SimpleXMQ before 3.4.0, as used in SimpleX Chat before 4.2, does not apply a key derivation function to intended data, which can interfere with forward secrecy and can have other impacts if there is a compromise of a single private key. This occurs in the X3DH key exchange for the double ratchet protocol.
https://simplex.chat/blog/20221108-simplex-chat-v4.2-security-audit-new-website.html
https://github.com/trailofbits/publications/blob/master/reviews/SimpleXChat.pdf
https://github.com/simplex-chat/simplexmq/pull/548
https://github.com/simplex-chat/simplexmq/compare/v3.3.0...v3.4.0