Apple released a 22.4 MB security update that they recommend for all users. You must restart your computer after the update is installed.
Twelve common vulnerabilities and exposures (CVE) are identified. Many of the updates are related to third-party software. Adobe's Flash Player plug-in is responsible for seven of them.
The open-source security library, OpenSSL has a man-in-the-middle vulnerability that exists in the SSL and TLS protocols. This update disables renegotiation in OpenSSL as a preventative security measure. The organization IETF, that is responsible for the protocol, is working on a change to the renegotiation protocol. This is discussed in the security podcast, Security Now with Steve Gibson and Leo Laporte: http://twit.tv/sn223
Full details are listed at: http://support.apple.com/kb/HT1222