Injection

Overview This code was originally published in the first issue of tmp.0ut zine - an ELF Research Group founded by me and a super talented group of friends in early 2021. This project was finished literally minutes before the deadline we set. Living on the edge! In general, it took me around a couple of months to complete it, most of the time was dedicated to its core infection routine since the auxiliary sections are common file I/O operations that I’m already familiar with.

Overview My interest in Assembly language started when I was a kid, mainly because of computer viruses of the DOS era. I’ve spent countless hours contemplating my first humble collection of source codes and samples (you can find it at https://github.com/guitmz/virii) and to me, it’s cool how flexible and creative one can get with Assembly, even if its learning curve is steep. I’m an independant malware researcher and wrote this virus to learn and have fun, expanding my knowledge on the several ELF attack/defense techniques and Assembly in general.

"Even for Elves, they were stealthy little twerps. They'd taken our measure before we'd even seen them." — Marshall Volnikov Last month I wrote a post about the memfd_create syscall and left some ideas in the end. Today I’m here to show an example of such ideas implemented in an ELF runtime crypter (kinda lame, I know, but good for this demonstration). What is it? Glad you asked.

Something that always fascinated me was running code directly from memory. From Process Hollowing (aka RunPE) to PTRACE injection. I had some success playing around with it in C in the past, without using any of the previous mentioned methods, but unfortunately the code is lost somewhere in the forums of VXHeavens (sadly no longer online) but the code was buggy and worked only with Linux 32bit systems (I wish I knew about shm_open back then, which is sort of an alternative for the syscall we are using in this post, mainly targeting older systems where memfd_create is not available).

A code cave is a piece of code that is written to a process's memory by another program. The code can be executed by creating a remote thread within the target process. The Code cave of a code is often a reference to a section of the code’s script functions that have capacity for the injection of custom instructions. For example, if a script’s memory allows for 5 bytes and only 3 bytes are used, then the remaining 2 bytes can be used to add external code to the script.