Question

Recently I have been reading up articles about DLL injection and I understand them fairly well.

However, what I don’t understand is why APIs such as CreateRemoteThread, WriteProcessMemory(in being able to write to the memory of another process) and VirtualAllocEx(in being able to allocat memory in the context of another process) were implemented in the first place.

What was the original need for such APIs? Just curious.

Answer 1

WriteProcessMemory was made for ring3 debuggers that need to securely write process memory, most commonly for INT 3 breakpoints or user provided memory edits.

along the same line, CreateRemoteThread can also be used for debugging purposes, however, MSDN can enlighten us on CreateRemoteThread a bit more:

A common use of this function is to inject a thread into a process
that is being debugged to issue a break. However, this use is not
recommended, because the extra thread is confusing to the person
debugging the application and there are several side effects to using
this technique:

• It converts single-threaded applications into
  multithreaded applications.
• It changes the timing and memory layout of
  the process.
• It results in a call to the entry point of each DLL in
  the process.

IIRC, CreateRemoteThread is also used by debuggers to hook application native expection handlers, commonly set by SetExceptionHandler, which requires call from the target process as the handler is stored in the PEB.

VirtualAllocEx is just how windows virtual memory system operates, it needs a context to allocate in, be it in the current process, a child process or a remote process. VirtualAlloc in fact is nothing more than a pass through wrapper of the Ex variant, it just passes a special constant that indicates the handle of the caller process is to be used.