matrix-sdk-crypto vulnerable to sender of encrypted events being spoofed by homeserver administrator

Summary

matrix-sdk-crypto since version 0.8.0 up to 0.11.0 does not correctly validate the sender of an encrypted event. Accordingly, a malicious homeserver operator can modify events served to clients, making those events appear to the recipient as if they were sent by another user.

Although the CVSS score is 4.9 (AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:H/A:N), we consider this a High Severity security issue.

Details

The Matrix specification requires that clients ensure that "the event’s sender, room_id, and the recorded session_id match a trusted session". The vulnerable matrix-sdk-crypto versions check that the room_id matches that of the session denoted by session_id, but do not check the sender.

Patches

The issue is resolved by 13c1d20, included in versions 0.11.1 and 0.12.0 of matrix-sdk-crypto.

Workarounds

Since a successful attack requires administrator access to the homeserver, users who trust the administrators of their local homeserver are not affected.

References

• https://spec.matrix.org/v1.14/client-server-api/#mmegolmv1aes-sha2