Exception Driven "Debugging": Getting behind anti debugging tricks.

Of course, every debugging is exception driven. At least because a breakpoint generates debug exception wich is passed to debugger. In this article, however, I will refer to regular exceptions.

There are tens if not hundreds of software protectors used by software vendors around the globe. Some are good, some are less good, in either case, vendors rarely use them in a proper way, thinking that simply enabling anti-debugging features, provided by protector of their choice, is enough. I have seen it myself - a widely known commercial application, protected using Themida (which is one of the most complicated protectors) remains SOOO unprotected, that Themida is not even notices during the extraction of relatively sensitive information using the application itself.

However, the purpose of this article is not to discuss pros and cons of Themida or any other protector, nor do I have any intention to disgrace any of the software vendors. The purpose is to describe a relatively easy way of bypassing common anti debugging tricks (including Windows DRM protection)  with DLL injection.

As the term "anti debugging" states - such methods target modern debuggers. There are several commonly known tricks:

1. IsDebuggerPresent() - you would be surprised to know how many vendors rely on this API alone;
2. Additional methods of debugger presence detection;
3. IAT modification - which is not really worth trying;
4. Redirection of debugging API (e.g. to an infinite loop).
5. And some more.

Point #4 does not let you to implement your own debugger in a hope that it would not be noticed by the victim program (many beginners fall out at this point).

Point #3 - how much can you modify the IAT? I mean, system loader has to still be able to parse it, thus, if system loader can - everyone can.

Point #1 is not even worth further mentioning here.

In this article I am going to describe a simple way (although, some may cry and say it is a hard way) to get around most of anti debugging tricks without even noticing their presence by implementing a simple pseudo debugger dll, which is to be injected into the target process.

Step #1. Preparations

In order to use any debugger, you have to know where to set your breakpoints. Otherwise, the whole process is meaningless. But how can you define proper locations if the executable on disc is encrypted (e.g. with Themida) and you still cannot attach a debugger to see what is going on inside?

The solution is quite simple. Simple in deed. Windows provides us with all the instruments to read the memory of another process (given that you have sufficient access rights) with OpenProcess(), ReadProcessMemory() and NtQueryInformationProcess() API functions. Using those, you can simply dump the decrypted executable and any of its modules (DLLs) to a separate file on disc.

NtQueryInformationProcess() provides you with the address of the PEB (see this post for more information on PEB) of the target process. Then you simply parse the linked list of loaded modules, get the base address (module handle) and the image size for each, then use ReadProcessMemory to copy the image to a file. One complication, though, you will have to use ReadProcessMemory in order to access the PEB of the remote process.

Once you have dumped the target image to a file, such file can be easily loaded into IDA Pro, disassembled and researched statically.

Step #2. Injector and DLL

I do not see any reason to describe the DLL injection process here, as it has been described many times, even in this blog. You are free to use standard injection method, advanced DLL injection method or use this method if you have problems with the two previously mentioned.

DllMain()

It is suggested not to perform any heavy action in this function, however, we do not really have a choice (although, you can launch a separate thread). First thing to do is to suspend all running threads (except the current one of course). The problem is that Windows has no API function that would allow you to enumerate threads of a single process, instead, it lets you go through all the threads in the system. See MSDN pages for Thread32First and Thread32Next - there should be a perfect example of getting threads of the current process. Once all the threads are suspended, you are ready to proceed.

Installation of breakpoints 

No, we are not going to use regular 0xCC software breakpoints, neither are we going to make any use of hardware breakpoints here. Instead, we are going to place an instruction that would raise an exception to the location of desired breakpoint. To keep such instruction short and to avoid changing the values of the registers, 'AAM 0' seems to be a perfect candidate. It only takes two bytes 0xD4 0x00 and raises the EXCEPTION_INT_DIVIDE_BY_ZERO exception (exception code 0xC0000094).

Use the VirtualProtect() function to change the access rights of the target address, so you can alter its content, backup the original two bytes from that address and overwrite them with 0x00D4. 

VirtualProtect((LPVOID)(target & ~0xFFF), 0x1000, PAGE_EXECUTE_READWRITE, (PDWORD)&prevProtect);

*((unsigned short*)target) = 0x00D4;

VirtualProtect((LPVOID)(target & ~0xFFF), 0x1000, prevProtect, (PDWORD)&prevProtect);

Now the victim process is almost ready to be continued. One thing left - exception handler. We will use vectored exception handling mechanism as it allows our handler to be (at least among) the first to handle an exception. Once the handler has been added with AddVectoredExceptionHandler(), you may resume the suspended threads of the process.

Handler

One important thing to do once your handler gets control, is to check for the address where the exception occurred and for the exception code, as we have no intention to deal with irrelevant exceptions:

LONG CALLBACK handler(PEXCEPTION_POINTERS ep)

{

   if(ep->ContextRecord->Eip == target && ep->ExceptionRecord->ExceptionCode == 0xC0000094)

   {

      // Do your stuff here

   }

   else

      // Optionally log other exceptions

      return EXCEPTION_CONTINUE_SEARCH;

   return EXCEPTION_CONTINUE_EXECUTION;

}

Your Stuff

One of the parameters you get with your handler is the pointer to the CONTEXT structure, which provides you with the content of all the registers at the time of the exception. Needless to mention, that you have the access to the process' memory as well. Just as you were in a debugger with the only difference - you have to implement the routine that would show you the data you are interested in. Do not forget to emulate the original instruction replaced by the pseudo breakpoint and advance the Eip accordingly before returning from handler.

One more thing to mention - it may be a good idea to suspend all other threads of the victim process while in the 'your stuff' portion of the handler.

Stability

I am not claiming this method to be bullet proof and I am more than sure ( I simply know) - there are ways to defeat it, however, personally, I have not yet met such software. In addition - this method is tested and stable.


Hope this article was helpful. See you at the next.

P.S. Lazy guys, nerds, etc., do not cry for sources. This method is really simple. Besides, if copy/paste is the only programming technique you are aware of, then, probably, this blog is not the right place for you.

CreateRemoteThread. Bypass Windows 7 Session Separation

Internet is full of programmers' forums and those forums are full with questions about CreateRemoteThread Windows API function not working on Windows 7 (when trying to inject a DLL). Those posts made by lucky people, somehow, redirect you to the MSDN page dedicated to this API, which says: "Terminal Services isolates each terminal session by design. Therefore, CreateRemoteThread fails if the target process is in a different session than the calling process." and, basically, means - start the process from your injector as suspended, inject your DLL and then resume the process' main thread. This works... Most of the time... But sometimes you really need to inject your code into a running process. Isn't there a way to do that? Well, there is. As a matter of fact, it is so easy, that I decided not to attach my source code to this article (mainly, because I am too lazy to make it look readable :) ). It appears to be that I am not the only one lazy here :), so I have uploaded the source code.

Let me start as usual, with a note for nerds in order to avoid meaningless comments and stupid discussions. 

The code provided within the article is for example purposes only. Error checks have been omitted on purpose. Yes, there may be another, probably even better, way of doing this. No, manual DLL mapping is not better unless you have plenty of time and nothing to do with it.

All others, let's get to business :)

Opening the Victim Process

This is the easiest part. At this stage you will see whether you are able to inject your code or not (in case of a system process, for example). Nothing unusual here - you simply invoke the good old OpenProcess API

HANDLE WINAPI OpenProcess(

       DWORD dwDesiredAccess, /* in our case PROCESS_ALL_ACCESS */

       BOOL  bInheritHandle, /* no need, so FALSE */

       DWORD dwProcessId /* self explanatory enough */

);

which opens the process specified by dwProcessId and returns a handle to that process, unless, you have no sufficient rights to access that process.

Reading the Shellcode

What you usually see in the examples of shellcode over the internet, is an unsigned char array of hexadecimal values somewhere in the C code. Helps to keep the amount of files smaller, but is not really comfortable to deal with. I decided to store the shellcode in a separate binary file, produced with FASM (Flat Assembler):

use32

   ; offset of the LoadLibraryA address within the shellcode

   dd    func

   ; save all registers

   push  eax ebx ecx edx ebp edi esi

   ; get your EIP

   call  next

next:

   pop   eax

   mov   ebx, eax

   ; get the address of the DLL name

   mov   eax, string - next

   ; do this to avoid possible negative values (due to sign extend)

   movzx eax, al

   add   eax, ebx

   ; pass it to the LoadLibraryA API

   push  eax

   ; get the address of the LoadLibraryA function

   mov   eax, func - next

   movzx eax, al

   add   eax, ebx

   mov   eax, [eax]

   ; call LoadLibraryA

   call  eax

   ; restore registers

   pop   esi edi ebp edx ecx ebx eax

   ; return

   ret

func     dd 0x12345678 ; placeholder for the address

string:

Compiling this code with FASM.EXE will produce a raw binary file, where all offsets are 0 - based. There are some parts in the code above, that may require some additional explanation (for example, why does it not end with ExitThread()). I am aware of this and I will provide you with the explanation a little bit later.

For now, allocate an unsigned char buffer for your shellcode. Make this buffer large enough to contain the shellcode and the name of the DLL (my assumption is, that you passed that name as a command line parameter to your injector). with it's terminating zero.

Once you have read the shellcode into that buffer - append the name of the DLL (which may be a full path to the DLL) to the end of the shellcode with, for example, memcpy() function. Half done with it. Now we still have to "tell" the shellcode where the LoadLibraryA API function is located in memory. Fortunately, the load address randomization in Windows is far from being perfect (addresses  of loaded modules may vary between subsequent reboots, but are the same for all processes). This means that, just as in usual DLL injection, we obtain the address of this API in our process by calling good old GetProcAddress(GetModuleHandleA("kernel32.dll"), "LoadLibraryA") and save it to the "func" variable of the shellcode. Due to the fact that our shellcode may vary in size from time to time (that depends on the needs), we saved the offset to that variable in the first four bytes of the shellcode, which eliminates the need to hardcode the offset. Simply do the following:

*(unsigned int*)(shellcode_ptr + *(int*)(shellcode_ptr)) = (unsigned int)LoadLibraryA_address;

Our shellcode is ready now.

"Create remote thread" without CreateRemoteThread()

As the title of this paragraph suggests - we are not going to use the CreateRemoteThread(). In fact, we are not going to create any thread in the victim process (well, the injected DLL may, but the shellcode won't).

Code Injection

Surely, we need to move our shellcode into the victim process' address space in order to load or library. We are doing it in the same manner, as we would copy the name of the DLL in regular DLL injection procedure:

1. Allocate memory in the remote process with
   LPVOID WINAPI VirtualAllocEx(
      HANDLE hProcess, /* the handle we obtained with OpenProcess */
      LPVOID lpAddress, /* preferred address; may be NULL */
      SIZE_T dwSize, /* size of the allocation in bytes */
      DWORD  flAllocationType, /* MEM_COMMIT */
      DWORD  flProtect /* PAGE_EXECUTE_READWRITE */
   );
   This function returns the address of the allocation in the address space of the victim process or NULL if it fails.
2. Copy the shellcode into the buffer we've just allocated in the address space of the victim process:
   BOOL WINAPI WriteProcessMemory(
      HANDLE   hProcess, /* same handle as above */
      LPVOID   lpBaseAddress, /* address of the allocation */
      LPCVOID  lpBuffer, /* address of the local buffer with the shellcode */
      SIZE_T   nSize, /* size of the shellcode together with the appended                                 NULL-terminated string */
3.    SIZE_T   *lpNumberOfBytesWritten /* if this is zero - check your code */
   );
   If the return value of this function is non zero - we have successfully copied our shellcode into the victim process' address space. It may also be a good idea to check the value returned in the lpNumberOfBytesWritten.

Make It Run

So, we have copied our shell code. The only thing left, is to make it run, but we cannot use the CreateRemoteThread() API... Solution is a bit more complicated.

