CVE-2024-37303

Synapse's unauthenticated writes to the media repository allow planting of problematic content

Workarounds

Though extremely limited, server operators can use more strict rate limits based on IP address.

References

• https://github.com/matrix-org/matrix-spec-proposals/pull/3916

For more information

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

Synapse before version 1.106 allows, by design, unauthenticated remote participants to trigger a download and caching of remote media from a remote homeserver to the local media repository. Such content then also becomes available for download from the local homeserver in an unauthenticated way. The implication is that unauthenticated remote adversaries can use this functionality to plant problematic content into the media repository.

A critical operation is accessible without requiring any authentication. Typical impact: any user can invoke the privileged function.

CVE-2024-37303 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no 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 (1.106); upgrading removes the vulnerable code path.

Kodem intelligence

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Synapse 1.106 introduces a partial mitigation in the form of new endpoints which require authentication for media downloads. The unauthenticated endpoints will be frozen in a future release, closing the attack vector.

1. What is CVE-2024-37303? CVE-2024-37303 is a medium-severity missing authentication for critical function vulnerability in matrix-synapse (pip), affecting versions < 1.106. It is fixed in 1.106. A critical operation is accessible without requiring any authentication.
2. How severe is CVE-2024-37303? CVE-2024-37303 has a CVSS score of 5.3 (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-2024-37303? matrix-synapse (pip) versions < 1.106 is affected.
4. Is there a fix for CVE-2024-37303? Yes. CVE-2024-37303 is fixed in 1.106. Upgrade to this version or later.
5. Is CVE-2024-37303 exploitable, and should I be worried? Whether CVE-2024-37303 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-2024-37303 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-2024-37303? Upgrade matrix-synapse to 1.106 or later.

Other vulnerabilities in matrix-synapse