CVE-2021-41281

Synapse is a package for Matrix homeservers written in Python 3/Twisted.
Prior to version 1.47.1, Synapse instances with the media repository
enabled can be tricked into downloading a file from a remote server into an
arbitrary directory. No authentication is required for the affected
endpoint. The last 2 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 with a federation whitelist are also unaffected, since Synapse
will check the remote hostname, including the trailing ../s, against the
whitelist. Server administrators should upgrade to 1.47.1 or later. Server
administrators using a reverse proxy could, at the expense of losing media
functionality, may block the certain endpoints as a workaround.
Alternatively, non-containerized deployments can be adapted to use the
hardened systemd config.