// i.e. request is NOT allowed.
+		return true, nil
+	}
+
+	return false, nil
+}
diff --git a/internal/middleware/headerfilter_test.go b/internal/middleware/headerfilter_test.go
new file mode 100644
index 000000000..a28644153
--- /dev/null
+++ b/internal/middleware/headerfilter_test.go
@@ -0,0 +1,299 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package middleware_test
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/db/bundb"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/headerfilter"
+	"github.com/superseriousbusiness/gotosocial/internal/id"
+	"github.com/superseriousbusiness/gotosocial/internal/middleware"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+	"github.com/superseriousbusiness/gotosocial/testrig"
+)
+
+func TestHeaderFilter(t *testing.T) {
+	testrig.InitTestLog()
+	testrig.InitTestConfig()
+
+	for _, test := range []struct {
+		mode   string
+		allow  []filter
+		block  []filter
+		input  http.Header
+		expect bool
+	}{
+		{
+			// Allow mode with expected 200 OK.
+			mode: config.RequestHeaderFilterModeAllow,
+			allow: []filter{
+				{"User-Agent", ".*Firefox.*"},
+			},
+			block: []filter{},
+			input: http.Header{
+				"User-Agent": []string{"Firefox v169.42; Extra Tracking Info"},
+			},
+			expect: true,
+		},
+		{
+			// Allow mode with expected 403 Forbidden.
+			mode: config.RequestHeaderFilterModeAllow,
+			allow: []filter{
+				{"User-Agent", ".*Firefox.*"},
+			},
+			block: []filter{},
+			input: http.Header{
+				"User-Agent": []string{"Chromium v169.42; Extra Tracking Info"},
+			},
+			expect: false,
+		},
+		{
+			// Allow mode with too long header value expecting 403 Forbidden.
+			mode: config.RequestHeaderFilterModeAllow,
+			allow: []filter{
+				{"User-Agent", ".*"},
+			},
+			block: []filter{},
+			input: http.Header{
+				"User-Agent": []string{func() string {
+					var buf strings.Builder
+					for i := 0; i < headerfilter.MaxHeaderValue+1; i++ {
+						buf.WriteByte(' ')
+					}
+					return buf.String()
+				}()},
+			},
+			expect: false,
+		},
+		{
+			// Allow mode with explicit block expecting 403 Forbidden.
+			mode: config.RequestHeaderFilterModeAllow,
+			allow: []filter{
+				{"User-Agent", ".*Firefox.*"},
+			},
+			block: []filter{
+				{"User-Agent", ".*Firefox v169\\.42.*"},
+			},
+			input: http.Header{
+				"User-Agent": []string{"Firefox v169.42; Extra Tracking Info"},
+			},
+			expect: false,
+		},
+		{
+			// Block mode with an expected 403 Forbidden.
+			mode:  config.RequestHeaderFilterModeBlock,
+			allow: []filter{},
+			block: []filter{
+				{"User-Agent", ".*Firefox.*"},
+			},
+			input: http.Header{
+				"User-Agent": []string{"Firefox v169.42; Extra Tracking Info"},
+			},
+			expect: false,
+		},
+		{
+			// Block mode with an expected 200 OK.
+			mode:  config.RequestHeaderFilterModeBlock,
+			allow: []filter{},
+			block: []filter{
+				{"User-Agent", ".*Firefox.*"},
+			},
+			input: http.Header{
+				"User-Agent": []string{"Chromium v169.42; Extra Tracking Info"},
+			},
+			expect: true,
+		},
+		{
+			// Block mode with too long header value expecting 403 Forbidden.
+			mode:  config.RequestHeaderFilterModeBlock,
+			allow: []filter{},
+			block: []filter{
+				{"User-Agent", "none"},
+			},
+			input: http.Header{
+				"User-Agent": []string{func() string {
+					var buf strings.Builder
+					for i := 0; i < headerfilter.MaxHeaderValue+1; i++ {
+						buf.WriteByte(' ')
+					}
+					return buf.String()
+				}()},
+			},
+			expect: false,
+		},
+		{
+			// Block mode with explicit allow expecting 200 OK.
+			mode: config.RequestHeaderFilterModeBlock,
+			allow: []filter{
+				{"User-Agent", ".*Firefox.*"},
+			},
+			block: []filter{
+				{"User-Agent", ".*Firefox v169\\.42.*"},
+			},
+			input: http.Header{
+				"User-Agent": []string{"Firefox v169.42; Extra Tracking Info"},
+			},
+			expect: true,
+		},
+		{
+			// Disabled mode with an expected 200 OK.
+			mode: config.RequestHeaderFilterModeDisabled,
+			allow: []filter{
+				{"Key1", "only-this"},
+				{"Key2", "only-this"},
+				{"Key3", "only-this"},
+			},
+			block: []filter{
+				{"Key1", "Value"},
+				{"Key2", "Value"},
+				{"Key3", "Value"},
+			},
+			input: http.Header{
+				"Key1": []string{"Value"},
+				"Key2": []string{"Value"},
+				"Key3": []string{"Value"},
+			},
+			expect: true,
+		},
+	} {
+		// Generate a unique name for this test case.
+		name := fmt.Sprintf("%s allow=%v block=%v => expect=%v",
+			test.mode,
+			test.allow,
+			test.block,
+			test.expect,
+		)
+
+		// Update header filter mode to test case.
+		config.SetAdvancedHeaderFilterMode(test.mode)
+
+		// Run this particular test case.
+		ok := t.Run(name, func(t *testing.T) {
+			testHeaderFilter(t,
+				test.allow,
+				test.block,
+				test.input,
+				test.expect,
+			)
+		})
+
+		if !ok {
+			return
+		}
+	}
+}
+
+func testHeaderFilter(t *testing.T, allow, block []filter, input http.Header, expect bool) {
+	var err error
+
+	// Create test context with cancel.
+	ctx := context.Background()
+	ctx, cncl := context.WithCancel(ctx)
+	defer cncl()
+
+	// Initialize caches.
+	var state state.State
+	state.Caches.Init()
+
+	// Create new database instance with test config.
+	state.DB, err = bundb.NewBunDBService(ctx, &state)
+	if err != nil {
+		t.Fatalf("error opening database: %v", err)
+	}
+
+	// Insert all allow filters into DB.
+	for _, filter := range allow {
+		filter := &gtsmodel.HeaderFilter{
+			ID:       id.NewULID(),
+			Header:   filter.header,
+			Regex:    filter.regex,
+			AuthorID: "admin-id",
+			Author:   nil,
+		}
+
+		if err := state.DB.PutAllowHeaderFilter(ctx, filter); err != nil {
+			t.Fatalf("error inserting allow filter into database: %v", err)
+		}
+	}
+
+	// Insert all block filters into DB.
+	for _, filter := range block {
+		filter := &gtsmodel.HeaderFilter{
+			ID:       id.NewULID(),
+			Header:   filter.header,
+			Regex:    filter.regex,
+			AuthorID: "admin-id",
+			Author:   nil,
+		}
+
+		if err := state.DB.PutBlockHeaderFilter(ctx, filter); err != nil {
+			t.Fatalf("error inserting block filter into database: %v", err)
+		}
+	}
+
+	// Gin test http engine
+	// (used for ctx init).
+	e := gin.New()
+
+	// Create new filter middleware to test against.
+	middleware := middleware.HeaderFilter(&state)
+	e.Use(middleware)
+
+	// Set the empty gin handler (always returns okay).
+	e.Handle("GET", "/", func(ctx *gin.Context) { ctx.Status(200) })
+
+	// Prepare a gin test context.
+	r := httptest.NewRequest("GET", "/", nil)
+	rw := httptest.NewRecorder()
+
+	// Set input headers.
+	r.Header = input
+
+	// Pass req through
+	// engine handler.
+	e.ServeHTTP(rw, r)
+
+	// Get http result.
