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CVE-2026-45076 — matrix-synapse

CVE-2026-45076 is a Improper Input Validation vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Synapse pagination denial of service

Also known asGHSA-6qf2-7x63-mm6vPYSEC-2026-194
Published
May 28, 2026
Updated
Aug 12, 2026
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Sep 27, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-45076.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs30th percentile — riskier than 30% of all scored CVEsHighest risk

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

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

In federated rooms, malicious homeservers can craft room events in such a way that prevents Synapse from providing full history to paginating clients.

Clients could therefore fail to display room history.

Patches

Update to Synapse 1.152.1 or later.

Workarounds

There are no known workarounds for this issue.

Identifiers

  • ELEMENTSEC-2025-1636

For more information

If you have any questions or comments about this advisory, please email us at security at element.io.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImatrix-synapseall versions1.152.1pip install --upgrade 'matrix-synapse==1.152.1'

Affected Products

1 product · 1 configurations
Application
synapseelement
< 1.152.1
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 1.152.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-45076 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact In federated rooms, malicious homeservers can craft room events in such a way that prevents Synapse from providing full history to paginating clients. Clients could therefore fail to display room history. ### Patches Update to Synapse 1.152.1 or later. ### Workarounds There are no known workarounds for this issue. ### Identifiers - ELEMENTSEC-2025-1636 ### For more information If you have any questions or comments about this advisory, please email us at [security at element.io](mailto:[email protected]).
O3 Security · Impact-Aware SCA

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CVE-2026-45076: matrix-synapse | O3 Security