diff --git a/changes/feature27367 b/changes/feature27367
new file mode 100644
index 0000000000..99c0839621
--- /dev/null
+++ b/changes/feature27367
@@ -0,0 +1,4 @@
+  o Minor features (parsing):
+    - Directory authorities now validate that router descriptors and ExtraInfo
+      documents are in a valid subset of UTF-8, and reject them if not.
+      Closes ticket 27367.
diff --git a/src/feature/dirauth/process_descs.c b/src/feature/dirauth/process_descs.c
index c379f25bdd..dca87b3eaf 100644
--- a/src/feature/dirauth/process_descs.c
+++ b/src/feature/dirauth/process_descs.c
@@ -519,7 +519,8 @@ WRA_MORE_SEVERE(was_router_added_t a, was_router_added_t b)
 /** As for dirserv_add_descriptor(), but accepts multiple documents, and
  * returns the most severe error that occurred for any one of them. */
 was_router_added_t
-dirserv_add_multiple_descriptors(const char *desc, uint8_t purpose,
+dirserv_add_multiple_descriptors(const char *desc, size_t desclen,
+                                 uint8_t purpose,
                                  const char *source,
                                  const char **msg)
 {
@@ -536,6 +537,11 @@ dirserv_add_multiple_descriptors(const char *desc, uint8_t purpose,
 
   r=ROUTER_ADDED_SUCCESSFULLY; /*Least severe return value. */
 
+  if (!string_is_utf8_no_bom(desc, desclen)) {
+    *msg = "descriptor(s) or extrainfo(s) not valid UTF-8 or had BOM.";
+    return ROUTER_AUTHDIR_REJECTS;
+  }
+
   format_iso_time(time_buf, now);
   if (tor_snprintf(annotation_buf, sizeof(annotation_buf),
                    "@uploaded-at %s\n"
@@ -552,7 +558,7 @@ dirserv_add_multiple_descriptors(const char *desc, uint8_t purpose,
 
   s = desc;
   list = smartlist_new();
-  if (!router_parse_list_from_string(&s, NULL, list, SAVED_NOWHERE, 0, 0,
+  if (!router_parse_list_from_string(&s, s+desclen, list, SAVED_NOWHERE, 0, 0,
                                      annotation_buf, NULL)) {
     SMARTLIST_FOREACH(list, routerinfo_t *, ri, {
         msg_out = NULL;
@@ -568,7 +574,7 @@ dirserv_add_multiple_descriptors(const char *desc, uint8_t purpose,
   smartlist_clear(list);
 
   s = desc;
-  if (!router_parse_list_from_string(&s, NULL, list, SAVED_NOWHERE, 1, 0,
+  if (!router_parse_list_from_string(&s, s+desclen, list, SAVED_NOWHERE, 1, 0,
                                      NULL, NULL)) {
     SMARTLIST_FOREACH(list, extrainfo_t *, ei, {
         msg_out = NULL;
diff --git a/src/feature/dirauth/process_descs.h b/src/feature/dirauth/process_descs.h
index ad9d5c3d4c..5a0914acd8 100644
--- a/src/feature/dirauth/process_descs.h
+++ b/src/feature/dirauth/process_descs.h
@@ -17,7 +17,8 @@ void dirserv_free_fingerprint_list(void);
 int dirserv_add_own_fingerprint(crypto_pk_t *pk);
 
 enum was_router_added_t dirserv_add_multiple_descriptors(
-                                     const char *desc, uint8_t purpose,
+                                     const char *desc, size_t desclen,
+                                     uint8_t purpose,
                                      const char *source,
                                      const char **msg);
 enum was_router_added_t dirserv_add_descriptor(routerinfo_t *ri,
diff --git a/src/feature/dircache/dircache.c b/src/feature/dircache/dircache.c
index dff4b85caa..4032223db4 100644
--- a/src/feature/dircache/dircache.c
+++ b/src/feature/dircache/dircache.c
@@ -1602,8 +1602,8 @@ directory_handle_command_post,(dir_connection_t *conn, const char *headers,
     const char *msg = "[None]";
     uint8_t purpose = authdir_mode_bridge(options) ?
                       ROUTER_PURPOSE_BRIDGE : ROUTER_PURPOSE_GENERAL;
-    was_router_added_t r = dirserv_add_multiple_descriptors(body, purpose,
-                                             conn->base_.address, &msg);
+    was_router_added_t r = dirserv_add_multiple_descriptors(body, body_len,
+                                           purpose, conn->base_.address, &msg);
     tor_assert(msg);
 
     if (r == ROUTER_ADDED_SUCCESSFULLY) {
diff --git a/src/lib/string/util_string.c b/src/lib/string/util_string.c
index b2b85d151d..e76e73046f 100644
--- a/src/lib/string/util_string.c
+++ b/src/lib/string/util_string.c
@@ -541,3 +541,16 @@ string_is_utf8(const char *str, size_t len)
   }
   return true;
 }
+
+/** As string_is_utf8(), but returns false if the string begins with a UTF-8
+ * byte order mark (BOM).
+ */
+int
+string_is_utf8_no_bom(const char *str, size_t len)
+{
+  if (len >= 3 && (!strcmpstart(str, "\uFEFF") ||
+                   !strcmpstart(str, "\uFFFE"))) {
+    return false;
+  }
+  return string_is_utf8(str, len);
+}
diff --git a/src/lib/string/util_string.h b/src/lib/string/util_string.h
index 746ece0d33..99467a27c3 100644
--- a/src/lib/string/util_string.h
+++ b/src/lib/string/util_string.h
@@ -53,5 +53,6 @@ const char *find_str_at_start_of_line(const char *haystack,
 int string_is_C_identifier(const char *string);
 
 int string_is_utf8(const char *str, size_t len);
+int string_is_utf8_no_bom(const char *str, size_t len);
 
 #endif /* !defined(TOR_UTIL_STRING_H) */
diff --git a/src/test/test_util.c b/src/test/test_util.c
index 1a71da2794..bcface64fd 100644
--- a/src/test/test_util.c
+++ b/src/test/test_util.c
@@ -4024,6 +4024,13 @@ test_util_string_is_utf8(void *ptr)
   tt_int_op(1, OP_EQ, string_is_utf8("ascii\x7f\n", 7));
   tt_int_op(1, OP_EQ, string_is_utf8("Risqu\u00e9=1", 9));
 
+  /* Test the utf8_no_bom function */
+  tt_int_op(0, OP_EQ, string_is_utf8_no_bom("\uFEFF", 3));
+  tt_int_op(0, OP_EQ, string_is_utf8_no_bom("\uFFFE", 3));
+  tt_int_op(0, OP_EQ, string_is_utf8_no_bom("\uFEFFlove", 7));
+  tt_int_op(1, OP_EQ, string_is_utf8_no_bom("loveandrespect",
+                                            strlen("loveandrespect")));
+
   // Validate exactly 'len' bytes.
   tt_int_op(0, OP_EQ, string_is_utf8("\0\x80", 2));
   tt_int_op(0, OP_EQ, string_is_utf8("Risqu\u00e9=1", 6));