streamengine.cpp in libzmq (aka ZeroMQ/C++)) 4.0.5 before 4.0.5 allows man-in-the-middle attackers to conduct downgrade attacks via a crafted connection request.

libzmq (aka ZeroMQ/C++) 4.0.x before 4.0.5 does not ensure that nonces are unique, which allows man-in-the-middle attackers to conduct replay attacks via unspecified vectors.

A flaw was found in the ZeroMQ server in versions before 4.3.3. This flaw allows a malicious client to cause a stack buffer overflow on the server by sending crafted topic subscription requests and then unsubscribing. The highest threat from this vulnerability is to confidentiality, integrity, as well as system availability.