CVE-2021-29471: Denial of service in Matrix Synapse
First published: Tue May 11 2021(Updated: )
### 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.
Credit: security-advisories@github.com security-advisories@github.com
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Matrix Synapse | <1.33.2 | |
| Fedoraproject Fedora | =34 | |
| pip/matrix-synapse | <1.33.2 | 1.33.2 |
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Reference Links
- 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@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
- https://nvd.nist.gov/vuln/detail/CVE-2021-29471
- https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-135.yaml
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
- https://github.com/advisories/GHSA-x345-32rc-8h85 (Advisory)
Frequently Asked Questions
What is CVE-2021-29471?
CVE-2021-29471 is a vulnerability in the Synapse Matrix reference homeserver that allows "Push rules" to match events under certain conditions.
What is the severity of CVE-2021-29471?
CVE-2021-29471 has a severity rating of 5.3 (medium).
Which software is affected by CVE-2021-29471?
The Matrix Synapse version up to 1.33.2 and Fedoraproject Fedora version 34 are affected by CVE-2021-29471.
How can I fix CVE-2021-29471?
To fix CVE-2021-29471, upgrade to Synapse version 1.33.2 or later.
Where can I find more information about CVE-2021-29471?
You can find more information about CVE-2021-29471 in the references provided: [Link 1](https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c), [Link 2](https://github.com/matrix-org/synapse/releases/tag/v1.33.2), [Link 3](https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85).
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