danielkrupinski/ VAC

Reverse Engineering

steam

CSS

csgo

tf2

disassembly

dota2

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danielkrupinski

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Description

Source code of Valve Anti-Cheat obtained from disassembly of compiled modules

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Daniel Krupiński danielkrupinski

# 2,323

csgo

steam

C++

cpp17

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Readme

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

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

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

)

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): - SystemTimeOfDayInformation - returns undocumented

SYSTEM_TIMEOFDAY_INFORMATION

struct, VAC uses two fields: - LARGEINTEGER CurrentTime - LARGEINTEGER BootTime - SystemCodeIntegrityInformation - returns

SYSTEM_CODEINTEGRITY_INFORMATION

, module saves

CodeIntegrityOptions

field - SystemDeviceInformation - returns

SYSTEM_DEVICE_INFORMATION

, module saves

NumberOfDisks

field - SystemKernelDebuggerInformation - returns

SYSTEM_KERNEL_DEBUGGER_INFORMATION

, VAC uses whole struct - SystemBootEnvironmentInformation - returns

SYSTEM_BOOT_ENVIRONMENT_INFORMATION

, VAC copies

BootIdentifier

GUID - SystemRangeStartInformation - returns

SYSTEM_RANGE_START_INFORMATION

which is just

void*

. Anti-cheat saves returned kernel space start address and sign bit of that address (to check if executable inside which VAC is running is linked with

LARGEADDRESSAWARE

option)

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

). 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

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:

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