matrix-js-sdk subject to user impersonation due to key/device identifier confusion in SAS verification

Impact

An attacker cooperating with a malicious homeserver could interfere with the verification flow between two users, injecting its own cross-signing user identity in place of one of the users’ identities, leading to the other device trusting/verifying the user identity under the control of the homeserver instead of the intended one.

The vulnerability is a bug in the matrix-js-sdk, caused by checking and signing user identities and devices in two separate steps, and inadequately fixing the keys to be signed between those steps.

Even though the attack is partly made possible due to the design decision of treating cross-signing user identities as Matrix devices on the server side (with their device ID set to the public part of the user identity key), no other examined implementations were vulnerable.

Patches

The matrix-js-sdk has been modified to double check that the key signed is the one that was verified instead of just referencing the key by ID. An additional check has been made to report an error when one of the device ID matches a cross-signing key.

Workarounds

As this attack requires coordination between a malicious homeserver and an attacker -- if you trust your homeserver no particular workaround is needed.

As a potential way of detecting compromise, it’s possible to review your device list or the device list of other users for devices with IDs in the form of a base64 cross-signing key (5XaczGNlfz0bl8R1IX5qn+tBoue2tWJqLMh+SDUuvCk) instead of classical device ID (SEHACYDHMG).

References