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Fencer Vulns
low 3.7

CVE-2021-29471 json

Denial of service in Matrix Synapse

published 2021-05-11 5 y ago modified 2026-07-08 4 mo ago also GHSA-x345-32rc-8h85 , PYSEC-2021-135 , EUVD-2021-0127

cvss
3.7
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.6%
EPSS 1.6% chance of exploitation in 30 days, percentile 76%
public exploits
0
none known

Summary

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.

Patch / fix

  • pypi matrix-synapse 1.33.2
  • matrix synapse 1.33.2
  • matrix-synapse openSUSE Tumbleweed 1.43.0-1.1

opensuse advisories openSUSE-SU-2024:11041-1

  • synapse Alpine 3.13 1.34.0-r0
  • synapse Alpine 3.24, Alpine 3.23, Alpine 3.22 +8 more 1.33.2-r0

alpine advisories ALPINE-CVE-2021-29471

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • matrix-synapse Arch Linux 1.33.2-1

arch linux advisories AVG-1943

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-29471 updated cvss 5.3

matrix-synapse: Denial-of-service when processing moderate length events

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.65% chance of exploitation in the next 30 days, higher than 76% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:N/A:P
GitHub 3.1 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Affected packages

ecosystem package affected fixed in
pypi matrix-synapse
<1.33.2
1.33.2

Affected products

  • matrix-org synapse

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption
  • CWE-331 Insufficient Entropy

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.