31 files changed, 811 insertions, 66 deletions

diff --git a/Documentation/RelNotes/2.14.6.txt b/Documentation/RelNotes/2.14.6.txt
new file mode 100644
index 0000000000..72b7af6799
--- /dev/null
+++ b/Documentation/RelNotes/2.14.6.txt
@@ -0,0 +1,54 @@
+Git v2.14.6 Release Notes
+=========================
+
+This release addresses the security issues CVE-2019-1348,
+CVE-2019-1349, CVE-2019-1350, CVE-2019-1351, CVE-2019-1352,
+CVE-2019-1353, CVE-2019-1354, and CVE-2019-1387.
+
+Fixes since v2.14.5
+-------------------
+
+ * CVE-2019-1348:
+   The --export-marks option of git fast-import is exposed also via
+   the in-stream command feature export-marks=... and it allows
+   overwriting arbitrary paths.
+
+ * CVE-2019-1349:
+   When submodules are cloned recursively, under certain circumstances
+   Git could be fooled into using the same Git directory twice. We now
+   require the directory to be empty.
+
+ * CVE-2019-1350:
+   Incorrect quoting of command-line arguments allowed remote code
+   execution during a recursive clone in conjunction with SSH URLs.
+
+ * CVE-2019-1351:
+   While the only permitted drive letters for physical drives on
+   Windows are letters of the US-English alphabet, this restriction
+   does not apply to virtual drives assigned via subst <letter>:
+   <path>. Git mistook such paths for relative paths, allowing writing
+   outside of the worktree while cloning.
+
+ * CVE-2019-1352:
+   Git was unaware of NTFS Alternate Data Streams, allowing files
+   inside the .git/ directory to be overwritten during a clone.
+
+ * CVE-2019-1353:
+   When running Git in the Windows Subsystem for Linux (also known as
+   "WSL") while accessing a working directory on a regular Windows
+   drive, none of the NTFS protections were active.
+
+ * CVE-2019-1354:
+   Filenames on Linux/Unix can contain backslashes. On Windows,
+   backslashes are directory separators. Git did not use to refuse to
+   write out tracked files with such filenames.
+
+ * CVE-2019-1387:
+   Recursive clones are currently affected by a vulnerability that is
+   caused by too-lax validation of submodule names, allowing very
+   targeted attacks via remote code execution in recursive clones.
+
+Credit for finding these vulnerabilities goes to Microsoft Security
+Response Center, in particular to Nicolas Joly. The `fast-import`
+fixes were provided by Jeff King, the other fixes by Johannes
+Schindelin with help from Garima Singh.
diff --git a/Documentation/git-fast-import.txt b/Documentation/git-fast-import.txt
index 3d3d219e58..ff71fc2962 100644
--- a/Documentation/git-fast-import.txt
+++ b/Documentation/git-fast-import.txt
@@ -50,6 +50,21 @@ OPTIONS
 	memory used by fast-import during this run.  Showing this output
 	is currently the default, but can be disabled with --quiet.
 
+--allow-unsafe-features::
+	Many command-line options can be provided as part of the
+	fast-import stream itself by using the `feature` or `option`
+	commands. However, some of these options are unsafe (e.g.,
+	allowing fast-import to access the filesystem outside of the
+	repository). These options are disabled by default, but can be
+	allowed by providing this option on the command line.  This
+	currently impacts only the `export-marks`, `import-marks`, and
+	`import-marks-if-exists` feature commands.
++
+	Only enable this option if you trust the program generating the
+	fast-import stream! This option is enabled automatically for
+	remote-helpers that use the `import` capability, as they are
+	already trusted to run their own code.
+
 Options for Frontends
 ~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/builtin/clone.c b/builtin/clone.c
index dbddd98f80..73d1c2b30f 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -758,7 +758,7 @@ static int checkout(int submodule_progress)
 
 	if (!err && (option_recurse_submodules.nr > 0)) {
 		struct argv_array args = ARGV_ARRAY_INIT;
-		argv_array_pushl(&args, "submodule", "update", "--init", "--recursive", NULL);
+		argv_array_pushl(&args, "submodule", "update", "--require-init", "--recursive", NULL);
 
