CVE-2025-61672 is a vulnerability identified in the Synapse Matrix homeserver software, specifically related to improper validation of device keys (CWE-1287). Synapse is widely used for decentralized, federated communication via the Matrix protocol. The vulnerability exists in versions before 1.138.3 and in version 1.139.0, where the software fails to properly validate the type of input for device keys. This flaw allows an attacker who is registered on the victim’s homeserver to manipulate device keys in a way that degrades federation functionality. Federation is the mechanism by which Matrix homeservers communicate and synchronize messages and state across different servers. By exploiting this vulnerability, the attacker can unpredictably break outbound federation connections, disrupting communication with other homeservers. This does not require user interaction or elevated privileges beyond being a registered user, making it relatively easy to exploit within the victim environment. The vulnerability impacts the integrity and availability of federated communications but does not affect confidentiality. The issue 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. No known exploits are currently reported in the wild. The CVSS 4.0 base score is 5.3, indicating medium severity with network attack vector, low attack complexity, no privileges or user interaction required, and limited impact on confidentiality but moderate impact on integrity and availability.

For European organizations relying on Synapse for Matrix-based federated communication, this vulnerability can cause significant disruption in cross-organization messaging and collaboration. The degradation or unpredictable breaking of outbound federation can lead to loss of message synchronization, delayed communications, and potential operational inefficiencies, especially in environments where Matrix is used for critical internal or inter-organizational communications. While confidentiality is not directly impacted, the integrity and availability of federated communications are at risk, potentially undermining trust in the communication platform. Organizations in sectors such as government, education, and technology that adopt Matrix for secure and decentralized communication may experience operational disruptions. The ease of exploitation by any registered user increases the risk from insider threats or compromised accounts. The lack of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits over time.

European organizations should prioritize upgrading Synapse installations directly to versions 1.138.4 or 1.139.2, skipping the intermediate patched versions 1.138.3 and 1.139.1 due to regressions. Administrators should audit registered users and device keys for anomalies that could indicate exploitation attempts. Implementing strict registration controls and monitoring federation health metrics can help detect early signs of federation degradation. Network segmentation and limiting access to the homeserver management interfaces reduce the risk of unauthorized configuration changes. Additionally, organizations should maintain up-to-date backups of federation state and configuration to enable rapid recovery if federation is disrupted. Engaging with the Synapse community and subscribing to security advisories ensures timely awareness of further patches or related vulnerabilities. Finally, consider deploying anomaly detection on federation traffic patterns to identify unusual disruptions potentially caused by exploitation attempts.