# Process Explorer – bloody hell of indefinite waiting bugs

long time ago I found a bug in Process Explorer pointing out that it uses wrong algorithm of retrieving Thread Start Address. all threads created with CreateRemoteThread has Thread Start Address pointing to KERNEL32.DLL. it’s true, but help- and sense-less. I sent my bug report to Mark, but got no answer and posted it on my old blog: “bug in Process Explorer (a gift for malware)

it means that Process Explorer is not reliable enough, we can’t trust it anymore and yesterday I found more bugs. they’re very common and almost _every_ application has numerous bugs like this. are you intrigued? well, lets begin!!!

download to_attach_ldr.exe (anti-attach trick, based on damaged PEB=>PPEB_LDR_DATA – read this post for more info), run it and… ask Process Explorer to display properties of “to_attach_ldr.ex” process (yeah! “ex”, not “exe” – another bug?!). now, go to “Threads” tab and…

ops!!! Process Explorer freezes falling into infinitive loop with 100% CPU load. ~60% is to_attach_ldr.exe and ~40% is CSRSS.EXE. ok, close to_attach_ldr.exe and Process Explorer immediately wakes up. this means it just was waiting the even that was not going to happen.

ok, another example. download to_attach_33.exe (anti-attach based on intercepted NtRequestWaitReplyPort, described here ). run it and ask Process Explorer to show properties, go to “Threads” tab and… you know what will happens in the next moment. freezing!!!

freezing Process Explorer

freezing Process Explorer

we just found out how buggy Process Explorer is. but why? what’s the issue? the answer: NT has numerous functions like WaitForSingleObject, WaitForDebugEvent, etc. they all take dwMilliseconds argument specifies time to wait for an event. if the parameter is INFINITE, the function does not return until an event has occurred. the problem is – almost every programmer uses INFINITE and does not handles time out error. admit, you did this too?

Ilfak does for sure and I pointed out it before (“another EnableTracing() bug“).

ok, we got two points. first – never use INFINITE unless you’re 100% sure it works. the second point: if malware creates a malicious thread inside a trusted application – anti-attach tricks help it to survive. many anti-viruses are unable to enumerate threads in this case and some of them just freezes. very effective DoS attack against pro-active protections!!!

note: I tested Process Explorer 11.4 under W2K SP4, not tested other versions yet, but guess the bug is there.



  1. Huh, do You ever rest? It’s the Nth interesting post in hmm.. 3 days? ;>
    Very interesting stuff btw ;> Great work ;>

  2. afterlife is for resting :) in real life I would rather work hard. more posts will follow. I’m working on new post now – another anti-attach trick based on BaseThreadStartThunk() => PAGE_NOACCESS and just found a bug in FAR manager :)

  3. Well, tested it both on xp 32bit and xp 64bit, both exes work/show fine under Process Explorer 11.33. (Where did you get 11.4? No such version on the official site ;).

  4. I was happy with the old version of Process Explorer, but you intrigued me and I downloaded 11.33 to test it under XP SP3 32-bit. well, I should disagree with you. to_attach_33.zip shows empty threads tab and causes 100% CPU load.
    to_attach_ldr.zip shows no threads and causes 100% load as well. so, even the newest version of Process Explorer is buggy.

    if you got another result – plz tell me: which version of XP do you use? which version of ms debugging tools is installed? thanks a lot for your feedback!!!

  5. Regarding “to_attach_ldr.ex”, check the ToolhelpSnapshot process list, and you’ll see that’s how it looks. It’s probably a length limitation.

  6. Interesting: I double checked, and seems I had old version of PE: 9.x, and it worked fine. I downloaded 11.4 and it bugs as you described. OS was XP SP3 US with all updates.

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