 		if (option_shallow_submodules == 1)
 			argv_array_push(&args, "--depth=1");
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 30e0bb876c..0d03fa8bf9 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -13,6 +13,7 @@
 #include "remote.h"
 #include "refs.h"
 #include "connect.h"
+#include "dir.h"
 
 static char *get_default_remote(void)
 {
@@ -616,6 +617,7 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 	char *p, *path = NULL, *sm_gitdir;
 	struct strbuf sb = STRBUF_INIT;
 	struct string_list reference = STRING_LIST_INIT_NODUP;
+	int require_init = 0;
 	char *sm_alternate = NULL, *error_strategy = NULL;
 
 	struct option module_clone_options[] = {
@@ -640,6 +642,8 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 		OPT__QUIET(&quiet, "Suppress output for cloning a submodule"),
 		OPT_BOOL(0, "progress", &progress,
 			   N_("force cloning progress")),
+		OPT_BOOL(0, "require-init", &require_init,
+			   N_("disallow cloning into non-empty directory")),
 		OPT_END()
 	};
 
@@ -667,6 +671,10 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 	} else
 		path = xstrdup(path);
 
+	if (validate_submodule_git_dir(sm_gitdir, name) < 0)
+		die(_("refusing to create/use '%s' in another submodule's "
+			"git dir"), sm_gitdir);
+
 	if (!file_exists(sm_gitdir)) {
 		if (safe_create_leading_directories_const(sm_gitdir) < 0)
 			die(_("could not create directory '%s'"), sm_gitdir);
@@ -678,6 +686,8 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 			die(_("clone of '%s' into submodule path '%s' failed"),
 			    url, path);
 	} else {
+		if (require_init && !access(path, X_OK) && !is_empty_dir(path))
+			die(_("directory not empty: '%s'"), path);
 		if (safe_create_leading_directories_const(path) < 0)
 			die(_("could not create directory '%s'"), path);
 		strbuf_addf(&sb, "%s/index", sm_gitdir);
@@ -726,6 +736,7 @@ struct submodule_update_clone {
 	int quiet;
 	int recommend_shallow;
 	struct string_list references;
+	unsigned require_init;
 	const char *depth;
 	const char *recursive_prefix;
 	const char *prefix;
@@ -741,7 +752,7 @@ struct submodule_update_clone {
 	int failed_clones_nr, failed_clones_alloc;
 };
 #define SUBMODULE_UPDATE_CLONE_INIT {0, MODULE_LIST_INIT, 0, \
-	SUBMODULE_UPDATE_STRATEGY_INIT, 0, 0, -1, STRING_LIST_INIT_DUP, \
+	SUBMODULE_UPDATE_STRATEGY_INIT, 0, 0, -1, STRING_LIST_INIT_DUP, 0, \
 	NULL, NULL, NULL, \
 	STRING_LIST_INIT_DUP, 0, NULL, 0, 0}
 
@@ -860,6 +871,8 @@ static int prepare_to_clone_next_submodule(const struct cache_entry *ce,
 		argv_array_pushl(&child->args, "--prefix", suc->prefix, NULL);
 	if (suc->recommend_shallow && sub->recommend_shallow == 1)
 		argv_array_push(&child->args, "--depth=1");
+	if (suc->require_init)
+		argv_array_push(&child->args, "--require-init");
 	argv_array_pushl(&child->args, "--path", sub->path, NULL);
 	argv_array_pushl(&child->args, "--name", sub->name, NULL);
 	argv_array_pushl(&child->args, "--url", url, NULL);
@@ -1011,6 +1024,8 @@ static int update_clone(int argc, const char **argv, const char *prefix)
 		OPT__QUIET(&suc.quiet, N_("don't print cloning progress")),
 		OPT_BOOL(0, "progress", &suc.progress,
 			    N_("force cloning progress")),
+		OPT_BOOL(0, "require-init", &suc.require_init,
+			   N_("disallow cloning into non-empty directory")),
 		OPT_END()
 	};
 
