1 files changed, 136 insertions, 0 deletions
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
+}