Debugging child processes - Olly 2.01h

A lot of malware creates new processes and injects the actual malicious code into the memory of that process and then runs it from there. Meaning..you could start debugging malware.exe...but then find out that it has called CreateProcess() and created a new child process boo.exe. Then it called VirtualAlloc() and allocated memory into some part of boo.exe.

The point being... malware.exe has nothing and you now need to debug boo.exe. There's a few different ways I have tried, all to varying degrees of success.

- Get PID of the Child process from Process Explorer. Then go to Olly and Attach to process.
- Set Olly as JIT debugger. Windows Task Manager. Right click - Debug
- Set Olly as JIT debugger. Set the entry point of the process to CC (a breakpoint) and run the process.
- Try and attach Immunity to the process while debugging the parent in Olly.

But none of that, for multiple reasons worked at all... or worked very sporadically. That sucked.

Luckily OllyDbg 2.01h has this new (well for me ;)) option to debug child processes created by Olly. Meaning, if I'm debugging malware.exe and it creates a new process..and you have this option checked, it'll automatically open a new window and load boo.exe into it :). Perfect.

You can find this option in Olly 2.01 (latest version) in Options - Events. Tick the box that says 'Debug child processes' and save yourself a lot of pain :).

Debugging threads - Olly

Recently I was debugging a piece of malware which launched numerous threads inside, after it ran. Now, after the thread spawned, I could no longer F7 or F8 my way through the malware and understand things. This was because it was the thread which was doing all the work. So somehow I needed to get into the thread.

The first thing I did was 'Right click' and then select a thread from the Threads sub menu. That though just seemed to take me to system space, which was kind of useless. I wanted to see what the Thread did in User Space.

I looked at the CreateThread API then, which was what was being used. The 3rd argument to the function was a start address for the thread. I did a Ctrl+G, went to that address in Olly and put a breakpoint there, and then restarted the program. Went on as normal till CreateThread and then F9'd to run till next breakpoint. The main thread still "hung" but I did break inside UserMode for the Thread and could debug it.. yay :)

If you want to break even before UserMode and want to track it the moment the thread is launched, you can set debugging options in Olly to break each time a new thread is started or stopped. There's simple check boxes under the Options menu. Go search :)

The last bit is when  the Thread itself exits..it just says Thread Terminated and you again cannot F7 or F8 because there is nothing left to F7 or F8 into. You need to get back to the main thread, where the CreateThread API was called. Makes sense ..rt? Main.. created a thread...I debugged thread...now I come back to main...once I finish debugging the thread.

To do so, pause the program(F12) after the thread terminates and hit Alt+F9 to return to user mode. This will bring you right to the spot after CreateThread was first called.


Hope this helps someone newish to reversing :). Have fun!!