diff --git a/compat/mingw.c b/compat/mingw.c
index 2d44d21aca..725cd6cb50 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -333,6 +333,12 @@ int mingw_mkdir(const char *path, int mode)
 {
 	int ret;
 	wchar_t wpath[MAX_PATH];
+
+	if (!is_valid_win32_path(path)) {
+		errno = EINVAL;
+		return -1;
+	}
+
 	if (xutftowcs_path(wpath, path) < 0)
 		return -1;
 	ret = _wmkdir(wpath);
@@ -345,13 +351,18 @@ int mingw_open (const char *filename, int oflags, ...)
 {
 	va_list args;
 	unsigned mode;
-	int fd;
+	int fd, create = (oflags & (O_CREAT | O_EXCL)) == (O_CREAT | O_EXCL);
 	wchar_t wfilename[MAX_PATH];
 
 	va_start(args, oflags);
 	mode = va_arg(args, int);
 	va_end(args);
 
+	if (!is_valid_win32_path(filename)) {
+		errno = create ? EINVAL : ENOENT;
+		return -1;
+	}
+
 	if (filename && !strcmp(filename, "/dev/null"))
 		filename = "nul";
 
@@ -413,6 +424,11 @@ FILE *mingw_fopen (const char *filename, const char *otype)
 	int hide = needs_hiding(filename);
 	FILE *file;
 	wchar_t wfilename[MAX_PATH], wotype[4];
+	if (!is_valid_win32_path(filename)) {
+		int create = otype && strchr(otype, 'w');
+		errno = create ? EINVAL : ENOENT;
+		return NULL;
+	}
 	if (filename && !strcmp(filename, "/dev/null"))
 		filename = "nul";
 	if (xutftowcs_path(wfilename, filename) < 0 ||
@@ -435,6 +451,11 @@ FILE *mingw_freopen (const char *filename, const char *otype, FILE *stream)
 	int hide = needs_hiding(filename);
 	FILE *file;
 	wchar_t wfilename[MAX_PATH], wotype[4];
+	if (!is_valid_win32_path(filename)) {
+		int create = otype && strchr(otype, 'w');
+		errno = create ? EINVAL : ENOENT;
+		return NULL;
+	}
 	if (filename && !strcmp(filename, "/dev/null"))
 		filename = "nul";
 	if (xutftowcs_path(wfilename, filename) < 0 ||
@@ -872,7 +893,7 @@ static const char *quote_arg(const char *arg)
 				p++;
 				len++;
 			}
-			if (*p == '"')
+			if (*p == '"' || !*p)
 				n += count*2 + 1;
 			continue;
 		}
@@ -894,16 +915,19 @@ static const char *quote_arg(const char *arg)
 				count++;
 				*d++ = *arg++;
 			}
-			if (*arg == '"') {
+			if (*arg == '"' || !*arg) {
 				while (count-- > 0)
 					*d++ = '\\';
+				/* don't escape the surrounding end quote */
+				if (!*arg)
+					break;
 				*d++ = '\\';
 			}
 		}
 		*d++ = *arg++;
 	}
 	*d++ = '"';
-	*d++ = 0;
+	*d++ = '\0';
 	return q;
 }
 
@@ -1965,6 +1989,30 @@ pid_t waitpid(pid_t pid, int *status, int options)
 	return -1;
 }
 
+int mingw_has_dos_drive_prefix(const char *path)
+{
+	int i;
+
+	/*
+	 * Does it start with an ASCII letter (i.e. highest bit not set),
+	 * followed by a colon?
+	 */
+	if (!(0x80 & (unsigned char)*path))
+		return *path && path[1] == ':' ? 2 : 0;
+
+	/*
+	 * While drive letters must be letters of the English alphabet, it is
+	 * possible to assign virtually _any_ Unicode character via `subst` as
+	 * a drive letter to "virtual drives". Even `1`, or `ä`. Or fun stuff
+	 * like this:
+	 *
+	 *      subst ֍: %USERPROFILE%\Desktop
+	 */
+	for (i = 1; i < 4 && (0x80 & (unsigned char)path[i]); i++)
+		; /* skip first UTF-8 character */
+	return path[i] == ':' ? i + 1 : 0;
+}
+
 int mingw_skip_dos_drive_prefix(char **path)
 {
 	int ret = has_dos_drive_prefix(*path);
@@ -2106,6 +2154,50 @@ static void setup_windows_environment(void)
 		setenv("TERM", "cygwin", 1);
 }
 
