UNIX systems are not necessarily imune, they just aren't exploited like Windows systems. Set UID as well as buffer overrun vulnerabilities have been identified on UNIX systems from time to time for all sorts of applications including Oracle 9i, which was touted as unbreakable/unhackable. Who remembers the Morris worm that exploited sendmail and crahsed countless systems accross the net more than a decade ago? This attack was the first one that caught the world's attention due to its substantial impact. CERT has advised that yet another vulnerability exists in sendmail (albeit through unauthorized distrobutions) can be compromised even through a firewall.

Thomas