Summary
Workarounds
The vulnerability can only be exploited by users registered on the victim homeserver.
Impact
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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
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, it is recommend to skip these releases and upgrading straight to 1.138.4 and 1.139.2.
Frequently Asked Questions
- What is CVE-2025-61672? CVE-2025-61672 is a medium-severity security vulnerability in matrix-synapse (pip), affecting versions < 1.138.3. It is fixed in 1.138.3, 1.139.1.
- Which versions of matrix-synapse are affected by CVE-2025-61672? matrix-synapse (pip) versions < 1.138.3 is affected.
- Is there a fix for CVE-2025-61672? Yes. CVE-2025-61672 is fixed in 1.138.3, 1.139.1. Upgrade to this version or later.
- Is CVE-2025-61672 exploitable, and should I be worried? Whether CVE-2025-61672 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-61672 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-61672?
- Upgrade
matrix-synapseto 1.138.3 or later - Upgrade
matrix-synapseto 1.139.1 or later
- Upgrade