Refuse to follow invalid paths in .git files

This corresponds to the fixes in TortoiseGit v2.19 that were announced
in https://groups.google.com/g/tortoisegit-announce/c/31zdmOBi4vY.

The threat surface in Git for Windows is a lot smaller than in
TortoiseGit because Git is not integrated directly into the Windows
Explorer and hence does not run automatically on any extracted archive.
A Git user would have to run Git operations on an extracted archive of
dubious origin, which is never a good idea. In all other circumstances,
the `.git` files Git would see would always be written by Git for
Windows itself, which will never write `.git` files containing invalid
paths (or paths pointing to other servers). For this reason, it was
deemed okay to integrate this change as a regular patch.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -3299,6 +3299,30 @@ static int acls_supported(const char *path)
 	return 0;
 }
 
+int is_valid_windows_path_element(wchar_t ch)
+{
+	// cf. https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file
+	// Windows disallows ASCII control characters 0x00–0x1F
+	if (ch < 0x1F)
+		return false;
+
+	// Windows reserved path characters
+	switch (ch) {
+	case L'<':
+	case L'>':
+	case L':':
+	case L'"':
+	case L'/':
+	case L'\\':
+	case L'|':
+	case L'?':
+	case L'*':
+		return 0;
+	default:
+		return 1;
+	}
+}
+
 int is_path_owned_by_current_sid(const char *path, struct strbuf *report)
 {
 	WCHAR wpath[MAX_PATH];
@@ -3327,6 +3351,15 @@ int is_path_owned_by_current_sid(const char *path, struct strbuf *report)
 	if (!wcsicmp(wpath, home))
 		return 1;
 
+	/* Do not leak NTLM hashes, UNC paths etc. are generally problematic */
+	if (wpath[0] == L'\\' && !is_valid_windows_path_element(wpath[1])) {
+		if (report)
+			strbuf_addf(report,
+				    "'%s' may refer to a non-local directory",
+				    path);
+		return 0;
+	}
+
 	/* Get the owner SID */
 	err = GetNamedSecurityInfoW(wpath, SE_FILE_OBJECT,
 				    OWNER_SECURITY_INFORMATION |

compat/mingw.h

@@ -46,6 +46,7 @@ char *mingw_query_user_email(void);
  */
 int is_path_owned_by_current_sid(const char *path, struct strbuf *report);
 #define is_path_owned_by_current_user is_path_owned_by_current_sid
+int is_valid_windows_path_element(wchar_t ch);
 
 /**
  * Verifies that the given path is a valid one on Windows.

setup.c

@@ -943,6 +943,35 @@ void read_gitfile_error_die(int error_code, const char *path, const char *dir)
 	}
 }
 
+#if (defined _WIN32 || defined __WIN32__)
+static int ensure_valid_ownership(const char *gitfile,
+				  const char *worktree, const char *gitdir,
+				  struct strbuf *report);
+
+static int is_invalid_dotgit_path(const char *gitfile, const char *potential_gitdir)
+{
+	WCHAR wpath[MAX_PATH];
+	struct strbuf dir = STRBUF_INIT;
+	int ret;
+
+	if (xutftowcs_path(wpath, potential_gitdir) < 0)
+		return 1;
+
+	/* Do not leak NTLM hashes, UNC paths etc. are generally problematic */
+	if (wpath[0] != L'\\' || is_valid_windows_path_element(wpath[1]))
+		return 0;
+
+	strbuf_addstr(&dir, gitfile);
+	strbuf_strip_file_from_path(&dir);
+
+	ret = ensure_valid_ownership(gitfile, dir.buf, potential_gitdir, NULL) ? 0 : 1;
+
+	strbuf_release(&dir);
+
+	return ret;
+}
+#endif
+
 /*
  * Try to read the location of the git directory from the .git file,
  * return path to git directory if found. The return value comes from
@@ -1016,6 +1045,13 @@ const char *read_gitfile_gently(const char *path, int *return_error_code)
 		free(buf);
 		buf = dir;
 	}
+#if (defined _WIN32 || defined __WIN32__)
+	if (is_dir_sep(dir[0]) && is_invalid_dotgit_path(path, dir)) {
+		strbuf_add(&realpath, dir, strlen(dir));
+		path = realpath.buf;
+		goto cleanup_return;
+	}
+#endif
 	if (!is_git_directory(dir)) {
 		error_code = READ_GITFILE_ERR_NOT_A_REPO;
 		goto cleanup_return;

t/t5580-unc-paths.sh

@@ -44,6 +44,8 @@ test_expect_success clone '
 '
 
 test_expect_success 'clone without file://' '
+	test_must_fail git clone "$UNCPATH" clone-without-file &&
+	git config set --global --append safe.directory "$UNCPATH/.git" &&
 	git clone "$UNCPATH" clone-without-file
 '