The document uses the Word remote template feature to retrieve a HTML file from a remote webserver, which in turn uses the ms-msdt MSProtocol URI scheme to load some code and execute some PowerShell.
While such an action should not be possible, theoretically, the code does it, when decoded.
There’s a lot going on here, but the first problem is Microsoft Word is executing the code via msdt (a support tool) even if macros are disabled.
Protected View does kick in, although if you change the document to RTF form, it runs without even opening the document (via the preview tab in Explorer) let alone Protected View.
It’s a zero day allowing code execution in Office products.
Historically, when there’s easy ways to execute code directly from Office, people use it to do all kind of things.
This breaks the boundary of having macros disabled.
Vendor detection is poor.
The vulnerability appears exploitable using .RTF files on all versions of Office 365.
The vulnerability has been proved in Office 2013, 2016, 2019, 2021, Office ProPlus and Office 365.
It also applies to Windows itself, e.g. it can be called from .lnk files – effectively there are two different issues in my opinion, Office itself using MS Protocol and allowing loading unfiltered from HTML Word templates and Outlook links, and MSDT allowing code execution.
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