First of all, we have to suspend all threads of the victim process. In general, suspending only one thread is enough, but, as we cannot know for sure what is going on there, we should suspend them all. There is no specific API that would provide us with the list of threads for a specified process, instead, we have to create a snapshot with CreateToolhelp32Snapshot, which provides us with the list of all currently running threads of all processes running in the system:

HANDLE WINAPI CreateToolhelp32Snapshot(

   DWORD dwFlags, /* TH32CS_SNAPTHREAD = 0x00000004 */

   DWORD th32ProcessID /* in this case may be 0 */

);

This function returns the handle to the snapshot, which contains information on all present threads. Once we have this, we "iterate through the list" with Thread32First and Thread32Next API functions:

BOOL WINAPI Thread32First(

   HANDLE hSnapshot, /* the handle to the snapshot */

   LPTHREADENTRY32 lpte /* pointer to the THREADENTRY32 structure */

);

The Thread32Next has the same prototype as Thread32First.

typedef struct tagTHREADENTRY32{

   DWORD dwSize; /* size of this struct; you have to initialize this field before use */

   DWORD cntUsage; 

   DWORD th32ThreadID; /* use this value to open thread for suspension */

   DWORD th32OwnerProcessID; /* compare this value against the PID of the victim 

                              to filter out threads of other processes */

   LONG  tpBasePri;

   LONG  tpDeltaPri;

   DWORD dwFlags;

} THREADENTRY32, *PTHREADENTRY32;

For each THREADENTRY32 with matching th32OwnerProcessID, open it with OpenThread() and suspend with SuspendThread:

HANDLE WINAPI OpenThread(

   DWORD dwDesiredAccess, /* THREAD_ALL_ACCESS */

   BOOL  bInheritHandle, /* FALSE */

   DWORD dwThreadId /* th32ThreadID field of THREADENTRY32 structure */

);

and

DWORD WINAPI SuspendThread(

   HANDLE hThread, /* Obtained by OpenThread() */

);

Don't forget to CloseHandle(openedThread) :)

Take the first thread, once it is opened (actually, you can do that with any thread that belongs to the victim process) and suspended, and get its CONTEXT (see "Community Additions" here) using the GetThreadContext API:

BOOL WINAPI GetThreadContext(

   HANDLE    hThread, /* handle to the thread */

   LPCONTEXT lpContext /* pointer to the CONTEXT structure */

);

Now, when all the threads of the victim process are suspended, we are may do our job. The idea is to redirect the execution flow of this thread to our shellcode, but make it in such a way, that the shellcode would return to where the suspended thread currently is. This is not a problem at all, as we have the CONTEXT of the thread. The following code does that just fine:

/* "push" current EIP of the thread onto its stack, so that the ret instruction in the shellcode returns the execution flow to this address (which is somewhere in WaitForSingleObject for suspended threads) */

ctx.Esp -= sizeof(unsigned int);

WriteProcessMemory(victimProcessHandle, 

                   (LPVOID)ctx.Esp, 

                   (LPCVOID)&ctx.Eip,

                   sizeof(unsigned int),

                   &bytesWritten);

/* Set the EIP to our injected shellcode; do not forget to skip the first four bytes */

ctx.Eip = remoteAddress + sizeof(unsigned int);

Almost there. All we have to do now, is resume the previously suspended threads in the same manner (iterating with Thread32First and Thread32Next with the same snapshot handle).

Don't forget to close the victim process' handle with CloseHandle() ;)

Shellcode

After all this, the execution flow in the selected thread of the victim process reaches our shellcode, which source code should be pretty clear now. It simply calls the LoadLibraryA() API function with the name/path of the DLL we want to inject.

One important note - it is a bad practice to do anything "serious" inside the DllMain() function. My suggestion is - create a new thread in DllMain() and do all the job there, so that it may return safely.

Hope this article was helpful.

Have fun injecting and see you at the next.

