There are a LOT of good sites out there for UNIX security in general and Linux security specifically. It's very important to subscribe to one (or more) of the security mailing lists and keep current on security fixes. Most of these lists are very low volume, and very informative.
CERT is the Computer Emergency Response Team. They often send out alerts of current attacks and fixes. cert.org
Replay has archives of many security programs. Since they are outside the US, they don't need to obey US crypto restrictions. replay.com
Matt Blaze is the author of CFS and a great security advocate. Matt Blaze's stuff
tue.nl is a great security ftp site in the Netherlands. ftp.win.tue.nl
The Hacker FAQ is a FAQ about hackers: The Hacker FAQ
The COAST archive has a large number of unix security programs and information: COAST
Rootshell.com is a great site for seeing what exploits are currently being used by crackers: rootshell.com exploits
BUGTRAQ puts out advisories on security issues: BUGTRAQ archives
CERT, the Computer Emergency Response Team, puts out advisories on common attacks on unix platforms: CERT home
Dan Farmer is the author of SATAN and many other security tools, his home site has some interesting security survey information as well as security tools: Dan Farmers trouble.org
The Linux security WWW is a good site for Linux security information: Linux Security WWW
Reptile has lots of good Linux security information on his site: Reptiles Linux Security Page
Infilsec has a vulnerability engine that can tell you what vunerabilities affect a specific platform: Infilsec vunerability engine
CIAC sends out periodic security bulitins on common exploits: CIAC bulitins
A good starting point for Linux Pluggable Authentication modules can be found at http://www.kernel.org/pub/linux/libs/pam/.
Bugtraq: To subscribe to bugtraq, send mail to listserv@netspace.org containing the message body subscribe bugtraq. (see links above for archives).
CIAC: Send e-mail to: majordomo@tholia.llnl.gov In the BODY (not subject) of the message put (either or both): subscribe ciac-bulletin
There are a number of good security books out there. This section lists a few of them. In addition to the security specify books, security is covered in a number of other books on system administration.
Building Internet Firewalls By D. Brent Chapman & Elizabeth D. Zwicky
1st Edition September 1995
ISBN: 1-56592-124-0
Practical UNIX & Internet Security, 2nd Edition By Simson Garfinkel & Gene Spafford
2nd Edition April 1996
ISBN: 1-56592-148-8
Computer Security Basics By Deborah Russell & G.T. Gangemi, Sr.
1st Edition July 1991
ISBN: 0-937175-71-4
Linux Network Administrator's Guide By Olaf Kirch
1st Edition January 1995
ISBN: 1-56592-087-2
PGP: Pretty Good Privacy By Simson Garfinkel
1st Edition December 1994
ISBN: 1-56592-098-8
Computer Crime A Crimefighter's Handbook By David Icove, Karl Seger & William VonStorch (Consulting Editor Eugene H. Spafford)
1st Edition August 1995
ISBN: 1-56592-086-4