Update

This issue was fixed by Mark Russinovich on 2018-07-06; that was pretty quick!

Old

Sysmon is an easy target, because it’s easily downloadable and everyone can poke around in its code or toy around with the system and see what sysmon logs. It’s obviously not fair – if other EDR code was that easily available I am pretty sure we would see a cascade of ‘funny stuff’ in these products as well.

Anyway…

In my older post I presented a simple technique that may fool parsers and their state machines into ‘thinking’ they are parsing correct records while in fact they are processing data some malicious software meticulously crafted for them. This is not necessarily sysmon’s problem, but who would read that old post if there was no clickbait value in the title, right?

Back to sysmon and poking around… once you start looking at it you can quickly discover that it can be run in a so-called debug mode – all we have to do is provide an undocumented command line switch ‘-t’ when we install it. When I first discovered it I got really excited, only to immediately get a bucket of cold water thrown by the Twitter post by @mattifestation who figured it out in… Jan 2018.

It’s a really cool feature.

When you run ‘sysmon -t -i’ the program will start throwing a lot messages to the console and some of them will eventually trigger your interest. Especially if you ‘help’ them a bit to appear 🙂

So… what we see in this error message is a very crucial information.

The sysmon had to truncate a very long command line which I have provided to a test process. It was so long that it had to be trimmed.

A-ha… but how long?

Well, it turns out sysmon doesn’t like command line longer than 0x2000 characters – i.e. this a number of wide characters it can swallow, before trimming down the rest.

Now this 0x2000 (Wide characters) is actually 16384 bytes only.

I was curious where the 0x2000 came from, because after reading various versions of MSDN pages about CreateProcess I know very well that the lpCommandLine argument can be much longer; as per the MSDN:

The maximum length of this string is 32,768 characters,

including the Unicode terminating null character.

So… this is an interesting discrepancy.

I have a hypothesis (and I am totally guessing it) that the sysmon author used the arbitrary limit imposed on cmd.exe command line arguments.

Such discrepancy is a nice gift and we can of course abuse it.

Since we can’t pass the command line arguments that are longer than 0x2000 characters to cmd.exe let’s try to use powershell instead.

If you run ‘powershell <0x2000 spaces> calc’ you will spawn Windows Calculator.

What will you see in the logs?

This:

And if you export it to TXT or XML you will get this:

So… using long command line arguments provided to executables that can work with such madness (e.g. powershell) can help to evade sysmon logs…

If you want to test it, grab this .exe.