CVE-2021-41281

https://github.com/matrix-org/synapse/releases/tag/v1.47.1

https://github.com/matrix-org/synapse/security/advisories/GHSA-3hfw-x7gx-437c

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.