REW-sploit Emulate and Dissect MSF and other Attacks
REW-sploit Emulate and Dissect MSF and other Attacks

REW-sploit: Emulate and Dissect MSF and *other* Attacks

The tool has been presented at Black-Hat Arsenal USA 2021. See here.

Slides of presentation are available at

Need help in analyzing Windows shellcode or attack coming from Metasploit Framework or Cobalt Strike (or may be also other malicious or obfuscated code)? Do you need to automate tasks with simple scripting? Do you want help to decrypt MSF generated traffic by extracting keys from payloads?

REW-sploit is here to help Blue Teams!

Here a quick demo:


Installation is very easy. I strongly suggest to create a specific Python Env for it:

# python -m venv <your-env-path>/rew-sploit
# source <your-env-path>/bin/activate
# git clone
# cd REW-sploit
# pip install -r requirements.txt
# ./ -f
# ./rew-sploit

If you prefer, you can use the Dockerfile. To create the image:

docker build -t rew-sploit/rew-sploit .

and then start it (sharing the /tmp/ folder):

docker run --rm -it --name rew-sploit -v /tmp:/tmp rew-sploit/rew-sploit

You see an script in the installation sequence. This is required to apply a small patch to the speakeasy-emulator ( to make it compatible with REW-sploit. You can easily revert the patch with ./ -r if required.

Optionally, you can also install Cobalt-Strike Parser:

# cd REW-sploit/extras
# git clone

Standing on the shoulder of giants

REW-sploit is based on a couple of great frameworks, Unicorn and speakeasy-emulator (but also other libraries). Thanks to everyone and thanks to the OSS movement!

How it works

In general we can say that whilst Red Teams have a lot of tools helping them in “automating” attacks, Blue Teams are a bit “tool-less”. So, what I thought is to build something to help Blue Team Analysis.

REW-sploit can get a shellcode/DLL/EXE, emulate the execution, and give you a set of information to help you in understanding what is going on. Example of extracted information are:

You can find several examples on the current capabilities here below:

Donut support

You know for sure the Donut package, able to create PIC from EXE, DLL, VBScript and JScript.

Donut, in order to evade detection, uses a API exports enumeration based on hashes computed on every API name, as many PIC do. This is very CPU intensive (especially in an emulated environment like REW-sploit). So, I implemented a sort of shortcut (changed from 0.3.3 release) to unhook some of the slowest parts of emulation when a Donut stub is detected.

Also, in order to be able to correctly complete the emulation, you need to give to Speakeasy the DLL to get the complete exports. To do it copy the following DLLs


in the Speakeasy folder winenv/decoys/amd64 and/or winenv/decoys/x86 (see Speakeasy README for details). If you don’t need them, don’t leave the DLLs there, in other case they can slow down emulation.


This Shikata Ga Nai implementation works just fine most of the times. In some cases it fails with an invalid read, so I implemented Fixup #4 for it.

A couple of words about performance

Obviously emulation slows down everything. Moreover, hooking every instruction in order to interact with the execution, make things even slower. In general this works fine with small shellcode, but have some issues with complex code. That’s why I added an option to turn off hooking to speed up execution:

emulate_payload -P <path_to_filename> -U 0

In this way you can get a picture of what the emulated code is doing (with API tracking), but nothing else will be done (no fixups, no key extractions, etc). If you specify something different than 0 the hooking will be re-enabled when the IP (instruction pointer) will reach the specified address (fixups will be applied from the same address).


In some cases emulation was simply breaking, for different reasons. In some cases obfuscation was using some techniques that was confusing the emulation engine. So I implemented some ad-hoc fixups (you can enable them by using -F option of the emulate_payload command). Fixups are implemented in modules/ Currently we have

Unicorn issue #1092:

    # Fixup #1
    # Unicorn issue #1092 (XOR instruction executed twice)
    #               #820 (Incorrect memory view after running self-modifying code)
    # Issue: self modfying code in the same Translated Block (16 bytes?)
    # Yes, I know...this is a huge kludge... :-/

FPU emulation issue:

    # Fixup #2
    # The "fpu" related instructions (FPU/FNSTENV), used to recover EIP, sometimes
    # returns the wrong addresses.
    # In this case, I need to track the first FPU instruction and then place
    # its address in STACK when FNSTENV is called

Trap Flag evasion:

    # Fixup #3
    # Trap Flag evasion technique
    # The call of the RDTSC with the trap flag enabled, cause an unhandled
    # interrupt. Example code:
    #        pushf
    #        or dword [esp], 0x100
    #        popf
    #        rdtsc
    # Any call to RDTSC with Trap Flag set will be intercepted and TF will
    # be cleared

Too few values on stack:

    # Fixup #4
    # Stack too small (not enough values stored)
    # Some obfuscator/evasion technique try to access some values on the stack
    # (like for example SGN
    #     cmovne ax, word ptr [esp + 0xfa]
    # In this case the emulation fails with an "invalid_read" since ESP is too
    # close to the top of the stack. This creates some 'fake' values.

Customize YARA rules

File modules/ contains the YARA rules used to intercept the interesting part of the code, in order to implement instrumentation. I tried to comment as much as possible these sections in order to let you create your own rule (please share them with a pull request if you think they can help others). For example:

# Payload Name: [MSF] windows/meterpreter/reverse_tcp_rc4
# Search for  : mov esi,dword ptr [esi]
#               xor esi,0x<const>
# Used for    : this xor instruction contains the constant used to
#               encrypt the lenght of the payload that will be sent as 2nd
#               stage
# Architecture: x32
yara_reverse_tcp_rc4_xor_32 = 'rule reverse_tcp_rc4_xor {                \
                               strings:                                  \
                                   $opcodes_1 = { 8b 36                  \
                                                  81 f6 ?? ?? ?? ?? }    \
                               condition:                                \
                                   $opcodes_1 }'


Please, open Issues if you find something that not work or that can be improved. Thanks!