PYSEC-2026-842 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 inse...