Arch Linux Security Advisory ASA-202106-51
=========================================
Severity: Medium
Date    : 2021-06-22
CVE-ID  : CVE-2021-32659
Package : matrix-appservice-irc
Type    : insufficient validation
Remote  : Yes
Link    : https://security.archlinux.org/AVG-2076

Summary
======
The package matrix-appservice-irc before version 0.27.0-1 is vulnerable
to insufficient validation.

Resolution
=========
Upgrade to 0.27.0-1.

# pacman -Syu "matrix-appservice-irc>=0.27.0-1"

The problem has been fixed upstream in version 0.27.0.

Workaround
=========
As a workaround, disabling the automatic room upgrade handling can be
done by removing the `roomUpgradeOpts` key from the `Bridge` class
options.

Description
==========
In versions 2.6.0 and earlier of matrix-appservice-bridge, as included
in matrix-appservice-irc before version 0.27.0, 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 malicious admin
of a bridged room can repoint the traffic to a different room without
the new room being aware. Versions 2.6.1 and greater are patched. As a
workaround, disabling the automatic room upgrade handling can be done
by removing the `roomUpgradeOpts` key from the `Bridge` class options.

Impact
=====
A malicious admin of a bridged room can repoint the traffic to a
different room without the new room being aware.

References
=========
https://github.com/matrix-org/matrix-appservice-bridge/security/advisories/GHSA-35g4-qx3c-vjhx
https://github.com/matrix-org/matrix-appservice-irc/releases/tag/0.27.0
https://github.com/matrix-org/matrix-appservice-bridge/pull/330
https://github.com/matrix-org/matrix-appservice-bridge/commit/b69e745584a34fcfd858df33e4631e420da07b9f
https://security.archlinux.org/CVE-2021-32659