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.

libzmq before 4.0.6 and 4.1.x before 4.1.1 allows remote attackers to conduct downgrade attacks and bypass ZMTP v3 protocol security mechanisms via a ZMTP v2 or earlier header.

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.

stream_engine.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.