CVE-2021-41281

Path traversal in Matrix Synapse

Workarounds

Server administrators using a reverse proxy could, at the expense of losing media functionality, block the following endpoints:

• /_matrix/media/r0/download/{serverName}/{mediaId}
• /_matrix/media/r0/download/{serverName}/{mediaId}/{fileName}
• /_matrix/media/r0/thumbnail/{serverName}/{mediaId}

Alternatively, non-containerized deployments can be adapted to use the hardened systemd config, located at contrib/systemd/override-hardened.conf.

References

n/a

For more information

If you have any questions or comments about this advisory, e-mail us at [email protected].

Synapse instances with the media repository enabled can be tricked into downloading a file from a remote server into an arbitrary directory, potentially outside the media store directory.

The last two directories and file name of the path are chosen randomly by Synapse and cannot be controlled by an attacker, which limits the impact.

Homeservers with the media repository disabled are unaffected. Homeservers configured with a federation whitelist are also unaffected.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2021-41281 has a CVSS score of 7.5 (High). 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 (1.47.1); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Server administrators should upgrade to 1.47.1 or later.

1. What is CVE-2021-41281? CVE-2021-41281 is a high-severity path traversal vulnerability in matrix-synapse (pip), affecting versions < 1.47.1. It is fixed in 1.47.1. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2021-41281? CVE-2021-41281 has a CVSS score of 7.5 (High). 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-2021-41281? matrix-synapse (pip) versions < 1.47.1 is affected.
4. Is there a fix for CVE-2021-41281? Yes. CVE-2021-41281 is fixed in 1.47.1. Upgrade to this version or later.
5. Is CVE-2021-41281 exploitable, and should I be worried? Whether CVE-2021-41281 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-2021-41281 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-2021-41281? Upgrade matrix-synapse to 1.47.1 or later.

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