Impact
Attackers can specify a specific string of characters, which would confuse the bridge into combining an attacker-owned channel and an existing channel, allowing them to grant themselves permissions in the channel.
Patched
The vulnerability has been patched in matrix-appservice-irc 0.35.0.
Workarounds
Disable dynamic channel joining via dynamicChannels.enabled to prevent users from joining new channels, which prevents any new channels being bridged outside of what is already bridged, and what is specified in the config.
References
Credits
Discovered and reported by Val Lorentz.
For more information
If you have any questions or comments about this advisory email us at security@matrix.org.