DLL Injection and API Hooking

Situations that require breaking through process boundary walls to access another process’ address space include the following:

  • When you want to subclass a window created by another process
  • When you need debugging aids—for example, when you need to determine which dynamic-link libraries (DLLs) another process is using
  • When you want to hook other processes

In computer programming, DLL injection is a technique used to run code within the address space of another process by forcing it to load a dynamic-link library. DLL injection is often used by third-party developers to influence the behavior of a program in a way its authors did not anticipate or intend. For example, the injected code could trap system function calls, or read the contents of password textboxes, which cannot be done the usual way.

Common DLL injection approaches:

  • Using the registry
  • Using windows hooks
  • Using remote threads
  • Using Trojans
  • As a debugger
  • Injecting code with CreateProcess

Of all the methods for injecting a DLL, using the regsitry by far the easiest. All you do is add two values to an already existing registry key. But this technique also has some disadvantages:

  • Your DLL is mapped only into processes that use User32.dll. All GUI-based applications use User32.dll, but most CUI-based applications do not. So if you need to inject your DLL into a compiler or linker, this method won’t work.
  • Your DLL is mapped into every GUI-based application, but you probably need to inject your library into only one or a few processes. The more processes your DLL is mapped into, the greater the chance of crashing the “container” processes. After all, threads running in these processes are executing your code. If your code enters an infinite loop or accesses memory incorrectly, you affect the behavior and robustness of the processes in which your code runs. Therefore, it is best to inject your library into as few processes as possible.
  • Your DLL is mapped into every GUI-based application for its entire lifetime. This is similar to the previous problem. Ideally, your DLL should be mapped into just the processes you need, and it should be mapped into those processes for the minimum amount of time. Suppose that when the user invokes your application, you want to subclass WordPad’s main window. Your DLL doesn’t have to be mapped into WordPad’s address space until the user invokes your application. If the user later decides to terminate your application, you’ll want to unsubclass WordPad’s main window. In this case, your DLL no longer needs to be injected into WordPad’s address space. It’s best to keep your DLL injected only when necessary

Following are steps to inject a DLL using Remote Threads:

  1. Use the VirtualAllocEx function to allocate memory in the remote process’ address space.
  2. Use the WriteProcessMemory function to copy the DLL’s pathname to the memory allocated in step 1.
  3. Use the GetProcAddress function to get the real address (inside Kernel32.dll) of the LoadLibraryW or LoadLibraryA function.
  4. Use the CreateRemoteThread function to create a thread in the remote process that calls the proper LoadLibrary function, passing it the address of the memory allocated in step 1. At this point, the DLL has been injected into the remote process’ address space, and the DLL’s DllMain function receives a DLL_PROCESS_ATTACH notification and can execute the desired code. When DllMain returns, the remote thread returns from its call to Load-LibraryW/A back to the BaseThreadStart function. BaseThreadStart then calls ExitThread, causing the remote thread to die.

    Now the remote process has the block of storage allocated in step 1 and the DLL still stuck in its address space. To clean this stuff up, we need to execute the following steps after the remote thread exists:

  5. Use the VirtualFreeEx function to free the memory allocated in step 1.
  6. Use the GetProcAddress function to get the real address (inside Kernel32.dll) of the FreeLibrary function.
  7. Use the CreateRemoteThread function to create a thread in the remote process that calls the FreeLibrary function, passing the remote DLL’s HMODULE.

Another way to inject a DLL is to replace a DLL that you know a process will load. For example, if you know that a process will load Xyz.dll, you can create your own DLL and give it the same filename. Of course, you must rename the original Xyz.dll to something else.

Inside your Xyz.dll, you must export all the same symbols that the original Xyz.dll exported. You can do this easily using function forwarders, which make it trivially simple to hook certain functions, but you should avoid using this technique because it is not version-resilient. If you replace a system DLL, for example, and Microsoft adds new functions in the future, your DLL will not have function forwarders for them. Applications that reference these new functions will be unable to load and execute.

