The focus of any malware research is on anticipating where an attack may go, or where it’s already been in order to develop and implement new prevention techniques. While reverse engineering some recent Linux malware samples, I found an interesting and novel technique being used that’s important to share with the broader community. A malicious actor had logged into a honeypot and attempted to download a file I hadn’t seen before. Loading the file into IDA Pro I was prompted by a “SHT table size or offset is invalid. Continue?” message – nothing to worry about as this is normal for every stripped executable. However, after continuing through this message I was prompted by a new warning I’d not seen before;
One of the program headers was pointing outside of the actual file. This is easy to fix, simply nulling this section out allowed IDA Pro to load the sample. Interestingly enough, it turned out this was an invalid binary and the section was misaligned only because the file was truncated. However, this error message lead me down the path to try and reconstruct this error – and it was simple to do. The steps were relatively easy to reproduce using the hex editor;
- Strip all sections from the ELF header
- Find a program header which is not required by the ELF file for loading
- Make this program header have the offset for this section pointing outside of the file
As long as the rest of the section headers are not found – IDA Pro will fail to load. After scripting this process, I decided to test a few scenarios with other disassemblers and debuggers. Radare (r2), Hopper and lldb handled the binary perfectly fine – however GDB failed to understand the file format;
Trying to take things a bit further I wanted to see if this would work as not only an anti-disassembly technique, but also an anti-analysis or obfuscation technique. The idea was that if I was so easily able to find this issue with a few disassemblers, it would be likely that some anti-virus applications may have also implemented the same issue in their parsing engines. From here I grabbed a relatively well detected malware sample from the Linux/XorDDos family;
9 different engines (Two appear to be owned by the same company? So I hesitate to say 10) failed to detect the same malware, they just recently detected. This was interesting to me, as I’m relatively new to the Linux side of malware, I would have assumed that these engines would have easily detected the malware and a simple change like this would not be such a simple evasion technique.
It seemed almost too easy to beat the disassemblers and engines – so I wanted to look across a large corpus of samples and see if anyone else has stumbled upon and implemented this technique. Using the rather simple YARA rule below, I was able to find over 6,000 samples which are currently utilizing this exact technique. Luckily, almost every single one of these samples was just a commercial Android packer attempting to protect it’s own code.
While we have yet to see any malicious actors use this technique in the wild, there are likely many other similar tricks being used in the wild. This is a good start at looking to see how ELF files might be abused to hide from analysis, and hopefully with the release of these scripts, people will be able to monitor for this technique being used and other similar ones in the future.
Prior to publishing this article, I’ve notified Hex-Rays and the 10 engines which failed to detect the slightly modified malware. The script for producing and fixing these modified binaries can be found on github here.