Ah, got it:
“The exploit takes advantage of two issues in JDK 7: The ClassFinder and MethodFinder.findMethod(). Both were newly introduced in JDK 7. ClassFinder is a replacement for classForName back in JDK 6. It allows untrusted code to obtain a reference and have access to a restricted package in JDK 7, which can be used to abuse sun.awt.SunToolkit (a restricted package). With sun.awt.SunToolkit, we can actually invoke getField() by abusing findMethod() in Statement.invokeInternal() (but getField() must be public, and that’s not always the case in JDK 6) in order to access Statement.acc’s private field, modify AccessControlContext, and then disable Security Manager. Once Security Manager is disabled, we can execute arbitrary Java code. Our exploit has been tested successfully against multiple platforms, including: IE, Firefox, Safari, Chrome; Windows, Ubuntu, OS X, Solaris, etc.”
So if you don’t have Java 7, but are running 6 or 5, then you are good.
Here is a full description of the vulnerability with sample code:
http://immunityproducts.blogspot.com/2012/08/java-0day-analysis-cve-2012-4681.html
They are basically using tricks to get access to a private field in the security context object and changing it.