Today we are disclosing a critical security issue affecting
multiple Matrix clients and libraries including Element
(Web/Desktop/Android), FluffyChat, Nheko, Cinny, and SchildiChat.
Specifically, in certain circumstances it may be possible to
trick vulnerable clients into disclosing encryption keys for
messages previously sent by that client to user accounts later
compromised by an attacker.
Exploiting this vulnerability to read encrypted messages requires
gaining control over the recipient’s account. This requires either
compromising their credentials directly or compromising their homeserver.