When matrix-nio receives forwarded room keys, the receiver doesn't check if it requested the key from the forwarder

When matrix-nio before 0.20 requests a room key from our devices, it correctly accepts key forwards only if they are a response to a previous request. However, it doesn't check that the device that responded matches the device the key was requested from.

This allows a malicious homeserver to insert room keys of questionable validity into the key store in some situations, potentially assisting in an impersonation attack.

For more information

If you have any questions or comments about this advisory, e-mail us at poljar@termina.org.uk.