ANY OS.
is vulnerable to rootkits.
specially the unix and unix-likes.*
The privilege escalation nature of administration for the unix and unix-like operating systems is exactly what rootkits were originally designed to exploit, get the root user login info. [ root being system admin account ]
*the BSD variants, GNU/Linux, HPUX, AIX, Irix, MACOS(n) for a quick listing of unix and unix-likes