Faking KERNEL32.DLL - an Amateur Sandbox

As a part of my work (read "fun") of maintaining this blog, I am constantly checking the statistic information on traffic sources and keywords (it's nice to know that people are getting here via Google) in order to see whether my readers are getting what they are looking for (personally, I see no reason in simply "streaming my consciousness to the masses" as this is not the point of this blog). Sometimes, it gives an idea of what is missing but still related to system and low level programming.

A couple of days ago, I saw that someone was looking for a way to load and use fake KERNEL32.dll and I realized that this information has not yet been covered here. There is no source code for this article as I am a bit short on time to write it, but I will do my best to provide as much information as possible so, those who would want to try it would have no problem doing that.

KERNEL32.DLL

First notable thing about KERNEL32.dll is that it is always loaded, regardless of whether a running executable imports anything from it (this is briefly covered here). Same as NTDLL.dll (well, KERNEL32.dll imports from it). This library provides interfaces for interaction with deeper levels of "user land" part of the operating system for the running executable and some of other dynamic link libraries loaded into process' memory. 

Knowing all that, the first thought may be: "how are we going to fake it if all the rest depends on it?". The solution is easier than one could think at first. However, we should keep in mind, that some programs may import from NTDLL.dll directly, bypassing the KERNEL32.dll (which used to happen quite often in the world of malware), meaning that once you faked KERNEL32.dll, you may have to fake NTDLL,as well.

Preparations

We should start with writing a good old simple DLL/code injector. It is easier to dissect the victim process from inside. This is the simplest part and it is covered in this and this posts of this blog. Roughly saying, the injector should be able to create a victim process in suspended state by passing the CREATE_SUSPENDED process creation flag to CreateProcess API.

Writing the code or the DLL we are going to inject is a harder task as this code is intended to perform the tasks described below in order of execution.

Load Fake KERNEL32.dll

Let's assume, that we already have a ready to use fake KERNEL32.dll (we'll get back to creation of fake dll a bit later). This is quite simple - call LoadLibrary function from your code. One thing worth mentioning is that MSDN is not suggesting to use LoadLibrary in your DllMain function. Therefore, if you decide to use DLL Injection instead of code injection, then better use the approach described in "Advanced DLL Injection" article. 

Fake KERNEL32.dll should simply import all API's from the original one. Don't be mistaken - import, not forward it's exports at least as long as we are talking about API functions, but you may safely forward exported objects and variables to the original one.

Resolve Victim's Imports

By the time we get our code/DLL running inside the suspended victim process, all of it's imports should already have been resolved. What we still have to do, is to replace all API addresses exported from the original KERNEL32.dll with corresponding addresses in our fake one.

Here is a link to Microsoft's specifications of MS PE and MS COFF file formats - would be useful digging through imports and export.

Hide the Original KERNEL32.dll

While performing the aforementioned actions may be enough in case of a regular application, we should take some precautions in case of malicious code. My suggestion is to hide the original KERNEL32.dll by replacing its entry in the list of LDR_MODULE structures in PEB with the one describing our fake KERNEL32.dll, just like we would hide an injected DLL in the "Hiding Injected DLL in Windows" article.

Creation of Fake KERNEL32.dll

This may sound scary, but there is no need to worry (at least not too much). All that we need in order to create one, is a C compiler (or whatever high level language you prefer) and any assembler (I use FASM as usual).

Dump KERNEL32.dll to ASM Source

No, of course we do not have to disassemble the whole DLL and dump it to a corresponding Assembly source. Instead, what we have to do, is write a small application in high level language (you may try to do it in Assembly if you want) that would parse the export table of the original KERNEL32.dll and create a set of Assembly source files: one for code, one for data (if needed), one for import and one for export sections.

Want it or not, but the application has to generate a bit of Assembly code for at least transferring the execution flow to an API function in the original KERNEL32.dll. For example, if we have no interest in, let's say, ExitProcess, then our fake ExitProcess should look similar to this:

fake_ExitProcess:

   ; As we are not tracing/logging this function, we simply let the

   ; original ExitProcess shoot

   jmp dword [real_ExitProcess]

However, the code would be different for APIs of interest. For example, the CreateFileA API would be implemented like this:

fake_CreateFileA:

   ; We pass control to a fake CreateFileA, which is implemented in

   ; a separate DLL imported by our fake KERNEL32.dll

   ; Parameters are already on the stack, so we simply jump.

