API calls interception is the task that allows you to get access to some parts of others’ programs. Lots of programmers spend time developing and describing various methods that allow that access. Such methods are used in many anti-virus and anti-spyware applications. Besides, sometimes, intercepting can help you to find errors in your application. However, it is not a secret that some viruses use it too. I spent much time finding and understanding the technique of interception. I would like to describe the results of my research.
First of all, you need to read the following article to understand the basics of the interception mechanism: HookSys (written by Ivo Ivanov). It was very helpful for me, and I used the sample code from it. However, it does not solve all my problems because Ivo’s samples sometimes miss very important API calls. It happens when the application starts up too fast and the intercepting service has no time to inject the DLL. After some research, I found the actual problem; it was related to using the kernel mode function: SetCreateProcessNotificationRoutine. This function is used to receive notification events about new process creation. Such a notification is often fired when the process has already been started. Therefore, I needed to find a way to improve Ivo’s code.
As far as I know, the execution of all Windows processes consists of the following steps:
- Loading the process initially
- Creating the main thread for the process in the suspended state
- Mapping the NT.DLL into the address space of the process
- Mapping all needed DLLs and calling their DllMain with DLL_PROCESS_ATTACH reason
- Resuming the main process’ thread.
The step right before the main thread resuming looks like the most comfortable for injection because the process is in suspended state and none of its instructions have been executed yet.
Most of the work on the process creation is done in the kernel mode. To change this algorithm, you need to intercept the kernel mode functions NtCreateProcess() and NtCreateThread(). The CONTEXT structure, the pointer to which is passed to the function NtCreateThread(), contains a member called EAX. I found that it equals the process’ start address in user mode, so if you can change it, you can get the control right after process creation and before starting. To solve this task, I wrote a kernel mode driver. It starts while the system starts up.
There are some initialization steps:
- Receiving configuration from the user mode
- Intercepting kernel mode functions such as: NtCreateProcess(), NtCreateThread(), NtTerminateProcess(), NewNtCreateProcessEx()—for Windows 2003 Server.
A handler to the NtCreateThread() function contains code that will do most of the interesting jobs. Here is a brief description of its algorithm:
- Allow access to the creating process by calling ObReferenceObjectByHandle()
- Remember the main thread start address (ThreadContext->EAX)
- “Jump” to the context of the creation process by calling KeAttachProcess()
- Allocate memory for my code by calling ZwAllocateVirtualMemory(), similar to the well-known technique for CreateRemoteThread() in user mode
- Copy the small code to the allocated memory that will load my DLL. This code looks like:
- “Jump” to the initial process
- Change the thread start address (ThreadContext->EAX) so it will point to the allocated memory.
mov ebx, LoadLibraryAddr
mov eax, Win32StartAddr
pszDllName: db ‘example.dll’;
That is all. You can download and compile the complete source code for this article.
Note: The sample is fully functional and quite enough for basic understanding, but for real usage it might be rewritten.
Compiling the Code
You need the NTDDK to be installed on your computer. I’m using MSVS 6.0 for compiling NtProcDrv, and MSVS 7.1 for the rest of the projects.
- 2006-05-31 – Submitted.