| Package | Affected versions | Patched versions |
|---|---|---|
| javascript sdk | < 19.7.0 | 19.7.0 |
| matrix-js-sdk | < 19.7.0 | 19.7.0 |
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.
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:
m.room_key, m.forwarded_room_key and m.secret.send to_device messages are discarded.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.
If you have any questions or comments about this advisory, e-mail us at [email protected].
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