CVE-2025-61672: PYSEC-2026-1612 - Synapse's invalid device keys degrade federation functionality

CVE-2025-61672 · Severity: medium · CVSS 4 · Published 2025-10-08

Synapse is an open source Matrix homeserver implementation. Lack of validation for device keys in Synapse before 1.138.3 and in Synapse 1.139.0 allow an attacker registered on the victim homeserver to degrade federation functionality, unpredictably breaking outbound federation to other homeservers. The issue is patched in Synapse 1.138.3, 1.138.4, 1.139.1, and 1.139.2. Note that even though 1.138.3 and 1.139.1 fix the vulnerability, they inadvertently introduced an unrelated regression. For this reason, the maintainers of Synapse recommend skipping these releases and upgrading straight to 1.138.4 and 1.139.2.

Technologies: matrix-synapse (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2025-61672 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2025-61672 being exploited in the wild.
How severe is CVE-2025-61672?
Medium severity, with a CVSS score of 4, published 8 October 2025.
Which products and versions are affected?
matrix-synapse (PyPI) ECOSYSTEM:introduced=0,fixed=1.138.3, ECOSYSTEM:introduced=0,fixed=1.138.3,introduced=1.139.0rc2,fixed=1.139.1.
Is there a fix for CVE-2025-61672?
Yes. matrix-synapse (PyPI): upgrade to 1.138.3, 1.139.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2025-61672?
Check whether you run matrix-synapse (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • matrix-synapse (PyPI): upgrade to 1.138.3, 1.139.1 or later
    pip install "matrix-synapse>=1.139.1"

Affected products

  • matrix-synapse (PyPI) ECOSYSTEM:introduced=0,fixed=1.138.3, ECOSYSTEM:introduced=0,fixed=1.138.3,introduced=1.139.0rc2,fixed=1.139.1

Identifiers

  • CVE-2025-61672
  • GHSA-fh66-fcv5-jjfr
  • PYSEC-2026-1612

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