fileserver: map invalid path errors to fs.ErrInvalid, and return 400 for any invalid path errors. (close #7008)

modules/caddyhttp/fileserver/staticfiles.go

@@ -300,8 +300,10 @@ func (fsrv *FileServer) ServeHTTP(w http.ResponseWriter, r *http.Request, next c
 	info, err := fs.Stat(fileSystem, filename)
 	if err != nil {
 		err = fsrv.mapDirOpenError(fileSystem, err, filename)
-		if errors.Is(err, fs.ErrNotExist) || errors.Is(err, fs.ErrInvalid) {
+		if errors.Is(err, fs.ErrNotExist) {
 			return fsrv.notFound(w, r, next)
+		} else if errors.Is(err, fs.ErrInvalid) {
+			return caddyhttp.Error(http.StatusBadRequest, err)
 		} else if errors.Is(err, fs.ErrPermission) {
 			return caddyhttp.Error(http.StatusForbidden, err)
 		}
@@ -611,6 +613,11 @@ func (fsrv *FileServer) mapDirOpenError(fileSystem fs.FS, originalErr error, nam
 		return originalErr
 	}
 
+	var pathErr *fs.PathError
+	if errors.As(originalErr, &pathErr) {
+		return fs.ErrInvalid
+	}
+
 	parts := strings.Split(name, separator)
 	for i := range parts {
 		if parts[i] == "" {