CVE-2021-41281: Path traversal in Matrix Synapse

Impact

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.

Patches

Server administrators should upgrade to 1.47.1 or later.

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 security@matrix.org.