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MEDIUM severity

Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpointsGHSA-jrh7-mhhx-6h88

MEDIUMFix: matrix-org/synapse#9321

GHSA-jrh7-mhhx-6h88 is a medium-severity (CVSS 5.3) Improper Input Validation vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Also known asCVE-2021-21393PYSEC-2021-26
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 12, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs75th percentile — riskier than 75% of all scored CVEsHighest risk
0.05%0.73%1.41%2.10%0.5%1.6%Apr 26Aug 26Oct 26

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

How urgent is this, really

GHSA-jrh7-mhhx-6h88 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐍matrix-synapse

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

Missing input validation of some parameters on the groups (also known as communities) endpoints could cause excessive use of disk space and memory leading to resource exhaustion. Additionally clients may have issues rendering large fields.

Patches

This issue is fixed by #9321 and #9393.

Workarounds

The groups feature can be disabled (by setting enable_group_creation to False) to mitigate this issue. Note that it is disabled by default.

Other information

Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImatrix-synapseall versions1.28.0pip install --upgrade 'matrix-synapse==1.28.0'

Affected Products

2 products · 2 configurations
OS
fedorafedoraproject
1 version
34
Application
synapsematrix
> 0.24.0 && < 1.28.0
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-synapse, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update matrix-synapse to 1.28.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-jrh7-mhhx-6h88 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 Missing input validation of some parameters on the groups (also known as communities) endpoints could cause excessive use of disk space and memory leading to resource exhaustion. Additionally clients may have issues rendering large fields. ### Patches This issue is fixed by #9321 and #9393. ### Workarounds The groups feature can be disabled (by setting `enable_group_creation` to `False`) to mitigate this issue. Note that it is disabled by default. ### Other information Note that the groups feature is not part of the [Matrix specification](https://matrix.org/docs/spec/) and the ch
O3 Security · Impact-Aware SCA

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Denial of service (via resource exhaustion) due to…