The Snow Leopard update addresses 40 security vulnerabilities, many of which occur in open source components of Mac OS X, such as Apache or Subversion.
Security Update 2009-006 describes fixes for Mac OS X 10.6.2 and for earlier Apple operating system versions. It includes 58 Common Vulnerabilities and Exposures (CVE) entries, 18 of which don't affect Snow Leopard.
The 10.6.2 update resolves a bug that could lead to the deletion of files in a user's home directory following a Guest account login. It also repairs an unexpected logout issue, a problem with Spotlight search that prevented Exchange contacts from being seen, and numerous other stability and usability issues, as explained on Apple's Web site.
Among the security flaws fixed is a login bypass vulnerability. "If an account on the system has no password, such as the Guest account, a user may log in to any account without supplying a password," Apple's update summary states. "This update addresses the issue through improved access checks."
Apple is disabling support for X.509 certificates with MD2 encryption for uses other than trusted root certificates. This is due to a weakness in the MD2 hash algorithm identified by IOACTIVE security researcher Dan Kaminsky and Microsoft Vulnerability Research.
Though Apple says an attack on this weakness isn't yet computationally feasible, the company is nonetheless limiting the use of X.509 certificates as "a proactive change to protect users in advance of improved attacks against the MD2 hash algorithm."
The update also addresses several QuickTime and CoreMedia flaws that could be exploited with maliciously crafted media files to allow remote code execution.
A flaw in CUPS, an open source printing system adopted by Apple, could allow a cross-site scripting attack to expose information about print system configuration and the titles of print jobs.
The update can be downloaded using Apple's Software Update preference panel, or from Apple's Web site.
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