If you have just a single application in which you want to use this technique, you can give your DLL a unique name and change the import section of the application’s .exe module. More specifically, the import section contains the names of the DLLs required by a module. You can rummage through this import section in the file and alter it so that the loader loads your own DLL. This technique is not too bad, but you have to be pretty familiar with the .exe and DLL file formats

API Hooking

Jeff mentioned an example that is very suitable for using API Hooking, he said:

“For example, I know of a company that produced a DLL that was loaded by a database product. The DLL’s job was to enhance and extend the capabilities of the database product. When the database product was terminated, the DLL received a DLL_PROCESS_DETACH notification and only then executed all of its cleanup code. The DLL would call functions in other DLLs to close socket connections, files, and other resources, but by the time it received the DLL_PROCESS_DETACH notification, other DLLs in the process’ address space had already gotten their DLL_PROCESS_DETACH notifications. So when the company’s DLL tried to clean up, many of the functions it called would fail because the other DLLs had already uninitialized.

The company hired me to help them solve this problem, and I suggested that we hook the ExitProcess function. As you know, calling ExitProcess causes the system to notify the DLLs with DLL_PROCESS_DETACH notifications. By hooking the ExitProcess function, we ensured that the company’s DLL was notified when ExitProcess was called. This notification would come in before any DLLs got a DLL_PROCESS_DETACH notification; therefore, all the DLLs in the process were still initialized and functioning properly. At this point, the company’s DLL would know that the process was about to terminate and could perform all of its cleanup successfully. Then the operating system’s ExitProcess function would be called, causing all the DLLs to receive their DLL_PROCESS_DETACH notifications and clean up. The company’s DLL would have no special cleanup to perform when it received this notification because it had already done what it needed to do”

Hook function is the function which replaces the original process function.

API Hooking Methodologies:

  • API Hooking by overwriting code.
  • API Hooking by Manipulating a Module’s Import Section

API Hooking by overwriting code steps:

  1. Save the first few bytes of this function in some memory of your own.
  2. You overwrite the first few bytes of this function with a JUMP CPU instruction that jumps to the memory address of your replacement function. Of course, your replacement function must have exactly the same signature as the function you’re hooking: all the parameters must be the same, the return value must be the same, and the calling convention must be the same.
  3. Now, when a thread calls the hooked function, the JUMP instruction will actually jump to your replacement function. At this point, you can execute whatever code you’d like.
  4. You unhook the function by taking the saved bytes (from step 2) and placing them back at the beginning of the hooked function.
  5. You call the hooked function (which is no longer hooked), and the function performs its normal processing.
  6. When the original function returns, you execute steps 2 and 3 again so that your replacement function will be called in the future.

This method was heavily used by 16-bit Windows programmers and worked just fine in that environment. Today, this method has several serious shortcomings, and I strongly discourage its use. First, it is CPU-dependent: JUMP instructions on x86, x64, IA-64, and other CPUs are different, and you must use hand-coded machine instructions to get this technique to work. Second, this method doesn’t work at all in a preemptive, multithreaded environment. It takes time for a thread to overwrite the code at the beginning of a function. While the code is being overwritten, another thread might attempt to call the same function. The results are disastrous! So this method works only if you know that no more than one thread will attempt to call a particular function at any given time.

API Hooking by Manipulating a Module’s Import Section

As it turns out, another API hooking technique solves both of the problems I’ve mentioned. This technique is easy to implement and is quite robust. But to understand it, you must understand how dynamic linking works. In particular, you must understand what’s contained in a module’s import section.

As you know, a module’s import section contains the set of DLLs that the module requires in order to run. In addition, it contains the list of symbols that the module imports from each of the DLLs. When the module places a call to an imported function, the thread actually grabs the address of the desired imported function from the module’s import section and then jumps to that address.

So, to hook a particular function, all you do is change the address in the module’s import section. That’s it. No CPU-dependent stuff. And because you’re not modifying the function’s code in any way, you don’t need to worry about any thread synchronization issues.

You should take into consideration delay loaded DLLs, in particular you can hook LoadLibrary* to first Load the DLL then hook its desired functions.


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