Dilip K Mahendra: Emulating Shellcodes

Sunday, May 28, 2023

Emulating Shellcodes - Chapter 2

Lets check different Cobalt Strike shellcodes and stages in the shellcodes emulator SCEMU.

This stages are fully emulated well and can get the IOC and the behavior of the shellcode.

But lets see another first stage big shellcode with c runtime embedded in a second stage.

In this case is loading tons of API using GetProcAddress at the beginning, then some encode/decode pointer and tls get/set values to store an address. And ends up crashing because is jumping an address that seems more code than address 0x9090f1eb.

Here there are two types of allocations:

Lets spawn a console on -c 3307548 and see if some of this allocations has the next stage.

The "m" command show all the memory maps but the "ma" show only the allocations done by the shellcode.

Dumping memory with "md" we see that there is data, and dissasembling this address with "d" we see the prolog of a function.

So we have second stage unpacked in alloc_e40064

With "mdd" we do a memory dump to disk we found the size in previous screenshot, and we can do some static reversing of stage2 in radare/ghidra/ida

In radare we can verify that the extracted is the next stage:

I usually do correlation between the emulation and ghidra, to understand the algorithms.

If wee look further we can realize that the emulator called a function on the stage2, we can see the change of code base address and is calling the allocated buffer in 0x4f...

And this stage2 perform several API calls let's check it in ghidra.

We can see in the emulator that enters in the IF block, and what are the (*DAT_...)() calls

Before a crash lets continue to the SEH pointer, in this case is the way, and the exception routine checks IsDebuggerPresent() which is not any debugger pressent for sure, so eax = 0;

So lets say yes and continue the emulation.

Both IsDebuggerPresent() and UnHandledExceptionFilter() can be used to detect a debugger, but the emulator return what has to return to not be detected.

Nevertheless the shellcode detects something and terminates the process.

Lets trace the branches to understand the logic:

target/release/scemu -f shellcodes/unsuported_cs.bin -vv | egrep '(\*\*|j|cmp|test)'