---
id: CVE-2021-29471
aliases:
  - GHSA-x345-32rc-8h85
  - PYSEC-2021-135
title: Denial of service attack via push rule patterns in matrix-synapse
summary: Denial of service attack via push rule patterns in matrix-synapse
severity: low
cvss: 3.7
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L'
vendor: matrix-synapse
product: matrix-synapse
ecosystem: pip
affected:
  - matrix-synapse < 1.33.2
patched:
  - matrix-synapse 1.33.2
published: '2021-05-13'
updated: '2026-07-08'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-x345-32rc-8h85'
references:
  - url: >-
      https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2021-29471'
  - url: >-
      https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c
  - url: 'https://github.com/matrix-org/synapse'
  - url: 'https://github.com/matrix-org/synapse/releases/tag/v1.33.2'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-135.yaml
  - url: >-
      https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
tags:
  - osv
  - pip
epss: 0.01647
epssPercentile: 0.75424
ingestedAt: '2026-07-08T18:25:54.062Z'
---

## Overview

### Impact

"Push rules" can specify [conditions](https://matrix.org/docs/spec/client_server/r0.6.1#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.

### Patches

The issue is patched by https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c.

### Workarounds

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

### For more information

If you have any questions or comments about this advisory, email us at security@matrix.org.

## Affected packages

- `matrix-synapse < 1.33.2`

## Remediation

Upgrade to a patched release:

- `matrix-synapse 1.33.2`