+int is_valid_win32_path(const char *path)
+{
+	int preceding_space_or_period = 0, i = 0, periods = 0;
+
+	if (!protect_ntfs)
+		return 1;
+
+	skip_dos_drive_prefix((char **)&path);
+
+	for (;;) {
+		char c = *(path++);
+		switch (c) {
+		case '\0':
+		case '/': case '\\':
+			/* cannot end in ` ` or `.`, except for `.` and `..` */
+			if (preceding_space_or_period &&
+			    (i != periods || periods > 2))
+				return 0;
+			if (!c)
+				return 1;
+
+			i = periods = preceding_space_or_period = 0;
+			continue;
+		case '.':
+			periods++;
+			/* fallthru */
+		case ' ':
+			preceding_space_or_period = 1;
+			i++;
+			continue;
+		case ':': /* DOS drive prefix was already skipped */
+		case '<': case '>': case '"': case '|': case '?': case '*':
+			/* illegal character */
+			return 0;
+		default:
+			if (c > '\0' && c < '\x20')
+				/* illegal character */
+				return 0;
+		}
+		preceding_space_or_period = 0;
+		i++;
+	}
+}
+
 /*
  * Disable MSVCRT command line wildcard expansion (__getmainargs called from
  * mingw startup code, see init.c in mingw runtime).
diff --git a/compat/mingw.h b/compat/mingw.h
index e03aecfe2e..17064665d9 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -394,8 +394,8 @@ HANDLE winansi_get_osfhandle(int fd);
  * git specific compatibility
  */
 
-#define has_dos_drive_prefix(path) \
-	(isalpha(*(path)) && (path)[1] == ':' ? 2 : 0)
+int mingw_has_dos_drive_prefix(const char *path);
+#define has_dos_drive_prefix mingw_has_dos_drive_prefix
 int mingw_skip_dos_drive_prefix(char **path);
 #define skip_dos_drive_prefix mingw_skip_dos_drive_prefix
 static inline int mingw_is_dir_sep(int c)
@@ -429,6 +429,20 @@ int mingw_offset_1st_component(const char *path);
 #endif
 
 /**
+ * Verifies that the given path is a valid one on Windows.
+ *
+ * In particular, path segments are disallowed which
+ *
+ * - end in a period or a space (except the special directories `.` and `..`).
+ *
+ * - contain any of the reserved characters, e.g. `:`, `;`, `*`, etc
+ *
+ * Returns 1 upon success, otherwise 0.
+ */
+int is_valid_win32_path(const char *path);
+#define is_valid_path(path) is_valid_win32_path(path)
+
+/**
  * Converts UTF-8 encoded string to UTF-16LE.
  *
  * To support repositories with legacy-encoded file names, invalid UTF-8 bytes
diff --git a/config.mak.uname b/config.mak.uname
index 685a80d138..d39a436848 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -381,7 +381,6 @@ ifeq ($(uname_S),Windows)
 	EXTLIBS = user32.lib advapi32.lib shell32.lib wininet.lib ws2_32.lib invalidcontinue.obj
 	PTHREAD_LIBS =
 	lib =
-	BASIC_CFLAGS += -DPROTECT_NTFS_DEFAULT=1
 ifndef DEBUG
 	BASIC_CFLAGS += -GL -Os -MD
 	BASIC_LDFLAGS += -LTCG
@@ -519,7 +518,6 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	COMPAT_OBJS += compat/mingw.o compat/winansi.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
 		compat/win32/dirent.o
-	BASIC_CFLAGS += -DPROTECT_NTFS_DEFAULT=1
 	EXTLIBS += -lws2_32
 	GITLIBS += git.res
 	PTHREAD_LIBS =
diff --git a/connect.c b/connect.c
index df56c0cbff..15eeb4ae75 100644
--- a/connect.c
+++ b/connect.c
@@ -264,7 +264,7 @@ int url_is_local_not_ssh(const char *url)
 	const char *colon = strchr(url, ':');
 	const char *slash = strchr(url, '/');
 	return !colon || (slash && slash < colon) ||
-		has_dos_drive_prefix(url);
+		(has_dos_drive_prefix(url) && is_valid_path(url));
 }
 