   ; Don't forget to declare the fake function as STDCALL 

   ; (remember #define WINAPI __declspec(stdcall) ? )

   jmp dword [our_CreateFileA]

The Assembly source file containing code for the import section would then contain the following:

section '.idata' import data readable writable

   library original, 'kernel32.dll',\            ;Import original KERNEL32.dll

           fake,     'our_dll_with_fake_api.dll' ;Import a DLL with fake APIs

   import original,\

      real_ExitProcess, 'ExitProcess'

   import fake,\

      our_CreateFileA, 'whatever you call it here'

Now, finally, we get to the export section's code:

section '.edata' export data readable

   export 'KERNEL32.dll',\              ;FASM does not care about what you type here, 

                                        ;so let's be fake to the end and pretend 

                                        ;to be KERNEL32.dll

      fake_ExitProcess, 'ExitProcess',\

      fake_CreateFileA, 'CreateFileA'

Finally the main source file, the one that would assemble all the rest together:

format PE DLL at 0x10000000

include 'code.asm'

include 'import.asm'

include 'export.asm'

section '.reloc' fixups data discardable

compile it with FASM and you have your fake KERNEL32.dll.

Implementation of Fake API

As it has been mentioned above, there are some functions we would want to trace. Those should have some custom implementation, preferably in a separate DLL (which would be loaded by Windows loader at the time it resolves our fake KERNEL32.dll's dependencies). Below is a diagram of the interactions between all the modules:

Interactions between modules involved in faking.

And here is an example of such fake API:

HANDLE WINAPI fake_CreateFileA(

                               LPCSTR lpFileName,

                               DWORD dwDesiredAccess,

                               DWORD dwShareMode,

                               LPSECURITY_ATTRIBUTES lpSA,

                               DWORD dwCreationDisposition,

                               DWORD dwFlagsAndAttributes,

                               HANDLE hTemplateFile)

{

   fprintf(log_file, "CreateFileA(list params here)\n", params);

   return CreateFileA(lpFileName, 

                      dwDesiredAccess, 

                      dwShareMode, 

                      lpSA, 

                      dwCreationDisposition,

                      dwFlagsAndAttributes,

                      hTemplateFile);

}

Of course, you may implement addition mechanisms within this DLL, e.g. let it communicate with another application via sockets or pipes, but this is deserves a separate article.

P.S. 

My personal suggestion is to insert more code into each function inside the fake KERNEL32.dll so that it would look more realistic to the victim process (should it try to do anything with it).

Hope this article was useful. 

See you at the next.

Executable Code Injection the Interesting Way

So. Executable code injection. In general, this term is associated with malicious intent. It is true in many cases, but in, at least, as many, it is not. Being malware researcher for the most of my career, I can assure you, that this technique appears to be very useful when researching malicious software, as it allows (in most cases) to defeat its protection and gather much of the needed information. Although, it is highly recommended not to use such approach, sometimes it is simply unavoidable.

There are several ways to perform code injection. Let's take a look at them.

DLL Injection

The most simple way to inject a DLL into another process is to create a remote thread in the context of that process by passing the address of the LoadLibrary API as a ThreadProc. However, it appears to be unreliable in modern versions of Windows due to the address randomization (which is currently not true, but who knows, may be once it becomes real randomization).

Another way, a bit more complicated, implies a shell code to be injected into the address space of another process and launched as a remote thread. This method offers more flexibility and is described here.

Manual DLL Mapping

Unfortunately, it has become fashionable to give new fancy names to the old good techniques. Manual DLL Mapping is nothing more than a complicated code injection. Why complicated, you may ask - because it involves implementation of custom PE loader, which should be able to resolve relocations. Adhering the Occam's Razor principle, I take the responsibility to claim, that it is much easier and makes more sense to simply allocate memory in another process using VirtualAllocEx API and inject the position independent shell code. 

