The Introduction Client in Briar through 1.5.3 does not implement out-of-band verification for the public keys of introducees. An introducer can launch man-in-the-middle attacks against later private communication between two introduced parties.

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.

Briar before 1.4.22 allows attackers to spoof other users' messages in a blog, forum, or private group, but each spoofed message would need to be an exact duplicate of a legitimate message displayed alongside the spoofed one.

Bramble Synchronisation Protocol (BSP) in Briar before 1.4.22 allows attackers to cause a denial of service (repeated application crashes) via a series of long messages to a contact.