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VAC

This repository contains parts of source code of Valve Anti-Cheat for Windows systems recreated from machine code.

Introduction

Valve Anti-Cheat (VAC) is user-mode noninvasive anti-cheat system developed by Valve. It is delivered in form of modules (dlls) streamed from the remote server. steamservice.dll loaded into SteamService.exe (or Steam.exe if run as admin) prepares and runs anti-cheat modules. Client VAC infrastructure is built using C++ (indicated by many thiscall convention functions present in disassembly) but this repo contains C code for simplicity. Anti-cheat binaries are currently 32-bit.

Modules

ID Purpose .text section raw size Source folder
1 Collect information about system configuration.
This module is loaded first and sometimes even before any VAC-secured game is launched.
0x5C00 Modules/SystemInfo
2 Enumerate running processes and handles.
This module is loaded shortly after game is launched but also repeatedly later.
0x4A00 Modules/ProcessHandleList
3 Collect VacProcessMonitor data from filemapping created by steamservice.dll. It's the first module observed to use virtual methods (polymorphism). 0x6600 Modules/ProcessMonitor

Encryption / Hashing

VAC uses several encryption / hashing methods:

  • MD5 - hashing data read from process memory
  • ICE - decryption of imported functions names and encryption of scan results
  • CRC32 - hashing table of WinAPI functions addresses
  • Xor - encryption of function names on stack, e.g NtQuerySystemInformation. Strings are xor-ed with ^ or > or & char.

Module Description

#1 - SystemInfo

This module is loaded first and sometimes even before any VAC-secured game is launched.

At first module invokes GetVersion function to retrieve major and build system version e.g 0x47BB0A00 - which means:

  • 0x47BB - build version (decimal 18363)
  • 0x0A00 - major version (decimal 10)

The module calls GetNativeSystemInfo function and reads fields from resultant SYSTEM_INFO struct:

  • wProcessorArchitecture
  • dwProcessorType

Then it calls NtQuerySystemInformation API function with following SystemInformationClass values (in order they appear in code):

For more information about SYSTEM_INFORMATION_CLASS enum see Geoff Chappell's page.

Next, anti-cheat calls GetProcessImageFileNameA function to retrieve path of current executable and reads last 36 characters (e.g. \Program Files (x86)\Steam\Steam.exe).

Later VAC retrieves system directory path (e.g C:\WINDOWS\system32) using GetSystemDirectoryW, converts it from wide-char to multibyte string, and stores it (max length of multibyte string - 200). Anti-cheat queries folder FileID (using GetFileInformationByHandleEx) and volume serial number (GetVolumeInformationByHandleW). Further it does the same with windows directory got from GetWindowsDirectoryW API.

Module reads NtDll.dll file from system directory and does some processing on it (not reversed yet).

VAC saves handles (base addresses) of imported system dlls (max 16, this VAC module loads 12 dlls) and pointers to WINAPI functions (max 160, module uses 172 functions). This is done to detect import address table hooking on anti-cheat module, if function address is lower than corresponding module base, function has been hooked.

Anti-cheat gets self module base by performing bitwise and on return address (_ReturnAddress() & 0xFFFF0000). Then it collects:

  • module base address
  • first four bytes at module base address (from DOS header)
  • DWORD at module base + 0x114
  • DWORD at module base + 0x400 (start of .text section)

Next it enumerates volumes using FindFirstVolumeW / FindNextVolumeW API. VAC queries volume information by calling GetVolumeInformationW, GetDriveTypeW and GetVolumePathNamesForVolumeNameW functions and fills following struct with collected data:

struct VolumeData {
    UINT volumeGuidHash;
    DWORD getVolumeInformationError;
    DWORD fileSystemFlags;
    DWORD volumeSerialNumber;
    UINT volumeNameHash;
    UINT fileSystemNameHash;
    WORD driveType;
    WORD volumePathNameLength;
    DWORD volumePathNameHash;
}; // sizeof(VolumeData) == 32

VAC gathers data of max. 10 volumes.

If this module was streamed after VAC-secured game had started, it attemps to get handle to the game process (using OpenProcess API).

Eventually, module encrypts data (2048 bytes), DWORD by DWORD XORing with key received from server (e.g 0x1D4855D3)

#2 - ProcessHandleList

To be disclosed...

#3 - ProcessMonitor

This module seems to be relatively new or was disabled for a long time. First time I saw this module in January 2020. It has an ability to perform many different types of scans (currently 3). Further scans depends on the results of previous ones.

Each scan type implements four methods of a base class.

Initially VAC server instructs client to perform scan #1.

Scan #1 - VacProcessMonitor filemapping

First scan function attemps to open Steam_{E9FD3C51-9B58-4DA0-962C-734882B19273}_Pid:%000008X filemapping. The mapping has following layout:

struct VacProcessMonitorMapping {
    DWORD magic; // when initialized - 0x30004
    PVOID vacProcessMonitor;
}; // sizeof(VacProcessMonitorMapping) == 8

VacProcessMonitorMapping::vacProcessMonitor is a pointer to the VacProcessMonitor object (size of which is 292 bytes).

VAC then reads the whole VacProcessMonitor object (292 bytes) and its VMT (Virtual Method Table) containing pointers to 6 methods (24 bytes). The base address of steamservice.dll is also gathered.

These data are probably used on VAC servers to detect hooking VacProcessMonitor. The procedure may be following:

if (method_ptr & 0xFFFF0000 != steamservice_base)
    hook_detected();
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