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.

2 files changed, 7 insertions, 0 deletions

diff --git a/changelog.d/10956.bugfix b/changelog.d/10956.bugfix
new file mode 100644

index 0000000000..13b8e5983b
--- /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 7a1adc2750..ca0293a3dc 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
     #