DLL Injector

include /masm32/include/masm32rt.inc

.data
    kernel32    db "kernel32.dll"           ,0
    loadlib     db "LoadLibraryA"           ,0
    library     db "C:\msg.dll"             ,0
    windowname  db "Untitled - Notepad"     ,0
    injected    db "Successfully injected!" ,0

.data?
    procid      dd ?
    prochandle  dd ?
    procaddress dd ?
    handle      dd ?
    address     dd ?
    libsize     dd ?
    thread      dd ?

.code

start:
    push offset kernel32
    call GetModuleHandle

    mov handle,eax
    push offset library
    call lstrlen

    add eax,1
    mov libsize,eax
    push offset loadlib
    push handle
    call GetProcAddress

    mov procaddress,eax

    push offset windowname
    push NULL
    call FindWindow

    push offset procid
    push eax
    call GetWindowThreadProcessId

    push procid
    push 0
    push PROCESS_ALL_ACCESS
    call OpenProcess

    .if eax <1
        ; error on opening process
    .endif

    mov prochandle,eax

    push PAGE_READWRITE
    push MEM_COMMIT
    push libsize
    push NULL
    push prochandle
    call VirtualAllocEx

    mov address,eax

    push NULL
    push libsize
    push offset library
    push address
    push prochandle
    call WriteProcessMemory

    .if eax <1
        ; error on wpm
    .endif

    push NULL
    push 0
    push address
    push procaddress
    push 0
    push NULL
    push prochandle
    call CreateRemoteThread

    mov thread,eax

    .if thread != 0
        ;thread successfully created.
    .elseif
        jmp close
    .endif

    push INFINITE
    push offset thread
    call WaitForSingleObject

    push offset thread
    call CloseHandle

    push MEM_RELEASE
    push libsize
    push offset address
    push prochandle
    call VirtualFreeEx

    close:
        push 0
        call ExitProcess
end start