After finishing up with my certification training/exams, I finally had free time to dive more in depth with an interest of mine, malware development. I wanted to get a better understanding of how to be able to get a meterpreter against a system that had an active AV/AM solution running. One source of inspiration was from IRed.team’s article about remote process shellcode injection.
I am not great enough at C++ to hit this goal just yet, so I wrote this out in C#. Source can be found here, and I’ll explain the steps needed here.
We can now focus on our main function. I commented on what each portion does to give everyone an idea of the flow, as well so I can remember what it does in the future. Running this just like this does work, but if you do so with just Microsoft Defender running, the shellcode will be detected and stop the execution.
We all know if there’s a will, there is a way. So let’s find a way to get past the defense and get our shellcode executed. One method of evasion is to obfuscate certain values and build them later, whether this is done by encoding, encryption, or web requests, the goal is the same. Hide the initial and deal with it later. I wanted to start simple and work my way up and find as many methods as I could, and to my surprise the first run was the golden ticket. While it isn’t hard to make your malware undetected if it’s home built, I found it intriguing that something as simple as Base64 encoding a value would throw off not only static scans, but as well as real time scans.
To get the base64 value, all we need to do is set a breakpoint on the WritePorcessMemory function. (Do this on a VM without AV/AM running).
Now that we have this value copy it, stop the current execution, edit a few lines. We can start off by removing our shellcode portion completely, un-commenting the b64DecodeMe line and the one below it, and lastly remove/comment out getB64FromByteArray.