2 files changed, 13 insertions, 1 deletions
diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index 8547a63535..c7bf95b426 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -23,6 +23,9 @@ MAX_DEPTH = 2**63 - 1
 # the maximum length for a room alias is 255 characters
 MAX_ALIAS_LENGTH = 255
 
+# the maximum length for a user id is 255 characters
+MAX_USERID_LENGTH = 255
+
 
 class Membership(object):
 
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index a51d11a257..e83ee24f10 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -19,7 +19,7 @@ import logging
 from twisted.internet import defer
 
 from synapse import types
-from synapse.api.constants import LoginType
+from synapse.api.constants import MAX_USERID_LENGTH, LoginType
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -123,6 +123,15 @@ class RegistrationHandler(BaseHandler):
 
         self.check_user_id_not_appservice_exclusive(user_id)
 
+        if len(user_id) > MAX_USERID_LENGTH:
+            raise SynapseError(
+                400,
+                "User ID may not be longer than %s characters" % (
+                    MAX_USERID_LENGTH,
+                ),
+                Codes.INVALID_USERNAME
+            )
+
         users = yield self.store.get_users_by_id_case_insensitive(user_id)
         if users:
             if not guest_access_token: