Arch Linux Security Advisory ASA-202105-19
=========================================
Severity: Medium
Date    : 2021-05-25
CVE-ID  : CVE-2021-29471
Package : matrix-synapse
Type    : denial of service
Remote  : Yes
Link    : https://security.archlinux.org/AVG-1943

Summary
======
The package matrix-synapse before version 1.33.2-1 is vulnerable to
denial of service.

Resolution
=========
Upgrade to 1.33.2-1.

# pacman -Syu "matrix-synapse>=1.33.2-1"

The problem has been fixed upstream in version 1.33.2.

Workaround
=========
A potential workaround might be to prevent users from making custom
push rules, by blocking such requests at a reverse-proxy.

Description
==========
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.

Impact
=====
A remote attacker could cause high CPU usage on the server using
crafted push rules, leading to denial of service.

References
=========
https://bugs.archlinux.org/task/70801
https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85
https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c
https://security.archlinux.org/CVE-2021-29471