What is CVE-2021-29471?

CVE-2021-29471 is a low-severity vulnerability in Matrix Synapse, classified under Uncontrolled Resource Consumption. CVSS score: 3.7/10. Published 2021-05-11.

How severe is CVE-2021-29471?

Low severity. CVSS v3 base score is 3.7 out of 10.

Resource exhaustion in Matrix Synapse

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.

Vulnerability class: DoS (Denial of Service)

Published 2021-05-11 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 3.7 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L.

EPSS exploit prediction

EPSS: 0.016 (75.5th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 76th percentile — 75.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.016 · newest: 0.016 · change: 0.000

Affected products

Matrix Synapse
Matrix-Org Synapse — versions < 1.33.2
Fedoraproject Fedora — versions 34

Weakness classification (CWE)

References

github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85 (Patch, Third Party Advisory)
github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c (Patch, Third Party Advisory)
github.com/matrix-org/synapse/releases/tag/v1.33.2 (Third Party Advisory, Release Notes)
lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/… (vendor-advisory)

Frequently asked questions

What is CVE-2021-29471?: CVE-2021-29471 is a low-severity vulnerability in Matrix Synapse, classified under Uncontrolled Resource Consumption. CVSS score: 3.7/10. Published 2021-05-11.
How severe is CVE-2021-29471?: Low severity. CVSS v3 base score is 3.7 out of 10.