CVE-2025-61672 in synapse
Summary
by MITRE • 10/08/2025
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.
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Analysis
by VulDB Data Team • 10/08/2025
The vulnerability identified as CVE-2025-61672 affects Synapse, an open source Matrix homeserver implementation that serves as a core component of the Matrix communication protocol. This security flaw represents a critical degradation in federation functionality that undermines the integrity of distributed communication networks. The issue stems from insufficient validation mechanisms for device keys within the Synapse software, creating a pathway for malicious actors to exploit the system and compromise federation operations. Matrix homeservers rely heavily on federation to maintain connectivity across different server instances, making this vulnerability particularly dangerous for maintaining network integrity and user communication continuity.
The technical flaw manifests in the improper handling of device keys during federation operations, specifically allowing authenticated attackers who have registered accounts on the targeted homeserver to manipulate the device key validation process. This weakness enables attackers to intentionally disrupt outbound federation connections to other Matrix homeservers, causing unpredictable breakdowns in communication between federated networks. The vulnerability operates at the protocol level where device keys are used to establish trust relationships between homeservers, and without proper validation, malicious actors can inject forged or corrupted device key information that propagates through the federation network.
The operational impact of this vulnerability extends beyond simple service disruption to encompass potential data integrity issues and communication failures across federated Matrix networks. When an attacker successfully exploits this vulnerability, they can cause intermittent or complete breakdowns in outbound federation, affecting users' ability to communicate with contacts on other homeservers. This creates a cascading effect where the compromised homeserver becomes isolated from the broader Matrix ecosystem, potentially leading to fragmented user experiences and loss of communication capabilities that can persist until the vulnerability is patched and the federation network is restored.
Security practitioners should note that this vulnerability aligns with CWE-20, "Improper Input Validation," and represents a specific implementation weakness in the device key handling mechanism within the Matrix federation protocol. The patching strategy requires careful consideration due to the introduction of regressions in the initial fixed versions. The maintainers recommend skipping releases 1.138.3 and 1.139.1 due to these unintended side effects, emphasizing that only versions 1.138.4 and 1.139.2 provide both the vulnerability fix and stable functionality. Organizations should implement immediate mitigation strategies by upgrading to the recommended patched versions while monitoring for any regression issues that might affect their specific deployment configurations. The vulnerability demonstrates the importance of thorough testing in security patches and highlights the complex interdependencies within federated communication systems where a single validation flaw can compromise entire network operations.