The diffiehellmansha256 function in kex.c in libssh2 before 1.7.0 improperly truncates secrets to 128 or 256 bits, which makes it easier for man-in-the-middle attackers to decrypt or intercept SSH sessions via unspecified vectors, aka a "bits/bytes confusion bug."

In libssh2 before 1.9.0, kexmethoddiffiehellmangroupexchangesha256keyexchange in kex.c has an integer overflow that could lead to an out-of-bounds read in the way packets are read from the server. A remote attacker who compromises a SSH server may be able to disclose sensitive information or cause a denial of service condition on the client system when a user connects to the server. This is related to an libssh2checklength mistake, and is different from the various issues fixed in 1.8.1, such as CVE-2019-3855.