A Simple Linux ELF Runtime Crypter Using memfd_create Syscall
"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.
Executing ELF binary files from memory with memfd_create syscall
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.