From 3a5b0cbe7ade000245695ec97c13ab5cb3565dc2 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Tue, 5 Oct 2021 13:23:29 +0100
Subject: [PATCH] Ensure that we reject events which use rejected events for
 auth (#10956)

When we consider whether to accept events, we should not accept those which
depend on rejected events for their auth events.

This (together with earlier changes such as
https://github.com/matrix-org/synapse/pull/10771 and
https://github.com/matrix-org/synapse/pull/10896) forms a partial fix to
https://github.com/matrix-org/synapse/issues/9595. There still remain code
paths where we do not check the `auth_events` at all.
---
 changelog.d/10956.bugfix | 1 +
 synapse/event_auth.py    | 6 ++++++
 2 files changed, 7 insertions(+)
 create mode 100644 changelog.d/10956.bugfix

diff --git a/changelog.d/10956.bugfix b/changelog.d/10956.bugfix
new file mode 100644
index 000000000..13b8e5983
--- /dev/null
+++ b/changelog.d/10956.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug which meant that events received over federation were sometimes incorrectly accepted into the room state.
diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index 7a1adc275..ca0293a3d 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -155,6 +155,12 @@ def check_auth_rules_for_event(
                 "which is in room %s"
                 % (event.event_id, room_id, auth_event.event_id, auth_event.room_id),
             )
+        if auth_event.rejected_reason:
+            raise AuthError(
+                403,
+                "During auth for event %s: found rejected event %s in the state"
+                % (event.event_id, auth_event.event_id),
+            )
 
     # Implementation of https://matrix.org/docs/spec/rooms/v1#authorization-rules
     #