diff --git a/synapse/events/validator.py b/synapse/events/validator.py
index 514273c792..711af512b2 100644
--- a/synapse/events/validator.py
+++ b/synapse/events/validator.py
@@ -15,8 +15,8 @@
from six import string_types
-from synapse.api.constants import EventTypes, Membership
-from synapse.api.errors import SynapseError
+from synapse.api.constants import MAX_ALIAS_LENGTH, EventTypes, Membership
+from synapse.api.errors import Codes, SynapseError
from synapse.api.room_versions import EventFormatVersions
from synapse.types import EventID, RoomID, UserID
@@ -56,6 +56,17 @@ class EventValidator(object):
if not isinstance(getattr(event, s), string_types):
raise SynapseError(400, "'%s' not a string type" % (s,))
+ if event.type == EventTypes.Aliases:
+ if "aliases" in event.content:
+ for alias in event.content["aliases"]:
+ if len(alias) > MAX_ALIAS_LENGTH:
+ raise SynapseError(
+ 400,
+ ("Can't create aliases longer than"
+ " %d characters" % (MAX_ALIAS_LENGTH,)),
+ Codes.INVALID_PARAM,
+ )
+
def validate_builder(self, event):
"""Validates that the builder/event has roughly the right format. Only
checks values that we expect a proto event to have, rather than all the
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