From 0c1d6f65d7c65efd8491adf4efc2620148e2841a Mon Sep 17 00:00:00 2001
From: Azrenbeth <77782548+Azrenbeth@users.noreply.github.com>
Date: Mon, 23 Aug 2021 16:25:33 +0100
Subject: [PATCH] Enforce the max length for per-room display names / avatar
 URLs. (#10654)

To match the maximum lengths allowed for profile data.
---
 changelog.d/10654.bugfix        |  1 +
 synapse/handlers/room_member.py | 17 ++++++++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/10654.bugfix

diff --git a/changelog.d/10654.bugfix b/changelog.d/10654.bugfix
new file mode 100644
index 000000000..b0bd78453
--- /dev/null
+++ b/changelog.d/10654.bugfix
@@ -0,0 +1 @@
+Enforce the maximum length for per-room display names and avatar URLs.
\ No newline at end of file
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index ba1319621..401b84aad 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -36,6 +36,7 @@ from synapse.api.ratelimiting import Ratelimiter
 from synapse.event_auth import get_named_level, get_power_level_event
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
+from synapse.handlers.profile import MAX_AVATAR_URL_LEN, MAX_DISPLAYNAME_LEN
 from synapse.types import (
     JsonDict,
     Requester,
@@ -79,7 +80,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         self.account_data_handler = hs.get_account_data_handler()
         self.event_auth_handler = hs.get_event_auth_handler()
 
-        self.member_linearizer = Linearizer(name="member")
+        self.member_linearizer: Linearizer = Linearizer(name="member")
 
         self.clock = hs.get_clock()
         self.spam_checker = hs.get_spam_checker()
@@ -556,6 +557,20 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
             content.pop("displayname", None)
             content.pop("avatar_url", None)
 
+        if len(content.get("displayname") or "") > MAX_DISPLAYNAME_LEN:
+            raise SynapseError(
+                400,
+                f"Displayname is too long (max {MAX_DISPLAYNAME_LEN})",
+                errcode=Codes.BAD_JSON,
+            )
+
+        if len(content.get("avatar_url") or "") > MAX_AVATAR_URL_LEN:
+            raise SynapseError(
+                400,
+                f"Avatar URL is too long (max {MAX_AVATAR_URL_LEN})",
+                errcode=Codes.BAD_JSON,
+            )
+
         effective_membership_state = action
         if action in ["kick", "unban"]:
             effective_membership_state = "leave"