Impact

If a bridge has room upgrade handling turned on in the configuration (the roomUpgradeOpts key when instantiating a new Bridge instance.), any m.room.tombstone event it encounters will be used to unbridge the current room and bridge into the target room. However, the target room m.room.create event is not checked to verify if the predecessor field contains the previous room. This means that any mailcious admin of a bridged room can repoint the traffic to a different room without the new room being aware.

Patches

Versions 2.6.1 and greater are patched.

Workarounds

Disabling the automatic room upgrade handling can be done by removing the roomUpgradeOpts key from the Bridge class options.

References

The issue is patched by matrix-org/matrix-appservice-bridge#330

For more information]

If you have any questions or comments about this advisory, email us at security@matrix.org.

References

• GHSA-35g4-qx3c-vjhx
• https://nvd.nist.gov/vuln/detail/CVE-2021-32659
• matrix-org/matrix-appservice-bridge#330
• matrix-org/matrix-appservice-bridge@b69e745
• https://github.com/matrix-org/matrix-appservice-bridge/releases/tag/2.6.1