Race condition in the HFS vfs sysctl interface in XNU 1228.8.20 and earlier on Apple Mac OS X 10.5.6 and earlier allows local users to cause a denial of service (kernel memory corruption) by simultaneously executing the same HFS_SET_PKG_EXTENSIONS code path in multiple threads, which is problematic because of lack of mutex locking for an unspecified global variable.

More... (http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2009-1238)