Dereference

A type of web app attack.

Dereferencing a pointer is retrieving the value stored in memory. Pointers store memory addresses. Page protections protect the kernel from user access. However, it can be exploited by a DoS attack through a null pointer reference. If the kernel isn't smart, DMA drivers can release user pages that are associated with a null pointer. 

In some x86 systems, depending on the OS, 0 is a valid memory address. This is sometimes exploitable.