CVE-2025-61672
MediumCVSS 5.3Exploitation Probability (EPSS)
Low risk38th percentile - higher than 38% of all known CVEs
Summary
Synapse, an open source Matrix homeserver implementation, lacks validation for device keys before version 1.138.3 and in version 1.139.0, allowing an attacker registered on the victim homeserver to degrade federation functionality and unpredictably break outbound federation to other homeservers. The issue is patched in versions 1.138.3, 1.138.4, 1.139.1, and 1.139.2, but 1.138.3 and 1.139.1 introduced an unrelated regression, so skipping them is recommended.
Risk Assessment
An attacker registered on the victim's homeserver can disrupt federated communication, leading to service unavailability and reliability issues in the Matrix environment. This may affect trust in the organization's communication infrastructure.
Recommendation
Upgrade Synapse directly to version 1.138.4 or 1.139.2, skipping 1.138.3 and 1.139.1. Test the update in a staging environment first.
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Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

