02 June, 2023

Fast Emulator For Shellcodes In Rust

I have developed a fast emulator for modern shellcodes, that perform huge loops of millions of instructions emulated for resolving API or for other stuff.

The emulator is in Rust and all the few dependencies as well, so the rust safety is good for emulating malware.  

There are shellcodes that can be emulated from the beginning to the end, but when this is not possible the tool has many features that can be used like a console, a memory tracing, register tracing, and so on.

https://github.com/sha0coder/scemu



In less than two seconds we have emulated 7 millions of instructions arriving to the recv. 

At this point we have some  IOC like  the ip:port where it's connecting and other details.

Lets see what happens after the recv() spawning a console at position: 7,012,204


target/release/scemu -f shellcodes/shikata.bin -vv -c 7012204



In the console, pressing "enter" several times to emulate  step into several steps and we arrive to a return instruction.


Let's see the stack in this moment:


The "ret" instruction is going to jump to the buffer read with recv() so is a kind of stager.

The option "-e" or "--endpoint" is not ready for now, but it will allow to proxy the calls to get the next  stage automatically, but for now we have the details to get the stage.


SCEMU also identify all the Linux  syscalls for 32bits shellcodes:



The encoder used in shellgen is also supported https://github.com/MarioVilas/shellgen

Let's check with cobalt-strike:


We can see where is connecting and which headers is using, so right now we can replicate the communications.



In verbose mode we could do several greps to see the calls and correlate with ghidra/ida/radare or  for example grep the branches to study the emulation flow.


target/release/scemu -f shellcodes/rshell_sgn.bin -vv | grep j


target/release/scemu -f shellcodes/rshell_sgn.bin -vv -c 44000 -l


The -l --loops options makes the emulation a bit slower but track the number of iterations.

Is possible to print all the registers in every step with  -r or --registers  but also is possible to track  specific register for example with --reg esi


target/release/scemu -f shellcodes/shikata.bin --reg esi 


In this case ESI register points to the API name, if we track EAX or ECX will see that are the counters of the loop. These shellcodes  contains a hard loop to locate the API names.

The flag -i or --inspect allow to monitor memory using expressions like "dword ptr [eax + 0xa]"

target/release/scemu -f shellcodes/shikata.bin -i 'dword ptr [esi]'

And more things to come...  find a demo below:

https://www.youtube.com/watch?v=qTYmMjW3DFs





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