Anytime that a professional pen tester, or red teamer, uses a tool on an assessment, your customer is trusting you to not introduce additional vulnerabilities into their network. When you create a backdoor through Veil-Evasion, or any other payload generator, you need to be sure that the payload you are creating connects back to you and only you. So how can you ensure that the payloads generated by Veil-Evasion only connects to where it is explicitly specified to connect to? One way would be to perform a complete source code review of Veil-Evasion and perform an environmental analysis of the generated payload. This method would provide the highest level of assurance.
There is also another way. There are a number of scripts that can be used to extract the Python source code from a PyInstaller executable. One such script is called PyInstaller Extractor and written by extremecoders. This script will extract all files within the PyInstaller executable, including the Python source code. We can then compare the extracted Python source code with the source code file that’s created alongside of the PyInstaller executable when using Veil-Evasion. The two Python source code files will match up and prove that there is not any additional code added inside the “compiled” payload, thus the original source code output can be reviewed and trusted as being the actual source code in the resulting payload. Ironically, as long as the original source code is reviewed and understood to be non-malicious, then you can trust Veil-Evasion payloads. So, let’s get along with proving this!
Note: This article only discusses PyInstaller payloads.
First, download the PyInstaller Extractor script and place it in the same directory as your Python payload. Call the PyInstaller Extractor script and pass the Veil-Evasion payload in as the script’s only option. The script will now extract the files from the PyInstaller executable. When you ls you should see the different files that were contained within the payload.
In this case, since the payload we are looking at was named veilpayload.exe, the extracted file we are looking for is therefore veilpayload. The veilpayload file contains the Python code that is carried out when running the original executable. Now, lets perform a comparison of the source code output from Veil-Evasion, and the extracted source code.
Original Source Code Output from Veil-Evasion
Extracted Source Code from Veil-Evasion Payload
Performing a quick visual check against the two scripts shows that they are virtually identical outside of small formatting differences and the additional null byte at the end of the extracted source code. Performing a diff of the two files also verifies this information.
In retrospect, this probably should have been among the first few published posts since Veil-Evasion’s initial release. However, hindsight is always 20/20, so we’re happy we’re able to provide some level of assurance that your Veil-Evasion payloads aren’t backdoored in any way. Please feel free to follow the steps outlined in this blog post to verify for yourself. If you have any additional questions, feel free to hit us up on Twitter, Github, E-mail, or on Freenode in #veil!