Hardening Linux

http://www.linux.com/security/

http://www.linuxsecurity.com/

http://www.linuxtoday.com/security/

http://lwn.net/security/

http://www.redhat.com/security/

http://www.sans.org/rr/catindex.php?cat_id=32

http://www.securityfocus.com/unix/

http://www.suse.com/de/security/