+	res := rw.Result()
+
+	switch {
+	case expect && res.StatusCode != http.StatusOK:
+		t.Errorf("unexpected response (should allow): %s", res.Status)
+
+	case !expect && res.StatusCode != http.StatusForbidden:
+		t.Errorf("unexpected response (should block): %s", res.Status)
+	}
+}
+
+type filter struct {
+	header string
+	regex  string
+}
+
+func (hf *filter) String() string {
+	return fmt.Sprintf("%s=%q", hf.header, hf.regex)
+}
diff --git a/internal/middleware/ratelimit.go b/internal/middleware/ratelimit.go
index 57055fe70..352a30c22 100644
--- a/internal/middleware/ratelimit.go
+++ b/internal/middleware/ratelimit.go
@@ -146,7 +146,7 @@ func RateLimit(limit int, exceptions []string) gin.HandlerFunc {
 			apiutil.Data(c,
 				http.StatusTooManyRequests,
 				apiutil.AppJSON,
-				apiutil.ErrorRateLimitReached,
+				apiutil.ErrorRateLimited,
 			)
 			c.Abort()
 			return
diff --git a/internal/middleware/useragent.go b/internal/middleware/useragent.go
index 6dc3e401f..38d28f4e5 100644
--- a/internal/middleware/useragent.go
+++ b/internal/middleware/useragent.go
@@ -18,21 +18,22 @@
 package middleware
 
 import (
-	"errors"
 	"net/http"
 
 	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
 )
 
 // UserAgent returns a gin middleware which aborts requests with
 // empty user agent strings, returning code 418 - I'm a teapot.
 func UserAgent() gin.HandlerFunc {
 	// todo: make this configurable
+	var rsp = []byte(`{"error": "I'm a teapot: no user-agent sent with request"}`)
 	return func(c *gin.Context) {
 		if ua := c.Request.UserAgent(); ua == "" {
-			code := http.StatusTeapot
-			err := errors.New(http.StatusText(code) + ": no user-agent sent with request")
-			c.AbortWithStatusJSON(code, gin.H{"error": err.Error()})
+			apiutil.Data(c,
+				http.StatusTeapot, apiutil.AppJSON, rsp)
+			c.Abort()
 		}
 	}
 }
diff --git a/internal/middleware/util.go b/internal/middleware/util.go
new file mode 100644
index 000000000..82850fd6d
--- /dev/null
+++ b/internal/middleware/util.go
@@ -0,0 +1,51 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	apiutil "github.com/superseriousbusiness/gotosocial/internal/api/util"
+)
+
+// respondBlocked responds to the given gin context with
+// status forbidden, and a generic forbidden JSON response,
+// finally aborting the gin handler chain.
+func respondBlocked(c *gin.Context) {
+	apiutil.Data(c,
+		http.StatusForbidden,
+		apiutil.AppJSON,
+		apiutil.StatusForbiddenJSON,
+	)
+	c.Abort()
+}
+
+// respondInternalServerError responds to the given gin context
+// with status internal server error, a generic internal server
+// error JSON response, sets the given error on the gin context
+// for later logging, finally aborting the gin handler chain.
+func respondInternalServerError(c *gin.Context, err error) {
+	apiutil.Data(c,
+		http.StatusInternalServerError,
+		apiutil.AppJSON,
+		apiutil.StatusInternalServerErrorJSON,
+	)
+	_ = c.Error(err)
+	c.Abort()
+}
diff --git a/internal/processing/admin/headerfilter.go b/internal/processing/admin/headerfilter.go
new file mode 100644
index 000000000..13105d191
--- /dev/null
+++ b/internal/processing/admin/headerfilter.go
@@ -0,0 +1,215 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package admin
+
+import (
+	"context"
+	"errors"
+	"net/textproto"
+	"regexp"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/headerfilter"
+	"github.com/superseriousbusiness/gotosocial/internal/id"
+	"github.com/superseriousbusiness/gotosocial/internal/util"
+)
+
+// GetAllowHeaderFilter fetches allow HTTP header filter with provided ID from the database.
