Injecting a DLL with a Trojan DLL

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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 (described in Chapter 20), 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.