In this article, I am going to list half a dozen DLL injection techniques that can be used by a user mode process running on MS Windows. There could be more techniques but I am sharing with you the techniques that I had first hand experience with.
People used to rely on the AppInit_DLLs registry key. The OS loader queries this value and loads the DLLs specified there when a process is created. I have not used this technique in a long while (last time I used it was on Windows XP) and I heard it is now restricted or discontinued because it was widely used by malware.
2. SetWindowsHookEx API
The SetWindowsHookEx API installs an application-defined hook procedure into a given hook chain. There are various supported hook chains (CBT, Journal, Window messages, keyboard, mouse, etc).
When using the SetWindowsHookEx API, you are instructing the operating system to inject your custom hook DLL into other process where it is relevant. The Windows hooks work when the other processes import / use functionality from USER32.dll.