The NTLMSSP authentication implementation in Samba 3.x and 4.x before 4.2.11, 4.3.x before 4.3.8, and 4.4.x before 4.4.2 allows man-in-the-middle attackers to perform protocol-downgrade attacks by modifying the client-server data stream to remove application-layer flags or encryption settings, as demonstrated by clearing the NTLMSSP_NEGOTIATE_SEAL or NTLMSSP_NEGOTIATE_SIGN option to disrupt LDAP security.