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MEDIUM severity

matrix-synapse vulnerable to denial of service due to malicious server ACL eventsGHSA-5chr-wjw5-3gq4

MEDIUMFix: matrix-org/synapse#16360

GHSA-5chr-wjw5-3gq4 is a medium-severity (CVSS 4.9) CWE-770 vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Also known asCVE-2023-45129PYSEC-2023-199
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs72th percentile — riskier than 72% of all scored CVEsHighest risk
0.00%0.63%1.26%1.89%0.3%1.4%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-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

Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. Prior to version 1.94.0, a malicious server ACL event can impact performance temporarily or permanently leading to a persistent denial of service. Homeservers running on a closed federation (which presumably do not need to use server ACLs) are not affected. Server administrators are advised to upgrade to Synapse 1.94.0 or later. As a workaround, rooms with malicious server ACL events can be purged and blocked using the admin API.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImatrix-synapseall versions1.94.0pip install --upgrade 'matrix-synapse==1.94.0'

Affected Products

2 products · 3 configurations
OS
fedorafedoraproject
2 versions
3738
Application
synapsematrix
< 1.94.0
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 1.94.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-5chr-wjw5-3gq4 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 A malicious server ACL event can impact performance temporarily or permanently leading to a persistent denial of service. Homeservers running on a closed federation (which presumably do not need to use server ACLs) are not affected. ### Patches Server administrators are advised to upgrade to Synapse 1.94.0 or later. ### Workarounds Rooms with malicious server ACL events can be [purged and blocked](https://matrix-org.github.io/synapse/latest/admin_api/rooms.html#version-2-new-version) using the admin API.
O3 Security · Impact-Aware SCA

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matrix-synapse vulnerable to denial of service due to…