Support legacy servers which use less secure dialects

configname: CONFIG_CIFS_ALLOW_INSECURE_LEGACY

Linux Kernel Configuration
└─>File systems
└─>Network File Systems
└─>Support legacy servers which use less secure dialects
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Modern dialects, SMB2.1 and later (including SMB3 and 3.1.1), have
additional security features, including protection against
man-in-the-middle attacks and stronger crypto hashes, so the use
of legacy dialects (SMB1/CIFS and SMB2.0) is discouraged.

Disabling this option prevents users from using vers=1.0 or vers=2.0
on mounts with cifs.ko

If unsure, say Y.