Windows Heap Exploitation Vulnerability With Record’s Size Field Leads to Arbitrary R/W

Windows Heap Exploitation Vulnerability

A recent analysis reveals a critical vulnerability in Windows heap management, where improper handling of record-size fields allows for arbitrary memory read and write operations. This vulnerability primarily exploits the Low Fragmentation Heap (LFH) mechanism.

The Windows NT Heap utilizes both FrontEnd and BackEnd allocators. The FrontEnd allocator handles small allocations under 16KB using LFH, while larger requests are managed by the BackEnd.

Activation of LFH occurs after 18 consecutive allocations of similar sizes, creating predictable memory layouts that attackers can exploit. The vulnerability is found in applications using private heaps created via the HeapCreate() function.

These private heaps often have fewer security mitigations compared to default process heaps accessed through GetProcessHeap() . The core issue arises during record updates, where applications reuse previous record sizes when reading new data.

Exploitation begins with the activation of LFH through repeated allocations, followed by the creation of controlled memory layouts. Attackers manipulate the target->size field, which remains unchanged during updates, facilitating heap overflow conditions when new data exceeds allocated boundaries.

Arbitrary read capabilities are achieved by filling UserBlocks through LFH activation, creating memory holes via record removal, and reusing chunks with crafted data structures. This method allows for reading sensitive memory regions, including heap base addresses, ntdll base locations, and Process Environment Block (PEB) structures.

For arbitrary write primitives, attackers exploit Windows chunk structures containing FLink and BLink pointers in free chunks. By crafting fake chunks and manipulating freelist pointers, researchers achieved FILE structure exploitation involving controlled _base , _file , _flag , and _bufsiz fields.

The exploitation of the FILE structure necessitates specific flag combinations, such as _IOBUFFER_USER (0x0080) and _IOALLOCATED (0x2000), to bypass validation checks. Setting _base to target memory addresses and _file to stdin enables writing arbitrary data to controlled locations.

Final exploitation involves constructing Return-Oriented Programming (ROP) chains utilizing Windows APIs, including ReadFile , VirtualProtect , and WriteFile , to load and execute shellcode. This technique leverages the Microsoft x64 calling convention, using registers RCX , RDX , R8 , and R9 for passing arguments through ROP gadgets in ntdll.

This vulnerability highlights the importance of proper heap management and size validation. Organizations are advised to implement robust input validation, utilize modern heap implementations, and employ comprehensive memory protection mechanisms to mitigate sophisticated exploitation techniques targeting Windows heap internals.

Based on reporting by Cyber Security News.