Distributors maintain mailing lists and sites for fixes that relate to their Linux distributions. Here are some useful addresses for general information on security alerts:
Visit http://www.securityfocus.com/ for general security issues. From this site, you can also link to the BugTraq mailing list for the discussion and announcement of computer security vulnerabilities.
CERT Coordination Center
Visit http://www.cert.org/ for general information, and http://www.cert.org/contact_cert/certmaillist.html for a mailing list with current security fixes.
Visit http://www.securiteam.com/ for a mailing list with news on current security issues and tools.
Visit http://www.securitytracker.com/ for news on security. You can list the issues according to vendors, impacts, and other categories.
Visit http://www.sans.org/top20/ for a list of the top vulnerabilities of computers.