Apple Security Advisory 2011-07-20-1 – A large amount of security issues have been addressed in Safari versions 5.1 and 5.0.6. These range from cross site scripting, possible arbitrary code execution, accidental trust in a disable root certificate, buffer and integer overflows, and more.
See more here:
Safari 5.1 / 5.0.6 XSS / Code Execution / SSL Trust Issue