Advisories and patches relating to security issues.
Please submit sites offering comprehensive listings of advisories and patches to this category.

Sites relating to groups researching new vulnerabilities/exploits should be submitted to: Computers/Hacking/Groups.

You should be on the security-alert mailing list of the vendor of every operating system you admin. Typically these are extremely low-volume mailing lists, and applying the security patches they recommend is frequently urgent if your machine is connected to the internet.
Advisories and patches from the OS vendor. Usually includes all historical advisories and patches for all currently-supported OS versions.