 static const char *prot_name(enum protocol protocol)
diff --git a/environment.c b/environment.c
index 8289c25b44..bca118772b 100644
--- a/environment.c
+++ b/environment.c
@@ -73,7 +73,7 @@ enum log_refs_config log_all_ref_updates = LOG_REFS_UNSET;
 int protect_hfs = PROTECT_HFS_DEFAULT;
 
 #ifndef PROTECT_NTFS_DEFAULT
-#define PROTECT_NTFS_DEFAULT 0
+#define PROTECT_NTFS_DEFAULT 1
 #endif
 int protect_ntfs = PROTECT_NTFS_DEFAULT;
 
diff --git a/fast-import.c b/fast-import.c
index d5e4cf0bad..b7167ab20e 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -367,6 +367,7 @@ static uintmax_t next_mark;
 static struct strbuf new_data = STRBUF_INIT;
 static int seen_data_command;
 static int require_explicit_termination;
+static int allow_unsafe_features;
 
 /* Signal handling */
 static volatile sig_atomic_t checkpoint_requested;
@@ -1862,6 +1863,12 @@ static void dump_marks(void)
 	if (!export_marks_file || (import_marks_file && !import_marks_file_done))
 		return;
 
+	if (safe_create_leading_directories_const(export_marks_file)) {
+		failure |= error_errno("unable to create leading directories of %s",
+				       export_marks_file);
+		return;
+	}
+
 	if (hold_lock_file_for_update(&mark_lock, export_marks_file, 0) < 0) {
 		failure |= error_errno("Unable to write marks file %s",
 				       export_marks_file);
@@ -3229,7 +3236,6 @@ static void option_import_marks(const char *marks,
 	}
 
 	import_marks_file = make_fast_import_path(marks);
-	safe_create_leading_directories_const(import_marks_file);
 	import_marks_file_from_stream = from_stream;
 	import_marks_file_ignore_missing = ignore_missing;
 }
@@ -3270,7 +3276,6 @@ static void option_active_branches(const char *branches)
 static void option_export_marks(const char *marks)
 {
 	export_marks_file = make_fast_import_path(marks);
-	safe_create_leading_directories_const(export_marks_file);
 }
 
 static void option_cat_blob_fd(const char *fd)
@@ -3313,10 +3318,12 @@ static int parse_one_option(const char *option)
 		option_active_branches(option);
 	} else if (skip_prefix(option, "export-pack-edges=", &option)) {
 		option_export_pack_edges(option);
-	} else if (starts_with(option, "quiet")) {
+	} else if (!strcmp(option, "quiet")) {
 		show_stats = 0;
-	} else if (starts_with(option, "stats")) {
+	} else if (!strcmp(option, "stats")) {
 		show_stats = 1;
+	} else if (!strcmp(option, "allow-unsafe-features")) {
+		; /* already handled during early option parsing */
 	} else {
 		return 0;
 	}
@@ -3324,6 +3331,13 @@ static int parse_one_option(const char *option)
 	return 1;
 }
 
