Synapse instances before version 1.47.1 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.
Synapse instances before version 1.47.1 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.
https://github.com/matrix-org/synapse/security/advisories/GHSA-3hfw-x7gx-437c https://github.com/matrix-org/synapse/commit/91f2bd0907f1d05af67166846988e49644eb650c
Vulmon
Workaround
==========
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, which is enabled by default for the Arch Linux package.