In short
- CVE-2021-32659 is a MEDIUM (CVSS 4.9) vulnerability affecting matrix-appservice-bridge (matrix), matrix-appservice-bridge.
- Chance of exploitation in the next 30 days (EPSS): 0.94%, higher than 60.8% of all vulnerabilities.
- Fix: update to 2.6.1 or later.
Description
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 https://github.com/matrix-org/matrix-appservice-bridge/pull/330
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NVD description
Matrix-appservice-bridge is the bridging service for the Matrix communication program's application services. In versions 2.6.0 and earlier, 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.
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