+static void check_unsafe_feature(const char *feature, int from_stream)
+{
+	if (from_stream && !allow_unsafe_features)
+		die(_("feature '%s' forbidden in input without --allow-unsafe-features"),
+		    feature);
+}
+
 static int parse_one_feature(const char *feature, int from_stream)
 {
 	const char *arg;
@@ -3331,10 +3345,13 @@ static int parse_one_feature(const char *feature, int from_stream)
 	if (skip_prefix(feature, "date-format=", &arg)) {
 		option_date_format(arg);
 	} else if (skip_prefix(feature, "import-marks=", &arg)) {
+		check_unsafe_feature("import-marks", from_stream);
 		option_import_marks(arg, from_stream, 0);
 	} else if (skip_prefix(feature, "import-marks-if-exists=", &arg)) {
+		check_unsafe_feature("import-marks-if-exists", from_stream);
 		option_import_marks(arg, from_stream, 1);
 	} else if (skip_prefix(feature, "export-marks=", &arg)) {
+		check_unsafe_feature(feature, from_stream);
 		option_export_marks(arg);
 	} else if (!strcmp(feature, "get-mark")) {
 		; /* Don't die - this feature is supported */
@@ -3461,6 +3478,20 @@ int cmd_main(int argc, const char **argv)
 	avail_tree_table = xcalloc(avail_tree_table_sz, sizeof(struct avail_tree_content*));
 	marks = pool_calloc(1, sizeof(struct mark_set));
 
+	/*
+	 * We don't parse most options until after we've seen the set of
+	 * "feature" lines at the start of the stream (which allows the command
+	 * line to override stream data). But we must do an early parse of any
+	 * command-line options that impact how we interpret the feature lines.
+	 */
+	for (i = 1; i < argc; i++) {
+		const char *arg = argv[i];
+		if (*arg != '-' || !strcmp(arg, "--"))
+			break;
+		if (!strcmp(arg, "--allow-unsafe-features"))
+			allow_unsafe_features = 1;
+	}
+
 	global_argc = argc;
 	global_argv = argv;
 
diff --git a/fsck.c b/fsck.c
index 032699e9ac..0b2b93aabd 100644
--- a/fsck.c
+++ b/fsck.c
@@ -551,7 +551,7 @@ static int fsck_tree(struct tree *item, struct fsck_options *options)
 
 	while (desc.size) {
 		unsigned mode;
-		const char *name;
+		const char *name, *backslash;
 		const struct object_id *oid;
 
 		oid = tree_entry_extract(&desc, &name, &mode);
@@ -565,6 +565,15 @@ static int fsck_tree(struct tree *item, struct fsck_options *options)
 			       is_hfs_dotgit(name) ||
 			       is_ntfs_dotgit(name));
 		has_zero_pad |= *(char *)desc.buffer == '0';
+
+		if ((backslash = strchr(name, '\\'))) {
+			while (backslash) {
+				backslash++;
+				has_dotgit |= is_ntfs_dotgit(backslash);
+				backslash = strchr(backslash, '\\');
+			}
+		}
+
 		if (update_tree_entry_gently(&desc)) {
 			retval += report(options, &item->object, FSCK_MSG_BAD_TREE, "cannot be parsed as a tree");
 			break;
diff --git a/git-compat-util.h b/git-compat-util.h
index 0d270ffc27..be8e4296fd 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -370,6 +370,10 @@ static inline int git_offset_1st_component(const char *path)
 #define offset_1st_component git_offset_1st_component
 #endif
 
+#ifndef is_valid_path
+#define is_valid_path(path) 1
+#endif
+
 #ifndef find_last_dir_sep
 static inline char *git_find_last_dir_sep(const char *path)
 {
diff --git a/git-submodule.sh b/git-submodule.sh
index 8f260fbd9c..e4843a5874 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -34,6 +34,7 @@ reference=
 cached=
 recursive=
 init=
+require_init=
 files=
 remote=
 nofetch=
@@ -528,6 +529,10 @@ cmd_update()
 		-i|--init)
 			init=1
 			;;
+		--require-init)
+			init=1
+			require_init=1
+			;;
 		--remote)
 			remote=1
 			;;
@@ -606,6 +611,7 @@ cmd_update()
 		${update:+--update "$update"} \
 		${reference:+"$reference"} \
 		${depth:+--depth "$depth"} \
+		${require_init:+--require-init} \
 		${recommend_shallow:+"$recommend_shallow"} \
 		${jobs:+$jobs} \
 		"$@" || echo "#unmatched" $?
diff --git a/path.c b/path.c
index 4c4a751539..84e88243a9 100644
--- a/path.c
+++ b/path.c
@@ -1289,37 +1289,77 @@ int daemon_avoid_alias(const char *p)
 	}
 }
 
