CVE-2021-29471

Synapse is a Matrix reference homeserver written in python (pypi package
matrix-synapse). Matrix is an ecosystem for open federated Instant
Messaging and VoIP. In Synapse before version 1.33.2 “Push rules” can
specify conditions under which they will match, including event_match,
which matches event content against a pattern including wildcards. Certain
patterns can cause very poor performance in the matching engine, leading to
a denial-of-service when processing moderate length events. The issue is
patched in version 1.33.2. A potential workaround might be to prevent users
from making custom push rules, by blocking such requests at a
reverse-proxy.