GHSA-r48r-j8fx-mq2c

### Impact

An attacker cooperating with a malicious homeserver can construct messages that legitimately appear to have come from another person, without any indication such as a grey shield.

Additionally, a sophisticated attacker cooperating with a malicious homeserver could employ this vulnerability to perform a targeted attack in order to send fake to-device messages appearing to originate from another user. This can allow, for example, to inject the key backup secret during a self-verification, to make a targeted device start using a malicious key backup spoofed by the homeserver.

These attacks are possible due to a protocol confusion vulnerability that accepts to-device messages encrypted with Megolm instead of Olm.

### Patches

matrix-js-sdk has been modified to only accept Olm-encrypted to-device messages.

Out of caution, several other checks have been audited or added: - Cleartext `m.room_key`, `m.forwarded_room_key` and `m.secret.send` to_device messages are discarded. - Secrets received from untrusted devices are discarded. - Key backups are only usable if they have a valid signature from a trusted device (no more local trust, or trust-on-decrypt). - The origin of a to-device message should only be determined by observing the Olm session which managed to decrypt the message, and not by using claimed sender_key, user_id, or any other fields controllable by the homeserver.

### Workarounds

As this attack requires coordination between a malicious home server and an attacker, if you trust your home server no particular workaround is needed. Notice that the backup spoofing attack is a particularly sophisticated targeted attack.

We are not aware of this attack being used in the wild, though specifying a false positive-free way of noticing malicious key backups key is challenging.

As an abundance of caution, to avoid malicious backup attacks, you should not verify your new logins using emoji/QR verifications methods until patched. Prefer verifying with your security passphrase instead.

### References Blog post: https://matrix.org/blog/2022/09/28/upgrade-now-to-address-encryption-vulns-in-matrix-sdks-and-clients

### For more information If you have any questions or comments about this advisory, e-mail us at [security@matrix.org](mailto:security@matrix.org).