/usr/lib/mono/4.5/Mono.Security.dll is in libmono-security4.0-cil 3.2.8+dfsg-4ubuntu1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/Mono.Security/4.0.0.0__0738eb9f132ed756/Mono.Security.dll