Simple Code Injection

As the title of this section states, this is the simplest way. Allocate a couple of memory blocks in the address space of the remote process using VirtualAllocEx (one for code and one for data), copy your shell code and its data into those blocks and launch it as a remote thread.

All the methods listed above are covered well on the Internet. You may just google for "code injection" and you will get thousands of well written tutorials and articles. My intention is to describe a more complex, but also a more interesting way of code injection (in a hope that you have nothing else to do but try to implement this).

Before we start:

Another note for nerds. 

• The code in this article does not contain any security checks unless it is needed as an example.
• This is not malware writing tutorial, so I do not care whether the AV alerts when you try to use this method.
• No, manual DLL mapping is not better ;-).
• Neither do I care about how stable this solution is. If you decide to implement this, you will be doing it at your own risk.

Now, let's have some fun.

Disk vs Memory Layout

Before we proceed with the explanation, let's take a look at the PE file layout, whether on disk or in memory, as our solution relies on that.

This layout is logically identical for both PE files on disk and PE files in memory. The only differences are that some parts may not be present in memory and, the most important for us, on disk items are aligned by "File Alignment" while in memory they are aligned by "Page Alignment" values, which, in turn may be found in the Optional Header. For full PE COFF format reference check here.

Right now, we are particularly interested in sections that contain executable code ((SectionHeader.characteristics & 0x20000020) != 0). Usually, the actual code does not fill the whole section, leaving some parts simply padded by zeros. For example, if our code section only contains 'ExitProcess(0)', which may be compiled into 8 bytes, it will still occupy FileAlignment bytes on disk (usually 0x200 bytes). It will take even more space in memory, as the next section may not be mapped closer than this_section_virtual_address + PageAlignement  (in this particular case), which means that if we have 0x1F8 free bytes when the file is on disk, we'll have 0xFF8 free bytes when the file is loaded in memory.

The "formula" to calculate available space in code section is next_section_virtual_address - (this_section_virtual_address + this_section_virtual_size) as virtual size is (usually) the amount of actual data in section. Remember this, as that is the space that we are going to use as our injection target.

It may happen, that the target executable does not have enough spare space for our shell code, but let this not bother you too much. A process contains more than one module (the main executable and all the DLLs). This means that you can look for spare space in the code sections of all modules. Why only code sections? Just in order not to mess too much with memory protection.

Shellcode

The first and the most important rule for shellcode - it MUST be position independent. In our case, this rule is especially unavoidable (if you may say so) as it is going to be spread all over the memory space (depends on the size of your shell code, of course). 

The second, but not less important rule - carefully plan your code according to your needs. The less space it takes, the easier the injection process would be.

Let's keep our shell code simple. All it would do is interception of a single API (does not matter which one, select whichever you want from the target executable's import section), and show a message box each time that API is called (you should probably select ExitProcess for interception if you do not want the message box popping up all the time).

Divide your shellcode into independent functional blocks. By independent, I mean that it should not have any direct or relative calls or jumps. Each block should have one data field, which would contain the address of the table containing addresses of all our functions (and data if needed). Such mechanism would allow us to spread the code all over the available space in different modules without the need to mess with relocations at all.

The picture on the left and the diagram below will help you to better understand the concept. 
Init - our initialization function. Once the code is injected, you would want to call this function as a remote thread.
Patch - this block is responsible for actually patching the import table with the address of our Fake.

The code in each of the above blocks will have to access Data in order to retrieve addresses of functions from other blocks.

Your initialization procedure would have to locate the KERNEL32.DLL in memory in order to obtain the addresses of LoadLibrary (yes, it would be better to use LoadLibrary rather then GetModuleHandle), GetProcAddress and VirtualProtect API functions which are crucial even for such a simple task as patching one API call. Those addresses would be stored in Data.

