Category Archives: Mitigations

Mitigating Null Pointer Exploitation on Windows

As part of a small research project, I recently looked into how exploitation of null pointer vulnerabilities could be mitigated on Windows. The problem with many of the recent vulnerabilities affecting Windows kernel components is that a large number of these issues can be exploited provided that the attacker maps and controls the contents of the null page. As many of you probably know, Windows allows non-privileged users to map the null page through functions such as NtAllocateVirtualMemory or NtMapViewOfFile.

Although there are multiple ways to approach the problem, the solution proposed relies on manipulation of virtual address descriptors (VADs) using a kernel-mode driver. As VADs are used to implement the PAGE_NOACCESS protection in Windows and contain special properties to secure address ranges in process memory, they can be used to deny null page access in both user and kernel space. The following paper details the proposed mitigation and suggests a possible implementation.

Locking Down the Windows Kernel: Mitigating Null Pointer Exploitation [PDF]

Abstract. One of the most prevalent bug classes affecting Windows kernel components today is undeniably NULL pointer dereferences. Unlike other platforms such as Linux, Windows (in staying true to backwards compatibility) allows non-privileged users to map the null page within the context of a user process. As kernel and user-mode components share the same virtual address space, an attacker may potentially be able to exploit kernel null dereference vulnerabilities by controlling the dereferenced data. In this paper, we propose a way to generically mitigate NULL pointer exploitation on Windows by restricting access to the lower portion of process memory using VAD manipulation. Importantly, as the proposed method employs features already present in the memory manager and does not introduce any offending hooks, it can be introduced on a wide range of Windows platforms. Additionally, because the mitigation only introduces minor changes at process creation-time, the performance cost is minimal.