Advchk (Advisory Check) reads security advisories so you don't have to.
Advisory Check gathers security advisories using RSS / RDF / XML feeds,
compares them against a list of known softwares, and alerts you if
you're vulnerable.

Software installed on remote systems can be monitored using SSH-Update.
SSH-Update retrieves packages list by executing the remote package
manager as an SSH ForcedCommand using Public-key authentication.

Quite a few CERTs and vendors offer security-relevant feeds. Mailing
lists can be monitored using feeds provided by some web archives. Some
usenet gateways like Google Groups also offer RSS subscriptions.