Microsoft® Windows® Script Interfaces
Windows Script Hosts
 Language Reference

When implementing Microsoft® Windows® Script host, you can safely assume that a scripting engine will only call the IActiveScriptSite interface in the context of the base thread as long as the host does the following:

These rules are automatically followed by all single-threaded hosts. The restricted model described above is intentionally loose enough to allow a host to abort a stuck script by calling IActiveScript::InterruptScriptThread from another thread (initiated by a CTRL+BREAK handler or the like), or to duplicate a script in a new thread using IActiveScript::Clone.

Also, none of these restrictions apply to a host that chooses to implement a free-threaded IActiveScriptSite interface and a free-threaded object model. Such a host can use the IActiveScript interface from any thread, without restriction.