Bramble Handshake Protocol (BHP) in Briar before 1.5.3 is not forward secure: eavesdroppers can decrypt network traffic between two accounts if they later compromise both accounts. NOTE: the eavesdropping is typically impractical because BHP runs over an encrypted session that uses the Tor hidden service protocol.
https://briarproject.org/news/2023-three-security-issues-found-and-fixed/