matrix-android-sdk2 vulnerable to Olm/Megolm protocol confusion

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. matrix-android-sdk2 would then additionally sign such a key backup with its device key, spilling trust over to other devices trusting the matrix-android-sdk2 device.

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

Patches

matrix-android-sdk2 has been modified to only accept Olm-encrypted to-device messages and to stop signing backups on a successful decryption.

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 using verify with passphrase.

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.