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HIGH severity

Sydent vulnerable to denial of service attack via memory exhaustionGHSA-wmg4-8cp2-hpg9

HIGHFix: matrix-org/sydent@0523511

GHSA-wmg4-8cp2-hpg9 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in matrix-sydent. A fix is available for matrix-sydent — see the affected versions and patch details below.

Also known asCVE-2021-29430PYSEC-2021-21
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.8%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs79th percentile — riskier than 79% of all scored CVEsHighest risk
0.87%1.36%1.84%2.33%1.4%1.8%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐍matrix-sydent

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

Sydent does not limit the size of requests it receives from HTTP clients. A malicious user could send an HTTP request with a very large body, leading to disk space exhaustion and denial of service.

Sydent also does not limit response size for requests it makes to remote Matrix homeservers. A malicious homeserver could return a very large response, again leading to memory exhaustion and denial of service.

This affects any server which accepts registration requests from untrusted clients.

Patches

Patched by 89071a1, 0523511, f56eee3.

Workarounds

Request sizes can be limited in an HTTP reverse-proxy.

There are no known workarounds for the problem with overlarge responses.

For more information

If you have any questions or comments about this advisory, email us at [email protected].

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImatrix-sydentall versions2.3.0pip install --upgrade 'matrix-sydent==2.3.0'

Affected Products

1 product · 1 configurations
Application
sydentmatrix
< 2.3.0
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-sydent, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update matrix-sydent to 2.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-wmg4-8cp2-hpg9 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Impact Sydent does not limit the size of requests it receives from HTTP clients. A malicious user could send an HTTP request with a very large body, leading to disk space exhaustion and denial of service. Sydent also does not limit response size for requests it makes to remote Matrix homeservers. A malicious homeserver could return a very large response, again leading to memory exhaustion and denial of service. This affects any server which accepts registration requests from untrusted clients. ### Patches Patched by 89071a1, 0523511, f56eee3. ### Workarounds Request sizes can be lim
O3 Security · Impact-Aware SCA

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Sydent vulnerable to denial of service attack via memory…