Synapse: Invalid device keys degrade federation functionality
Key facts
- Attack vector
- Network
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- Low
- Attack requirements
- None
- Impact type
- —
- Confidentiality · integrity · availability
- None · Low · Low
- Fix status
- Fixed
Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| element-hq | synapse | before 1.138.3; = 1.139.0 | 1.138.3 | Patch available | CNA |
| — | matrix-synapse (PyPI) | before 1.138.3 | 1.138.3 | Patch available | GHSA-fh66-fcv5-jjfr |
| — | matrix-synapse (PyPI) | 1.139.0rc2 to before 1.139.1 | 1.139.1 | Patch available | GHSA-fh66-fcv5-jjfr |
- synapse Vendorelement-hq Affected before 1.138.3; = 1.139.0 Fixed in 1.138.3 Patch available CNA
- matrix-synapse (PyPI) Vendor— Affected before 1.138.3 Fixed in 1.138.3 Patch available GHSA-fh66-fcv5-jjfr
- matrix-synapse (PyPI) Vendor— Affected 1.139.0rc2 to before 1.139.1 Fixed in 1.139.1 Patch available GHSA-fh66-fcv5-jjfr
Remediation
- Fix · CNA synapse (element-hq) Fixed in 1.138.3
- Fix · GHSA-fh66-fcv5-jjfr matrix-synapse (PyPI) Fixed in 1.138.3 Advisory →
- Fix · GHSA-fh66-fcv5-jjfr matrix-synapse (PyPI) Fixed in 1.139.1 Advisory →
Vendor assessments
Timeline
- 2026-10-02 Threatist record created from CVE-2025-61672
- 2026-10-03 Alias added: GHSA-fh66-fcv5-jjfr
References
Other 9
- 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
- https://github.com/element-hq/synapse
- https://github.com/element-hq/synapse/releases/tag/v1.138.4
- https://github.com/element-hq/synapse/releases/tag/v1.139.2
More element-hq vulnerabilities
- TI-2024-35327 Synapse allows a a malformed invite to break the invitee's `/sync` CVSS 4.0HIGH 8.7
- TI-2024-36056 Synapse can be forced to thumbnail unexpected file formats, invoking external, potentially untrustworthy decoders CVSS 4.0HIGH 8.2
- TI-2024-35318 Synapse allows unsupported content types to lead to memory exhaustion CVSS 4.0HIGH 8.2
- TI-2022-07650 Remote program execution with user interaction CVSS 3.1HIGH 8.8
- TI-2026-19372 ESS Community Helm Chart has a weak server key generation method CVSS 4.0CRITICAL 9.2
- TI-2024-24814 Synapse denial of service through media disk space consumption CVSS 3.1HIGH 7.5
Sources
- CVE-2025-61672 · CVE Program
- CVE-2025-61672 · NVD
- FIRST EPSS
- GHSA-fh66-fcv5-jjfr · GitHub Advisory Database · CC BY 4.0
- CVE-2025-61672 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences