Linux Security Patches - Security Patches and upgrades - http://www.linux-sec.net/Patches/
Debian: Security Information - Details of current and archive security advisories. Includes mailing list and a security FAQ. - http://www.debian.org/security/
Red Hat Errata - Errata for all releases of Red Hat Linux. - http://www.redhat.com/support/errata/
Debian Bug Tracking System - Automatic system tracking bugs reported by users until they are resolved. - http://www.debian.org/Bugs/