The Injector

While the shellcode is pretty trivial (at least in this particular case), the injector is not. It will not allocate memory in the address space of another process (if possible, of course). Instead, it will parse the the PEB (Process Environment Block) of the victim in order to get the list of loaded modules. Once that is done, it will parse section headers of every module in order to create list of available memory locations (remember, we prefer code sections only) and fill the Data block with appropriate addresses. Let's take a look at each step.

First of all, it may be a good idea to suspend the process by calling SuspendThread function on each of its threads. You may want to read this post about threads enumeration. One more thing to remember is to open the victim process with the following flags: PROCESS_VM_READ | PROCESS_VM_OPERATION | PROCESS_VM_WRITE | PROCESS_QUERY_INFORMATION | PROCESS_SUSPEND_RESUME in order to be able to perform all the following operations. The function itself is quite simple:

DWORD WINAPI SuspendThread(__in HANDLE hThread);

Don't forget to resume all threads with ResumeThread once the injection is done.

The next step would be calling the NtQueryInformationProcess function from the ntdll.dll. The only problem with it is that it has no associated import library and you will have to locate it with GetProcAddress(GetModuleHandle("ntdll.dll"), "NtQueryInformationProcess"), unless you have a way to explicitly specify it in the import table of your injector. Also, try LoadLibrary if the GetModuleHandle does not work for you.

NTSTATUS WINAPI NtQueryInformationProcess(
   __in      HANDLE ProcessHandle,
   __in      PROCESSINFOCLASS ProcessInformationClass, /* Use 0 in order to 
                                               get the PEB address */
   __out     PVOID ProcessInformation,  /* Pointer to the PROCESS_BASIC_INFORMATION
                                                       structure */
   __in      ULONG ProcessInformationLength, /* Size of the PROCESS_BASIC_INFORMATION
                                                     structure in bytes */
   __out_opt PULONG ReturnLength
);

typedef struct _PROCESS_BASIC_INFORMATION
{
   PVOID     ExitStatus;
   PPEB      PebBaseAddress;
   PVOID     AffinityMask;
   PVOID     BasePriority;
   ULONG_PTR UniqueProcessId;
   PVOID     InheritedFromUniqueProcessId;
} PROCESS_BASIC_INFORMATION;

The NtQueryInformationProces will provide you with the address of the PEB of the victim process. This post will explain you how to deal with PEB content. Of course, you will not be able to access that content directly (as it is in the address space of another process), so you will have to use WriteProcessMemory and ReadProcessMemory functions for that.

BOOL WINAPI WriteProcessMemory(
   __in   HANDLE   hProcess,
   __in   LPVOID   lpBaseAddress,  /* Address in another process */
   __in   LPCVOID  lpBuffer,  /* Local buffer */
   __in   SIZE_T   nSize,  /* Size of the buffer in bytes */
   __out  SIZE_T*  lpNumberOfBytesWritten
};

BOOL WINAPI ReadProcessMemory(
   __in   HANDLE   hProcess,
   __in   LPCVOID  lpBaseAddress, /* Address in another process */
   __out  LPVOID   lpBuffer,  /* Local buffer */
   __in   SIZE_T   nSize,  /* Size of the buffer in bytes */
   __out  SIZE_T*  lpNumberOfBytesRead
};

Due to the fact that you are going to deal with read only memory locations, you should call VirtualProtectEx in order to make those locations writable (PAGE_EXECUTE_READWRITE).  Don't forget to restore memory access permissions to PAGE_EXECUTE_READ when you are done. 

BOOL WINAPI VirtualProtectEx(
   __in  HANDLE hProcess,
   __in  LPVOID lpAddress, /* Address in another process */
   __in  SIZE_T dwSize,  /* Size of the range in bytes */
   __in  DWORD  flNewProtect, /* New protection */
   __out PDWORD lpflOldProtect
};

You may also want to change the VirtualSize of those sections of the victim process you used for injection in order to cover the injected code. Just adjust it in the headers in memory.

That's all folks. Let me leave the hardest part (writing the code) up to you this time. 

Hope this post was interesting and see you at the next.