A significant security flaw has been identified within the Windows operating system’s heap management, specifically involving the mishandling of record size fields. This vulnerability allows attackers to perform arbitrary read and write operations in memory, potentially leading to unauthorized code execution. Security researcher Suraj Malhotra has detailed an exploitation technique that leverages the Low Fragmentation Heap (LFH) mechanism to achieve code execution on Windows systems.
Understanding the Windows Heap Structure
The Windows NT Heap is divided into two primary allocators:
– FrontEnd Allocator: Manages small memory allocations (under 16KB) using the LFH.
– BackEnd Allocator: Handles larger memory requests.
The LFH is activated after 18 consecutive allocations of similar sizes, creating predictable memory layouts that can be exploited by attackers.
The Vulnerability Explained
The core issue arises in applications that utilize private heaps created through the `HeapCreate()` function. These private heaps often lack the security mitigations present in default process heaps accessed via `GetProcessHeap()`. The vulnerability is particularly evident in record update functionalities where applications reuse previous record sizes when reading new data.
Exploitation Process
The exploitation process involves several key steps:
1. LFH Activation: Attackers trigger the LFH by making repeated allocations, leading to a predictable memory layout.
2. Controlled Memory Layout Creation: By manipulating the `target->size` field, which remains unchanged during updates, attackers can induce heap overflow conditions when new data exceeds allocated boundaries.
3. Arbitrary Read Capabilities: By filling UserBlocks through LFH activation and creating memory holes via record removal, attackers can reuse chunks with crafted data structures. This enables reading sensitive memory regions, including heap base addresses, `ntdll` base locations, and Process Environment Block (PEB) structures.
4. Arbitrary Write Primitives: Attackers exploit Windows chunk structures containing `FLink` and `BLink` pointers in free chunks. By forging fake chunks and manipulating freelist pointers, they can achieve arbitrary write operations. This involves crafting `FILE` objects with controlled `_base`, `_file`, `_flag`, and `_bufsiz` fields.
5. Code Execution: The final step involves constructing Return-Oriented Programming (ROP) chains utilizing Windows APIs, such as `ReadFile`, `VirtualProtect`, and `WriteFile`, to load and execute shellcode. This technique leverages the Microsoft x64 calling convention, passing arguments through `RCX`, `RDX`, `R8`, and `R9` registers using ROP gadgets in `ntdll`.
Implications and Recommendations
This vulnerability underscores the critical importance of proper heap management and size validation in software development. 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.
