Search This Blog

Saturday, July 02, 2005

The Art of the Inside Job

Before we get into the really fun stuff (getting injected code running on a foreign process, which is really my specialty), we need to go over one more topic: passing data to your code in another process. At first thought you might wonder how that differs from the data injection we just went over, but really it's something else entirely.

Remember that whatever you stick in memory of another process has to be found by the process, otherwise there's no point in sticking it there to begin with. In some methods of executing code in foreign processes it's possible to pass parameters to your code, but not in others. In the latter case, you need another way of passing data across processes. There are a number of methods of accomplishing this, but we'll only discuss one, as it's so well suited to our purposes: memory mapped files.

That's right, it's what we just saw in the Windows 9x injection method; however, this time we're using it a bit differently; that is, we're using it like it's supposed to be used. In addition to being identified by a handle that's local to your process, memory mapped files can be identified by a name string. When named, most other processes in the system may open it by name. Note that, in this case, you must open the file mapping in all process that want to access it (the way shared file mappings are supposed to be done). As this is how they were made to work, this method works exactly the same on both Windows NT and Windows 9x.

So, what do we call the memory mapped file? Well, you can call it anything you like, provided it meets a couple of restrictions. First, it must be globally unique; things are gonna get really painful if two copies of your program try to use the same file mapping at the same time. Second, the foreign process must be able to determine the name without any outside help. That is, you can't simply choose a random string and tell the process what it is (if you could do that, we wouldn't need to use file mappings at all...). I'd recommend that you use a string that contains both the name of your program and the process ID of the process you're sticking code into in the string; this is a simple way of meeting both requirements.

td width="100%">const char *pszSectionNameFormat = "%s_xyz_%08X";

// Creates a memory mapped file of the specified size, giving it a name that can be found by the target process. Note that 'section' is another name for memory mapped file.
HANDLE CreateProcessSection(DWORD nSize, const char *lpszProgramName, DWORD nProcessID)

// We're using a fixed-size buffer, here. Care should be taken to not use an extremely large string for the program name that would overflow this buffer.
char szSectionName[128];

// Construct the name
sprintf(szSectionName, pszSectionNameFormat, lpszProgramName, nProcessID);

// Create the file mapping
return CreateFileMapping(NULL, NULL, PAGE_READWRITE, 0, nSize, szSectionName);

// Opens the file mapping which is targetted to this process
HANDLE OpenProcessSection(const char *lpszProgramName)

// Construct the name, same as before
char szSectionName[128];

sprintf(szSectionName, pszSectionNameFormat, lpszProgramName, GetCurrentProcessId());

return OpenFileMapping(FILE_MAP_WRITE, FALSE, szSectionName);

No comments: