CVE-2024-31208: Synapse's V2 state resolution weakness allows DoS from remote room members

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-04-23 · updated 2026-06-17
Weakness: CWE-770

Description

Synapse is an open-source Matrix homeserver. A remote Matrix user with malicious intent, sharing a room with Synapse instances before 1.105.1, can dispatch specially crafted events to exploit a weakness in the V2 state resolution algorithm. This can induce high CPU consumption and accumulate excessive data in the database of such instances, resulting in a denial of service. Servers in private federations, or those that do not federate, are not affected. Server administrators should upgrade to 1.105.1 or later. Some workarounds are available. One can ban the malicious users or ACL block servers from the rooms and/or leave the room and purge the room using the admin API.

Affected products

Fedoraproject Fedora: 38, 39, 40
matrix-org synapse: before 1.105.1 — fixed in 1.105.1

How to fix

Upgrade to a fixed version: matrix-org synapse 1.105.1. Check the vendor advisory for mitigations.

References

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