In July 2023, Microsoft released a patch for CVE-2023-33150, a vulnerability in Microsoft Office that allowed an attacker to create a malicious Word document which would not open in Protected View even though it had the Mark-of-the-Web ("MotW") set.
The first public detail about this vulnerability came from security researcher Eduardo B. Prado, noting that adding a non-breaking space character to the end of a Word document's extension prevents Word from opening the document in Protected View.
Subsequently, Will Dormann published his own research. Will noticed that in the process of opening a file with a non-breaking space in the extension, Word at some point - after normalizing the file path - tried to find the Mark-of-the-Web in a file without the non-breaking space, and failed because no such file existed. Using a flawed logic "no file, no Mark-of-the-Web", Word then decided that it was safe to open the document without Protected View.
To illustrate the issue, suppose a downloaded malicious file with Mark-of-the-Web is named PoC.doc<nbsp>, whereby <nbsp> denotes a non-breaking space. (One can create such file manually by editing the name of the file, placing the cursor at the end of the file name, holding down Alt and typing 255 on the numeric keypad.) Opening such file in Word would lead to Word normalizing the file name at some point (which removes the non-breaking space), and trying to read Mark-of-the-Web from PoC.doc. Since this file does not exist, Word would assume there was no Mark-of-the-Web, even though this mark existed on the malicious file. Believing there was no Mark-of-the-Web, Word would open the file without Protected View.
Microsoft's patch fixed this flawed logic: fully updated Word now still tries to open the file without the trailing non-breaking space, still fails (of course, such file is not there), but then defaults to "Mark-of-the-Web is present" and opens the document in Protected View.
The following video demonstrates the vulnerability on fully updated Office 2013, and shows that 0patch removes it. A PoC.doc file on the desktop has the Mark-of-the-Web and also has a non-breaking space appended to the end of the file name. Opening such file doesn't automatically launch Word, because this exact file extension is not associated with any application, but Windows very friendly offer Microsoft Word as the most likely candidate for opening the file. Word then opens this file without Protected View. With 0patch enabled, opening the same file results in the file being opened in Protected View.
Since users of Office 2010 and 2013 didn't receive Microsoft's patch for this issue, we created our own micropatches for these versions that fix CVE-2023-33150. All PRO and Enterprise users had these patches automatically applied without even having to relaunch Word.
And now, the 0day....
While working on the above vulnerability and its patch, our researchers noticed something strange in the patched version of Word. While Microsoft's patch for CVE-2023-33150 changed the flawed logic of "no file, no Mark-of-the-Web" to a more secure "no file, yes Mark-of-the-Web", the underlying assumption in both cases was that there was no file.
In our tests, while frequently moving, copying and renaming files, fully patched Word sometimes behaved strangely, seemingly randomly not opening the file in Protected View when it should have. It turned out there is another flaw in the other half of the above logic: the half where a file without the non-breaking space at the end happens to exist.
What happens in that case? Well, Word tries to read the Mark-of-the-Web from it and uses it for deciding whether to open the file in Protected View or not.
Suppose we have, like before, a malicious file with the Mark-of-the-Web named PoC.doc<nbsp>: a fully patched Word correctly opens it in Protected View. Suppose we then place another file named PoC.doc next to it without the Mark-of-the-Web and open the first file with Word: Word checks PoC.doc for Mark-of-the-Web and, not finding it, opens the malicious file without Protected View.
Is this a security issue? Let's discuss exploitability.
Could the attacker who tricked the user into opening a malicious PoC.doc<nbsp> (a file with Mark-of-the-Web) also plant PoC.doc (without Mark-of-the-Web) next to it to make Word open the former without Protected View? If they could, they might as well just plant PoC.doc and have the user open it for the same effect without having to exploit anything.
Alternatively, could the attacker plant a malicious MeetingMinutes.doc<nbsp> (a file with Mark-of-the-Web) next to a previously existing, legitimate MeetingMinutes.doc (without Mark-of-the-Web) on user's computer? Potentially yes: our best attack scenario is for the user to have downloaded a Word document from an intranet web server, which would end up in the Downloads folder without Mark-of-the-Web. The attacker would then trick the user to open MeetingMinutes.doc<nbsp> from their own web server on the Internet, which would result in Word opening this file from the Downloads folder, but would read the Mark-of-the-Web from the legitimate MeetingMinutes.doc, and decide to open the malicious file without Protected View.
This is arguably a pretty far-fetched scenario, and perhaps someone else will think of a better one. With this in mind we reported the issue to Microsoft and expect it to be fixed soon, but did not wait with publication due to very limited exploitability. We did, however, write a micropatch for all supported Word versions (Word 2016, 2019, 2021 and 365) and made it freely available until Microsoft has provided their official fix. Our CVE-2023-33150 patches for Word 2010 and 2013 also fix this vulnerability on these Office versions.
The following video demonstrates the vulnerability and how our patch removes it.Micropatch Availability
Micropatches were written for the following versions of Microsoft Office with all available updates installed:
- Office 2010 (PRO or Enterprise license required)
- Office 2013 (PRO or Enterprise license required)
- Office 2016 (free until Microsoft provides an official patch)
- Office 2019 (free until Microsoft provides an official patch)
- Office 2021 (free until Microsoft provides an official patch)
- Office 365 (free until Microsoft provides an official patch)
Vulnerabilities like this one get discovered on a regular basis, and
attackers know about them all. If you're using Windows that aren't
receiving official security updates anymore, 0patch will make sure these
vulnerabilities won't be exploited on your computers - and you won't
even have to know or care about these things.
If you're new to 0patch, create a free account in 0patch Central, then install and register 0patch Agent from 0patch.com, and email sales@0patch.com for a trial. Everything else will happen automatically. No computer reboot will be needed.
We would like to thank Eduardo B. Prado and Will Dormann for sharing their knowledge, which made it possible for us to create a
micropatch for this issue.
