Back to the Backdoor Factory

backdoorfactory setting up the man-in-the-middle with bettercap and injecting a binary inside of a tar.gz as it’s being downloaded by wget (courtesy of sblip)

Backdoor Factory documentation

Backdoor Factory source code

About six years ago, during a conversation with a red teamer friend of mine, I received “The Look”. You know the look I’m talking about. It’s the one that keeps you reading every PoC and threat feed and hacker blog trying to avoid. That look that says “What rock have you been under, buddy? Literally everyone already knows about this.

In this case, my transgression was my ignorance of The Backdoor Factory.

The Backdoor Factory was released by Josh Pitts in 2013 and took the red teaming world by storm. It let you set up a network man-in-the-middle attack using ettercap and then intercept any files downloaded over the web and inject platform-appropriate shellcode into them automatically.

Man-in-the-Middle Attack Using ARP Spoofing

In the days before binary signing was widely enforced and wifi security was even worse than it is now, this was BALLER. People were using this right and left to intercept installer downloads, pop boxes, and get on corpnet (via wifi) or escalate (via ARP). It was like a rap video, or that scene in Goodfellas before the shit hits the fan.

But nothing lasts forever. Operating systems made some subtle changes and entropy took over, and so the age of The Backdoor Factory came to an end. Some time later, the thing actually stopped working and red teamers sadly packed up their shit and lumbered off to the fields of Jenkins.

Fear not, gentle reader, for our tale does not end here.

For some reason, a year and change back, I found myself once again needing something very much like The Backdoor Factory and stumbled on this “end of support” announcement. Perhaps still motivated by my shameful ignorance years ago, I thought “maybe I owe this thing something for all the good times” and took a look into the code to see if something could be fixed easily.

No, no it couldn’t. Not at all. But the general design and the vast majority of the logic was all in there. It worked alongside ettercap to do ARP spoofing, then intercepted file downloads, determined what format they were, selected an appropriate shellcode if one was available, and then had a bunch of different configurable methods to inject shellcode into all binary formats.

…It’s just that it was heaps and heaps of prototype-grade Python and byte-banged files. I have heard a rumor, similar to On The Road, that the original version had been written in a single night. It clearly was going to take longer than that to port this to something maintainable, but… I mean… automatic backdooring of downloaded files! This needed to happen. This needed to be a capability that red teamers just had available in the future. Fuck entropy.

Around this time, I pitched the idea of an end-to-end rewrite to some others and we started a little group of enthusiasts.

For each of the abstract areas of functionality from the original, we made a separate Go library. The shellcode repository functions went into shellcode. The logic that handles how to inject shellcode into different binary formats went into binjection. To replace the binary parsing and writing logic, we forked the standard Golang debug library, which already parsed all binary formats, and we simply added the ability to write modified files back out.

This gives us a powerful tool to write binary analysis and modification programs in Go. All of these components work together to re-implement the original functionality of BDF, but since they’ve been broken into separate libraries, they can be re-used in other programs easily.

Finally, to replace the ailing ettercap, we used bettercap, the new Golang replacement, which supports both ARP spoofing and DNS poisoning attacks. bettercap allows for extension “caplet” scripts using an embedded Javascript interpreter, so we wrote the Binject caplet that intercepts file downloads and sends them to our local backdoorfactory service for injection via a named pipe and then passes the injected files along to the original downloader.

The flow of a file through the components of the Backdoor Factory, on its journey to infection

Injection methods have been updated to work on current OS versions for Mach-O, PE, and ELF formats, and will be much easier to maintain in the future, since they’re now written to an abstract “binary manipulation” API.

To put a little extra flair on it, we’ve added the ability to intercept archives being downloaded, decompress them on the fly, inject shellcode into any binaries inside, recompress them, and send them on. Just cuz. In the future, we’re planning on adding some extra logic to bypass signature checks on certain types of files and some other special handlers for things like RPMs.

Now you will have to provide your own shellcode, backdoorfactory only ships with some test code, but if you’re targeting Windows, I’ve also ported the Donut loader to Golang, so you can use go-donut to convert any existing Windows binary (EXE/DLL/.NET/native) to an injectable, encrypted shellcode. It even has remote stager capabilities.

We fully intend to get into a lot more detail about how to use Donut and BDF in future posts, but don’t wait for us to get it together for some vaporware future blog post that may never come… You can try it yourself right now!

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Author: awgh

ain't nothin like it once was

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