CVE-2021-32659: Automatic room upgrade handling can be used maliciously to bridge a room non-consentually

Severity: medium (CVSS 4.9 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-06-16 · updated 2026-06-17
Weakness: CWE-306

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.

Affected products

matrix-org matrix-appservice-bridge: before 2.6.1 — fixed in 2.6.1

How to fix

Upgrade to a fixed version: matrix-org matrix-appservice-bridge 2.6.1. Check the vendor advisory for mitigations.

References

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