Carlos Menezes
May 01, 2024

Hijack DLLs through proxying

Hijacking the execution flow of a binary by reexporting methods of a DLL it loads on it's startup procedure.



I delved into this topic because I wanted to create an ASI Loader for Grand Theft Auto: San Andreas. An ASI Loader, broadly speaking, enables the loading of custom libraries with the .asi extension into the game process. During game startup, the loader scans the game's root folder and loads all the files with the corresponding extensions, typically standard dynamic libraries (.dll) with renamed extensions. Injecting DLLs into a process is a fairly easy process if you use an injector (i.e. Extreme Injector), but that quickly becomes unpractical if you want to load multiple DLLs into a process, as you'd have to load them, by hand (automatically impossible through this route), on every start up.

DLL hijacking in a nutshell

A DLL is a library housing both code and data, accessible by multiple programs concurrently. Leveraging a DLL allows for the modularization of a program into distinct components, thereby enhancing its load time due to the separate module loading. When a binary executes on a Windows system, it may actively load functions and data from various DLLs during it's lifetime. DLL hijacking involves replacing a legitimate DLL with a another one, thereby intercepting the binary's execution flow. DLL proxying constitutes a form of DLL hijacking, wherein a genuine DLL, such as bar.dll, is renamed to bar1.dll. Subsequently, a DLL is introduced in place of bar.dll, bearing identical exported functions as bar1.dll. After the DLL is hijacked, the sequence of events when a program foo.exe invokes an exported function (such as ExportedFunction) from bar.dll unfolds as follows:
  1. bar.dll gets loaded into the calling process and it's code is executed;
  2. bar.dll forwards the call to ExportedFunction to bar1.dll;
  3. bar1.dll executes ExportedFunction.
Broadly speaking, the following diagram illustrates the execution flow of a program before and after the DLL is hijacked: Before hijacking After hijacking

Finding which DLLs the game loads

The first part of this process is finding a DLL to hijack. You could do guess work or use a debugger to view what DLLs are loaded, but I've opted for Sysinternals Procmon, "an advanced monitoring tool for Windows that shows real-time file system, registry and process/thread activity". Procmon accepts a list of display filters which allows you to filter the noise of the output. For this particular case, I want to include the following filters:
  • PROCCESS NAME IS gta_sa.exe INCLUDE (only output logs for the gta_sa.exe process)
  • PATH ENDS WITH .dll INCLUDE (only output logs where the path of the object subject to an operation ends in .dll)
  • PATH EXCLUDES <gta_sa.exe location> EXCLUDE (only output logs where the path for the DLL is in the same folder as gta_sa.exe)
Given these filters and upon launching gta_sa.exe, you will see this output in Procmon (enlarged): Procmon output We see that eax.dll and vorbis.dll are successfully loaded from the game's folder. One of these may very well be our target: we'll go for vorbisFile.dll.

Crafting a DLL

The Visual Studio IDE has a template for creating DLLs. If you start a new project with said template, the generator will create a dllmain.cpp file in the root directory. We won't do much with it for now, but it's a good start.

Retrieving exported functions

There are countless programs which will take a look at a DLL and tell you which functions it exports (to put it briefly, these programs parse the EAT, the Export Address Table, of a DLL file which follows the PE Format). I used DLL Export Viewer from Nirsoft. The executable allows one to export into an HTML file the exported functions from a DLL. Loading vorbisFile.dll into the program yields the following output: If you go into View -> HTML Report - All Functions, you will be presented with an HTML file which lists all the exported functions in tabular form:

Creating pragmas

We'll need to export each of these functions in our very own DLL. Otherwise, when the binary (i.e. gta_sa.exe) tries to call a function from vorbisFile.dll, that function won't be found in our DLL, resulting in the binary crashing. To achieve that, we'll use comment pragma. We're interested in the linker + exclude combo. To put it briefly, for every exported function, we'll need to add this line in our DLL template:
#pragma comment(linker,"/export:<function_name>=<filename>.<function_name>,@<ordinal>")
Because this is a tedious process, I created a Python script which operates on the file above (which you must save to your machine). It requires Beautiful Soup and Python 3.x. I'm going to rename vorbisFile to vorbisFileHooked:
from bs4 import BeautifulSoup import os def main(): page = open('./vf.html') # replace this with the file name soup = BeautifulSoup(, 'html.parser') # Find all the rows in the table rows = soup.find_all('tr') for row in rows: # Extract the function name, dll_orig, and ordinal data = row.find_all('td') if len(data) == 0: print("len 0") continue data = [t.getText().replace('<td bgcolor="#FFFFFF" nowrap="">', '').replace('</td>', '') for t in data] function_name = data[0] ordinal = data[3].split(" ")[0] full_path = data[5] filename_with_ext = os.path.basename(full_path) filename_without_ext = os.path.splitext(filename_with_ext)[0] dll_orig = "%sHooked" % filename_without_ext # vorbisFile --> vorbisFileHooked # Format the extracted data into the desired output format output = "#pragma comment(linker,\"/export:%s=%s.%s,@%s\")" % (function_name, dll_orig, function_name, ordinal) print(output) if __name__ == '__main__': main()
Quickly after, the output is spit:
#pragma comment(linker,"/export:ov_bitrate=vorbisFileHooked.ov_bitrate,@1") #pragma comment(linker,"/export:ov_bitrate_instant=vorbisFileHooked.ov_bitrate_instant,@2") #pragma comment(linker,"/export:ov_clear=vorbisFileHooked.ov_clear,@3") ... #pragma comment(linker,"/export:ov_time_total=vorbisFileHooked.ov_time_total,@34")

Putting it all together

To prove that the DLL hijacking worked, we'll show a message box with the text "Hello from the not-so-original vorbisFile.dll" when it is injected into gta_sa.exe and then resume the normal, expected execution flow. If we paste the output generated from the step above into dllmain.cpp (after every include) and make the change to display the message box, this is how the source should look:
#include <windows.h> #pragma comment(linker,"/export:ov_bitrate=vorbisFileHooked.ov_bitrate,@1") #pragma comment(linker,"/export:ov_bitrate_instant=vorbisFileHooked.ov_bitrate_instant,@2") #pragma comment(linker,"/export:ov_clear=vorbisFileHooked.ov_clear,@3") // All the other pragmas #pragma comment(linker,"/export:ov_time_total=vorbisFileHooked.ov_time_total,@34") BOOL APIENTRY DllMain ( HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved) { if (ul_reason_for_call == DLL_PROCESS_ATTACH) { MessageBoxW(NULL, L"Hello from the not-so-original vorbisFile.dll", L"", 0); } return TRUE; }
Compile for the 32-bit architecture and rename the build artifact to vorbisFile.dll (a). In the root folder, rename the original vorbisFile.dll to vorbisFileHooked.dll. Copy (a) into the root folder. Now, when we run gta_sa.exe:
I went on and implemented sa-asi-loader using this very same technique and it works flawlessly, loading mods like the Windowed Mode mod with no issues.
Tags: windows, systems
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