CVE-2025-61672 is a vulnerability classified under CWE-1287 (Improper Validation of Specified Type of Input) affecting the Synapse Matrix homeserver software developed by element-hq. The issue arises from insufficient validation of device keys before version 1.138.3 and in version 1.139.0, which allows an attacker who is registered on the victim homeserver to manipulate device key data in a way that degrades the federation functionality. Federation in Matrix enables communication between different homeservers, and this degradation unpredictably breaks outbound federation, disrupting inter-server message exchange. The vulnerability does not require user interaction and can be exploited remotely with low attack complexity, but the attacker must have at least a registered account on the target server, limiting the attack surface to insiders or malicious users with valid credentials. The vulnerability was patched in Synapse versions 1.138.3, 1.138.4, 1.139.1, and 1.139.2; however, versions 1.138.3 and 1.139.1 introduced unrelated regressions, so the maintainers recommend upgrading directly to 1.138.4 or 1.139.2. The CVSS 4.0 base score is 5.3 (medium), reflecting the moderate impact on availability and integrity of federation communications without affecting confidentiality. No known exploits have been reported in the wild as of the publication date. The vulnerability affects the core federation mechanism, which is critical for organizations relying on Matrix for decentralized communication.

For European organizations using Synapse as their Matrix homeserver, this vulnerability could lead to unpredictable disruptions in federated communication with other homeservers, impacting collaboration and real-time messaging across organizational boundaries. This degradation could affect operational continuity, especially for entities relying on Matrix for secure and decentralized communication, such as governmental agencies, research institutions, and privacy-conscious enterprises. The inability to maintain stable federation links could result in message delays, loss of synchronization, or partial denial of service in federated environments. While the vulnerability does not allow data exfiltration or direct compromise of confidentiality, the integrity and availability of federation are impacted, potentially undermining trust in the communication platform. Organizations with high reliance on Matrix federation for cross-organizational workflows may experience operational inefficiencies and increased support overhead until patched.

European organizations should immediately plan to upgrade Synapse deployments to versions 1.138.4 or 1.139.2, explicitly avoiding versions 1.138.3 and 1.139.1 due to known regressions. Prior to upgrade, conduct thorough testing in staging environments to ensure federation functionality remains stable post-patch. Implement strict access controls and monitoring on user registrations to reduce the risk of malicious insiders exploiting the vulnerability. Enable detailed federation logging to detect anomalies or degradation in outbound federation early. Consider deploying federation health monitoring tools that can alert administrators to unexpected federation failures. Regularly audit device key management processes and validate inputs where possible to detect malformed or suspicious device keys. Engage with the Synapse community and maintain awareness of further updates or patches addressing related issues. Finally, incorporate this vulnerability into incident response plans to quickly remediate any federation disruptions.