CVE-2024-31208

Synapse V2 state resolution weakness allows Denial of Service (DoS)

Workarounds

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

For more information

If you have any questions or comments about this advisory, please email us at security AT element.io.

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 how the auth chain cover index is calculated. 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.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2024-31208 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.105.1); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Server administrators should upgrade to 1.105.1 or later.

1. What is CVE-2024-31208? CVE-2024-31208 is a medium-severity allocation of resources without limits or throttling vulnerability in matrix-synapse (pip), affecting versions < 1.105.1. It is fixed in 1.105.1. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2024-31208? CVE-2024-31208 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of matrix-synapse are affected by CVE-2024-31208? matrix-synapse (pip) versions < 1.105.1 is affected.
4. Is there a fix for CVE-2024-31208? Yes. CVE-2024-31208 is fixed in 1.105.1. Upgrade to this version or later.
5. Is CVE-2024-31208 exploitable, and should I be worried? Whether CVE-2024-31208 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2024-31208 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2024-31208? Upgrade matrix-synapse to 1.105.1 or later.

Other vulnerabilities in matrix-synapse