-static int only_spaces_and_periods(const char *path, size_t len, size_t skip)
+/*
+ * On NTFS, we need to be careful to disallow certain synonyms of the `.git/`
+ * directory:
+ *
+ * - For historical reasons, file names that end in spaces or periods are
+ *   automatically trimmed. Therefore, `.git . . ./` is a valid way to refer
+ *   to `.git/`.
+ *
+ * - For other historical reasons, file names that do not conform to the 8.3
+ *   format (up to eight characters for the basename, three for the file
+ *   extension, certain characters not allowed such as `+`, etc) are associated
+ *   with a so-called "short name", at least on the `C:` drive by default.
+ *   Which means that `git~1/` is a valid way to refer to `.git/`.
+ *
+ *   Note: Technically, `.git/` could receive the short name `git~2` if the
+ *   short name `git~1` were already used. In Git, however, we guarantee that
+ *   `.git` is the first item in a directory, therefore it will be associated
+ *   with the short name `git~1` (unless short names are disabled).
+ *
+ * - For yet other historical reasons, NTFS supports so-called "Alternate Data
+ *   Streams", i.e. metadata associated with a given file, referred to via
+ *   `<filename>:<stream-name>:<stream-type>`. There exists a default stream
+ *   type for directories, allowing `.git/` to be accessed via
+ *   `.git::$INDEX_ALLOCATION/`.
+ *
+ * When this function returns 1, it indicates that the specified file/directory
+ * name refers to a `.git` file or directory, or to any of these synonyms, and
+ * Git should therefore not track it.
+ *
+ * For performance reasons, _all_ Alternate Data Streams of `.git/` are
+ * forbidden, not just `::$INDEX_ALLOCATION`.
+ *
+ * This function is intended to be used by `git fsck` even on platforms where
+ * the backslash is a regular filename character, therefore it needs to handle
+ * backlash characters in the provided `name` specially: they are interpreted
+ * as directory separators.
+ */
+int is_ntfs_dotgit(const char *name)
 {
-	if (len < skip)
+	char c;
+
+	/*
+	 * Note that when we don't find `.git` or `git~1` we end up with `name`
+	 * advanced partway through the string. That's okay, though, as we
+	 * return immediately in those cases, without looking at `name` any
+	 * further.
+	 */
+	c = *(name++);
+	if (c == '.') {
+		/* .git */
+		if (((c = *(name++)) != 'g' && c != 'G') ||
+		    ((c = *(name++)) != 'i' && c != 'I') ||
+		    ((c = *(name++)) != 't' && c != 'T'))
+			return 0;
+	} else if (c == 'g' || c == 'G') {
+		/* git ~1 */
+		if (((c = *(name++)) != 'i' && c != 'I') ||
+		    ((c = *(name++)) != 't' && c != 'T') ||
+		    *(name++) != '~' ||
+		    *(name++) != '1')
+			return 0;
+	} else
 		return 0;
-	len -= skip;
-	path += skip;
-	while (len-- > 0) {
-		char c = *(path++);
-		if (c != ' ' && c != '.')
+
+	for (;;) {
+		c = *(name++);
+		if (!c || c == '\\' || c == '/' || c == ':')
+			return 1;
+		if (c != '.' && c != ' ')
 			return 0;
 	}
-	return 1;
-}
-
-int is_ntfs_dotgit(const char *name)
-{
-	size_t len;
-
-	for (len = 0; ; len++)
-		if (!name[len] || name[len] == '\\' || is_dir_sep(name[len])) {
-			if (only_spaces_and_periods(name, len, 4) &&
-					!strncasecmp(name, ".git", 4))
-				return 1;
-			if (only_spaces_and_periods(name, len, 5) &&
-					!strncasecmp(name, "git~1", 5))
-				return 1;
-			if (name[len] != '\\')
-				return 0;
-			name += len + 1;
-			len = -1;
-		}
 }
 
 static int is_ntfs_dot_generic(const char *name,
@@ -1335,7 +1375,7 @@ static int is_ntfs_dot_generic(const char *name,
 only_spaces_and_periods:
 		for (;;) {
 			char c = name[i++];
-			if (!c)
+			if (!c || c == ':')
 				return 1;
 			if (c != ' ' && c != '.')
 				return 0;
diff --git a/read-cache.c b/read-cache.c
index 8cef7b6715..2a280b83cc 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -848,6 +848,9 @@ int verify_path(const char *path, unsigned mode)
 	if (has_dos_drive_prefix(path))
 		return 0;
 
