These two sites are an excellent reference for security alerts and links to security updates.
US Computer Emergency Readiness Team (US-CERT), a partnership between the Department of Homeland Security and the public and private sectors. Mandated to protect the US Internet infrastructure and coordinate defense against, and responses to, cyber attacks across the US.
Computer Emergency Response Team studies security vulnerabilities, researches long term changes in networked systems, and develops information and training to help improve security. They contribute to US-CERT.