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.
References6
- github.com/element-hq/synapse/commit/26aaaf9e48fff80cf67a20c691…
- github.com/element-hq/synapse/commit/7069636c2d6d1ef2022287addf…
- github.com/element-hq/synapse/pull/17097
- github.com/element-hq/synapse/releases/tag/v1.138.3
- github.com/element-hq/synapse/releases/tag/v1.139.1
- github.com/element-hq/synapse/security/advisories/GHSA-fh66-fcv…
Metrics
- CVSS 4.0
-
5.3
MEDIUM
Score source: CNA
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 0% percentile: 41 · as of 9 November 2026
- CWE
- CWE-1287
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.