Message impersonation via permissive key forwarding

Improper authentication in Matrix Android SDK 2 before 1.5.1 allows remote attackers cooperating with a malicious homeserver to impersonate senders. The key-forwarding strategy accepts forwarded Megolm session keys without requiring a prior request from the client or receipt from the client’s own verified devices, allowing forged encrypted messages to appear to originate from another person. Exploitation requires control or cooperation from a malicious homeserver; the primary consequence is message-integrity loss and sender impersonation, with no reported confidentiality or availability impact.