The SMB1 protocol implementation in Samba 4.x before 4.2.11, 4.3.x before 4.3.8, and 4.4.x before 4.4.2 does not recognize the server signing = mandatory setting, which allows man-in-the-middle attackers to spoof SMB servers by modifying the client-server data stream. http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2016-2114
