From what I see it is XML. As such it may accessible with a number of tools.
If you’re developing a product or tool, the current STIX reference implementation is in XML so any XML libraries are suitable for producing and consuming STIX XML. The project also maintains open-source Python bindings and other utilities to make working with STIX at the code level easier. Documentation and suggested practices as well as examples can help you understand how to use the STIX language conceptually (beyond just producing the XML).
https://stixproject.github.io/about/
digging in there...still looking