Windows Kernel CmpCleanupLightWeightPrepare Use-After-Free

In Windows Registry, security descriptors are shared by multiple keys, and thus reference counted via the _CM_KEY_SECURITY.ReferenceCount field. It is critical for system security that the kernel correctly keeps track of the references, so that the sum of the ReferenceCount fields is equal to the number of keys in the hive at all times (with small exceptions for things like transacted and not yet committed operations etc.). If the ReferenceCount of any descriptor drops below the true number of its active references, it may result in a use-after-free condition and memory corruption. Similarly, if the field becomes inadequately large, it may be possible to overflow it and also trigger a use-after-free. A bug of the latter type is described in this report.