+	if (!is_valid_path(path))
+		return 0;
+
 	goto inside;
 	for (;;) {
 		if (!c)
@@ -875,7 +878,15 @@ inside:
 			if ((c == '.' && !verify_dotfile(path, mode)) ||
 			    is_dir_sep(c) || c == '\0')
 				return 0;
+		} else if (c == '\\' && protect_ntfs) {
+			if (is_ntfs_dotgit(path))
+				return 0;
+			if (S_ISLNK(mode)) {
+				if (is_ntfs_dotgitmodules(path))
+					return 0;
+			}
 		}
+
 		c = *path++;
 	}
 }
diff --git a/submodule.c b/submodule.c
index 63e7094e16..2ae209f208 100644
--- a/submodule.c
+++ b/submodule.c
@@ -1794,6 +1794,47 @@ int merge_submodule(struct object_id *result, const char *path,
 	return 0;
 }
 
+int validate_submodule_git_dir(char *git_dir, const char *submodule_name)
+{
+	size_t len = strlen(git_dir), suffix_len = strlen(submodule_name);
+	char *p;
+	int ret = 0;
+
+	if (len <= suffix_len || (p = git_dir + len - suffix_len)[-1] != '/' ||
+	    strcmp(p, submodule_name))
+		BUG("submodule name '%s' not a suffix of git dir '%s'",
+		    submodule_name, git_dir);
+
+	/*
+	 * We prevent the contents of sibling submodules' git directories to
+	 * clash.
+	 *
+	 * Example: having a submodule named `hippo` and another one named
+	 * `hippo/hooks` would result in the git directories
+	 * `.git/modules/hippo/` and `.git/modules/hippo/hooks/`, respectively,
+	 * but the latter directory is already designated to contain the hooks
+	 * of the former.
+	 */
+	for (; *p; p++) {
+		if (is_dir_sep(*p)) {
+			char c = *p;
+
+			*p = '\0';
+			if (is_git_directory(git_dir))
+				ret = -1;
+			*p = c;
+
+			if (ret < 0)
+				return error(_("submodule git dir '%s' is "
+					       "inside git dir '%.*s'"),
+					     git_dir,
+					     (int)(p - git_dir), git_dir);
+		}
+	}
+
+	return 0;
+}
+
 /*
  * Embeds a single submodules git directory into the superprojects git dir,
  * non recursively.
@@ -1802,7 +1843,7 @@ static void relocate_single_git_dir_into_superproject(const char *prefix,
 						      const char *path)
 {
 	char *old_git_dir = NULL, *real_old_git_dir = NULL, *real_new_git_dir = NULL;
-	const char *new_git_dir;
+	char *new_git_dir;
 	const struct submodule *sub;
 
 	if (submodule_uses_worktrees(path))
@@ -1820,10 +1861,14 @@ static void relocate_single_git_dir_into_superproject(const char *prefix,
 	if (!sub)
 		die(_("could not lookup name for submodule '%s'"), path);
 
-	new_git_dir = git_path("modules/%s", sub->name);
+	new_git_dir = git_pathdup("modules/%s", sub->name);
+	if (validate_submodule_git_dir(new_git_dir, sub->name) < 0)
+		die(_("refusing to move '%s' into an existing git dir"),
+		    real_old_git_dir);
 	if (safe_create_leading_directories_const(new_git_dir) < 0)
 		die(_("could not create directory '%s'"), new_git_dir);
 	real_new_git_dir = real_pathdup(new_git_dir, 1);