Sure. On Windows, you can allocate memory into an arbitrary process’s address space with VirtualAllocEx, write arbitrary code into those allocated pages using WriteProcessMemory, and start a thread there with CreateRemoteThread.
Now, of course you can’t do this for every process, or it would be trivial to escalate privileges. But I’m pretty sure you can do this for child processes running as the same user.
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Sure. On Windows, you can allocate memory into an arbitrary process’s address space with VirtualAllocEx, write arbitrary code into those allocated pages using WriteProcessMemory, and start a thread there with CreateRemoteThread.
Now, of course you can’t do this for every process, or it would be trivial to escalate privileges. But I’m pretty sure you can do this for child processes running as the same user.