What’s Vulnerable › Vulnerabilities
CVE-2021-29471: Denial of service in Matrix Synapse
- Severity: medium (CVSS 5.3 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 1.6% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2021-05-11 · updated 2026-06-17
- Weakness: CWE-400
Description
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.
Affected products
- Fedoraproject Fedora: 34
- matrix-org synapse: before 1.33.2 — fixed in 1.33.2
How to fix
Upgrade to a fixed version: matrix-org synapse 1.33.2. Check the vendor advisory for mitigations.
References
- https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c
- https://github.com/matrix-org/synapse/releases/tag/v1.33.2
- https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c
- https://github.com/matrix-org/synapse/releases/tag/v1.33.2
- https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
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