# NtRequestWaitReplyPort abuses IDA-Pro

good news first. my simple anti-anti-attaching plug-in is coming soon and it works very well. meanwhile, I’m experimenting with different anti-attaching technologies and wish to share a few new (old?) tricks with you.

well, the method based on NTDLL!DbgBreakPoint (see “try to attach to me: if you can!” ) is not good enough to hurt IDA-Pro. like Ilfak said – just set “Stop on debugging start” checkbox in the debugger options to stop IDA-Pro _before_ NTDLL!DbgBreakPoint. how it’s going to help?! nothing! but we will try. press F7 several times. go to Debug\Open subviews\Open threads. do you see two threads there? one is the main thread of the app, another – the system tread, created by DebugActiveProcess() API. the point is – we don’t need the system thread anymore. we should kill it. why? the debugged application might inject bad code into it. but IDA-Pro does not allow us to kill treats.

OllyDbg does. ok, run Olly, go to Options\Debugging options\Events\Break on new thread [x]. Attach to the to_attach_31.exe. ok, OllyDbg has been stopped. now, View\Threads. do you see two threads there? current thread is the system thread. kill it! (context menu\kill thread). um, the thread does not want to die. don’t worry it’s almost dead. now, click another thread (the main), context menu, actualize it and start tracing the main thread step-by-step or just press F9 to run. the system thread is disappeared. there was injected code displaying “shit happens”, but since the thread has been killed – no shit!!! everything is just fine!!!

this is universal technology. I tested it on large malware/protectors collection and it works well! at least for Olly. for IDA-Pro we need to write a script or plug-in, killing unwanted threads.

ok, forget about NTDLL!DbgBreakPoint. back to IDA-Pro. “Stop on debugging start” is set, IDA-Pro attaches to the process (any process you want) and stops. where it stops? let me see…

NTDLL!77F88B6C ZwRequestWaitReplyPort proc near
NTDLL!77F88B6C mov eax, 0B0h ; NtRequestWaitReplyPort
NTDLL!77F88B71 lea edx, [esp+arg_0]
NTDLL!77F88B75 int 2Eh
NTDLL!77F88B77 retn 0Ch ; << here NTDLL!77F88B77 ZwRequestWaitReplyPort endp

so, IDA-Pro stops at NTDLL!77F88B77, when NtRequestWaitReplyPort NTCALL has been executed, so NtRequestWaitReplyPort (called by CsrClientCallServer) is executed _before_ stop. thus, if we intercept NtRequestWaitReplyPort – it will be easy for us to abuse IDA-Pro or do something unexpected. and IDA-Pro has nothing to do this it.

the problem is: NtRequestWaitReplyPort is very popular function and it’s used not only by debugger. so, we can’t just intercept it. we have to check the caller – the thread ID.

for example:

mov eax, fs:[18h] ; // *TIB
mov eax, [eax+24h] ; // CurrentThreadId
sub eax, [our_tid] ; // ?another Thread
jz to_old ; // => no dbg

; // perform stack overflow
die: push eax
jmp die

; // all ok, passing control to the old func
to_old: jmp ds:[old_NtRequestWaitReplyPort]

I wrote a simple POC, abusing IDA-Pro and OllyDbg. download it, run exe. do you see message – “attach to me”? well, ask Olly to attach. Olly attaches without any problems, but… the string changes to “debugger is detected” and the process is still running. wow!!! of course, we can stop the process and continue tracing, but… the point is – the debugger has been detected. how? I just injected my code into NtRequestWaitReplyPort to set global flag if we’re under debugger. the main thread checks this flag and changes its behavior if we’re under debugger. of course, in this simple case we can fix it after attaching, but imagine what happens if the injected code will wipe out all code of the app or destroy the critical structures or just free a few memory blocks cause random crashes?

OllyDbg stops too late. our code injected into NtRequestWaitReplyPort executes before, and debugger has no control under it. what’s about IDA-Pro? try to attach to to_attach_33.exe (all check box in debug options are set). what do we see? the protection tell us “debugger is detected”, the process is running, but IDA-Pro… freezes. what she is waiting for? and how to save our database? we don’t want to kill IDA-Pro, don’t we? right! kill to_attach_33.exe with Process Explorer. IDA-Pro will return from dead to alive.

IDA-Pro 5.3 fails to attach

IDA-Pro 5.3 fails to attach

well, who will break this simple crack me? who will find the way how to attach to the process do not disturbing the protection?



  1. This technique doesn’t work in Vista. IDA attaches without trouble.

  2. It doesn’t work on my XP SP2, either.

    In case you’re interested, on Windows 2000 it’s not IDA but CreateToolhelp32Snapshot that is hanging. CreateToolhelp32Snapshot creates a thread inside the process, in order to gather the information.
    Can also hook any of NTDLL.RtlQueryProcessDebugInformation, NTDLL.NtUnmapViewOfSection, NTDLL.LdrShutdownThread, NTDLL.NtTerminateThread for the same effect.

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