matrix-js-sdk will freeze when a user sets a room with itself as a its predecessor

Impact

A malicious homeserver can craft a room or room structure such that the predecessors form a cycle. The matrix-js-sdk’s getRoomUpgradeHistory function will infinitely recurse in this case, causing the code to hang. This method is public but also called by the ‘leaveRoomChain()’ method, so leaving a room will also trigger the bug.

Even if the CVSS score would be 4.1 (AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:N/A:L) we classify this as High severity issue.

Patches

This was patched in matrix-js-sdk 34.3.1.

Workarounds

Sanity check rooms before passing them to the matrix-js-sdk or avoid calling either getRoomUpgradeHistory or leaveRoomChain.

References

N/A.