The patch notes only mention “security improvements,” but the security notes include the following:
Certificate Trust Policy
Available for: Mac OS X v10.5.8, Mac OS X Server v10.5.8, Mac OS X v10.6.7, Mac OS X Server v10.6.7
Impact: An attacker with a privileged network position may intercept user credentials or other sensitive information
Description: Several fraudulent SSL certificates were issued by a Comodo affiliate registration authority. This may allow a man-in-the- middle attacker to redirect connections and intercept user credentials or other sensitive information. This issue is addressed by blacklisting the fraudulent certificates.
Note: For iOS, this issue is addressed with iOS 4.3.2 and iOS 4.2.7. For Windows systems, Safari relies on the certificate store of the host operating system to determine if an SSL server certificate is trustworthy. Applying the update described in Microsoft Knowledge Base Article 2524375 will cause Safari to regard these certificates as untrusted.
The update is available through Software Update on your Mac, or you can download a standalone installer at the links below.