+func (p *Processor) GetAllowHeaderFilter(ctx context.Context, id string) (*apimodel.HeaderFilter, gtserror.WithCode) {
+	return p.getHeaderFilter(ctx, id, p.state.DB.GetAllowHeaderFilter)
+}
+
+// GetBlockHeaderFilter fetches block HTTP header filter with provided ID from the database.
+func (p *Processor) GetBlockHeaderFilter(ctx context.Context, id string) (*apimodel.HeaderFilter, gtserror.WithCode) {
+	return p.getHeaderFilter(ctx, id, p.state.DB.GetBlockHeaderFilter)
+}
+
+// GetAllowHeaderFilters fetches all allow HTTP header filters stored in the database.
+func (p *Processor) GetAllowHeaderFilters(ctx context.Context) ([]*apimodel.HeaderFilter, gtserror.WithCode) {
+	return p.getHeaderFilters(ctx, p.state.DB.GetAllowHeaderFilters)
+}
+
+// GetBlockHeaderFilters fetches all block HTTP header filters stored in the database.
+func (p *Processor) GetBlockHeaderFilters(ctx context.Context) ([]*apimodel.HeaderFilter, gtserror.WithCode) {
+	return p.getHeaderFilters(ctx, p.state.DB.GetBlockHeaderFilters)
+}
+
+// CreateAllowHeaderFilter inserts the incoming allow HTTP header filter into the database, marking as authored by provided admin account.
+func (p *Processor) CreateAllowHeaderFilter(ctx context.Context, admin *gtsmodel.Account, request *apimodel.HeaderFilterRequest) (*apimodel.HeaderFilter, gtserror.WithCode) {
+	return p.createHeaderFilter(ctx, admin, request, p.state.DB.PutAllowHeaderFilter)
+}
+
+// CreateBlockHeaderFilter inserts the incoming block HTTP header filter into the database, marking as authored by provided admin account.
+func (p *Processor) CreateBlockHeaderFilter(ctx context.Context, admin *gtsmodel.Account, request *apimodel.HeaderFilterRequest) (*apimodel.HeaderFilter, gtserror.WithCode) {
+	return p.createHeaderFilter(ctx, admin, request, p.state.DB.PutBlockHeaderFilter)
+}
+
+// DeleteAllowHeaderFilter deletes the allowing HTTP header filter with provided ID from the database.
+func (p *Processor) DeleteAllowHeaderFilter(ctx context.Context, id string) gtserror.WithCode {
+	return p.deleteHeaderFilter(ctx, id, p.state.DB.DeleteAllowHeaderFilter)
+}
+
+// DeleteBlockHeaderFilter deletes the blocking HTTP header filter with provided ID from the database.
+func (p *Processor) DeleteBlockHeaderFilter(ctx context.Context, id string) gtserror.WithCode {
+	return p.deleteHeaderFilter(ctx, id, p.state.DB.DeleteBlockHeaderFilter)
+}
+
+// getHeaderFilter fetches an HTTP header filter with
+// provided ID, using given get function, converting the
+// resulting filter to returnable frontend API model.
+func (p *Processor) getHeaderFilter(
+	ctx context.Context,
+	id string,
+	get func(context.Context, string) (*gtsmodel.HeaderFilter, error),
+) (
+	*apimodel.HeaderFilter,
+	gtserror.WithCode,
+) {
+	// Select filter by ID from db.
+	filter, err := get(ctx, id)
+
+	switch {
+	// Successfully found.
+	case err == nil:
+		return toAPIHeaderFilter(filter), nil
+
+	// Filter does not exist with ID.
+	case errors.Is(err, db.ErrNoEntries):
+		const text = "filter not found"
+		return nil, gtserror.NewErrorNotFound(errors.New(text), text)
+
+	// Any other error type.
+	default:
+		err := gtserror.Newf("error selecting from database: %w", err)
+		return nil, gtserror.NewErrorInternalError(err)
+	}
+}
+
+// getHeaderFilters fetches all HTTP header filters
+// using given get function, converting the resulting
+// filters to returnable frontend API models.
+func (p *Processor) getHeaderFilters(
+	ctx context.Context,
+	get func(context.Context) ([]*gtsmodel.HeaderFilter, error),
+) (
+	[]*apimodel.HeaderFilter,
+	gtserror.WithCode,
+) {
+	// Select all filters from DB.
+	filters, err := get(ctx)
+
+	if err != nil && !errors.Is(err, db.ErrNoEntries) {
+		// Only handle errors other than not-found types.
+		err := gtserror.Newf("error selecting from database: %w", err)
+		return nil, gtserror.NewErrorInternalError(err)
+	}
+
+	// Convert passed header filters to apimodel filters.
+	apiFilters := make([]*apimodel.HeaderFilter, len(filters))
+	for i := range filters {
+		apiFilters[i] = toAPIHeaderFilter(filters[i])
+	}
+
+	return apiFilters, nil
+}
+
+// createHeaderFilter inserts the given HTTP header
+// filter into database, marking as authored by the
+// provided admin, using the given insert function.
+func (p *Processor) createHeaderFilter(
+	ctx context.Context,
+	admin *gtsmodel.Account,
+	request *apimodel.HeaderFilterRequest,
+	insert func(context.Context, *gtsmodel.HeaderFilter) error,
+) (
+	*apimodel.HeaderFilter,
+	gtserror.WithCode,
+) {
+	// Convert header key to canonical mime header format.
+	request.Header = textproto.CanonicalMIMEHeaderKey(request.Header)
+
+	// Validate incoming header filter.
+	if errWithCode := validateHeaderFilter(
+		request.Header,
+		request.Regex,
+	); errWithCode != nil {
+		return nil, errWithCode
+	}
+
+	// Create new database model with ID.
+	var filter gtsmodel.HeaderFilter
+	filter.ID = id.NewULID()
+	filter.Header = request.Header
+	filter.Regex = request.Regex
+	filter.AuthorID = admin.ID
+	filter.Author = admin
+
+	// Insert new header filter into the database.
+	if err := insert(ctx, &filter); err != nil {
+		err := gtserror.Newf("error inserting into database: %w", err)
+		return nil, gtserror.NewErrorInternalError(err)
+	}
+
+	// Finally return API model response.
+	return toAPIHeaderFilter(&filter), nil
+}
+
+// deleteHeaderFilter deletes the HTTP header filter
+// with provided ID, using the given delete function.
+func (p *Processor) deleteHeaderFilter(
+	ctx context.Context,
+	id string,
+	delete func(context.Context, string) error,
+) gtserror.WithCode {
+	if err := delete(ctx, id); err != nil && !errors.Is(err, db.ErrNoEntries) {
+		err := gtserror.Newf("error deleting from database: %w", err)
+		return gtserror.NewErrorInternalError(err)
+	}
+	return nil
+}
+
+// toAPIFilter performs a simple conversion of database model HeaderFilter to API model.
+func toAPIHeaderFilter(filter *gtsmodel.HeaderFilter) *apimodel.HeaderFilter {
+	return &apimodel.HeaderFilter{
+		ID:        filter.ID,
+		Header:    filter.Header,
+		Regex:     filter.Regex,
+		CreatedBy: filter.AuthorID,
+		CreatedAt: util.FormatISO8601(filter.CreatedAt),
+	}
+}
+
+// validateHeaderFilter validates incoming filter's header key, and regular expression.
+func validateHeaderFilter(header, regex string) gtserror.WithCode {
+	// Check header validity (within our own bound checks).
+	if header == "" || len(header) > headerfilter.MaxHeaderValue {
+		const text = "invalid request header key (empty or too long)"
+		return gtserror.NewErrorBadRequest(errors.New(text), text)
+	}
+
+	// Ensure this is compilable regex.
+	_, err := regexp.Compile(regex)
+	if err != nil {
+		return gtserror.NewErrorBadRequest(err, err.Error())
+	}
+
+	return nil
+}