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Patch now! Microsoft issues unexpected Office fix

You might not have heard of FBX files... but the latest Office versions support them, so don't neglect this patch!

Microsoft just issued Security Advisory ADV200004, entitled Availability of updates for Microsoft software utilizing the Autodesk FBX library.
At first glance, you might be inclined to read just the headline and skip on by because you don’t use FBX files or you don’t have any Autodesk software products.
We’ll be honest and admit we hadn’t even heard of FBX files until now, let alone created one – the abbreviation is short for Filmbox, and it’s a proprietary format owned by Autodesk that is used to save motion capture data along with audio and video streams.
Autodesk is probably still best-known for its AutoCAD computer aided drawing software, but it has a huge range of products for video rendering, game creation and more, where the FBX file format is right at home.

Well, Autodesk just published its own Security Advisory ADSK-SA-2000-0002, “Vulnerabilities in the Autodesk FBX Software Development Kit“.
This advisory announces fixes for six different security bugs denoted CVE-2020-7080 to CVE-2020-7085 consecutively.
Announced as they are at the same time, these vulnerabilities sound like the sort of multi-bug fix that sometimes emerges after a concerted burst of reviewing and testing existing code to improve it, and if so it sounds as though the review was both extensive and worthwhile.
These vulnerabilities are due to a range of different programming errors that often creep into code that handles complex data objects, namely: buffer overflow, type confusion, use after free, integer overflow and null pointer dereference.
Well, here’s the thing: it seems that the Microsoft Office 2019 and Office 365 ProPlus products from Microsoft include support for FBX files – whether you use FBXes yourself or not – and that the code to process those files comes from Autodesk.
Therefore these versions of Office inherited the six CVE-tagged vulnerabilities from Autodesk, and five out of six of them are listed as allowing RCE, short for remote code execution.
Microsoft also lists the Windows 10 components 3D Viewer and Paint 3D as affected by these bugs, so those products have been updated too.

Click to run

As you probably know, an RCE bug that is present when a vulnerable application processes a booby-trapped file often means that simply opening up or previewing that file could allow crooks to implant malware on your computer.
You typically won’t see any of the usual “do you want to download?” or “this file wants to run a program, are you sure?” warnings, so opening the file will not only feel innocent – as opening up a data file is supposed to be – but also appear innocent, too.
In other words, a crook could email you an FBX file – a file that isn’t a program and isn’t supposed to be a program – that puts you at risk.
A bug requiring you to click on and open up a rogue file isn’t as dangerous as a security hole that can be exploited remotely even when no one’s logged in, because you have to be tempted at least to look at the offending item.
But a “click-to-own” attack like this is much more dangerous than, say, a document file containing macros that have to be authorised as a second step after the document is opened.
And even if you think you’d never open an FBX file because it sounds unimportant or irrelevant, remember that:

  • Crooks rarely send one phishing email at a time. Even if crooks aren’t directly targeting your company, their spam database probably contains multiple email entries for every company domain on the list anyway. The crooks don’t have to trick everyone – they can target anyone and win if they trick someone.
  • Windows doesn’t show file extensions by default. A file called something.text.fbx will typically be displayed as something.text, which has a deceivingly safe look to it.

What to do?

Microsoft’s advisory states that it has “not identified any mitigating factors [or] workarounds for [these vulnerabilities]”.
So you know what we’re going to say, so we’ll say it quickly: Patch early, patch often.
And if you are an Autodesk customer, don’t forget to check for updates to affected Autodesk products.
The Autodesk list includes various verisons of: the FBX Software Development Kit (which is presumably how these bugs ended up in Office), Maya, Motion Builder, Mudbox, 3ds Max, Fusion, Revit, Flame, Infraworks, Navisworks and Autodesk AutoCAD.

One more thing

While you’re about it – because you can! – we recommend telling Windows not to suppress file extensions.
You might not yet know that files ending .JS (JavaScript) are actually programs rather than data files, and are generally very risky to open up directly on your computer.
But there’s a irony that once you do know what .JS files are, Windows doesn’t make it easy for you to use that knowledge to protect yourself.
Type file explorer in the search bar and launch the Windows File Explorer app; go to the View menu and check the box labelled File Name Extensions.
If you’re a syadmin, you can make this change for all your users via Windows Group Policy.

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It does make you wonder if MS got .. a little .. overzealous with support for unusual file types in Office.
I mean, I can’t think of the last time I was writing a budget increase letter, and thought to myself, you know what would really punch this up? Some motion capture !
And that there are no mitigating actions means, we can’t even simply tell Office to stop TRYING to open these.


“you can make this change for all your users via Windows Group Policy.“ – I have never found any consistent documentation about this, especially across Windows 10 builds


This registry entry seems to do the trick:
HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\HideFileExt = DWORD(0)
Annoyingly, this setting goes OFF to turn extensions ON, whereas the menu item in the Explorer application goes ON to turn extensions ON.


Why wouldn’t an alternative workaround to associate *.fbx files with Notepad (or anything else but office) mitigate the issue?


Good question – I’m not sure. Perhaps Microsoft’s definition of “mitigation” here is a bit more specific than just a rest-of-OS tweak, and refers only to changes that you can make to Office itself so that _even if it does open an FBX file_, the bug nevertheless won’t trigger. I guess that might make your proposal a workaround of sorts…
…but AFAIK, there are some file types that Office components will sometimes open by type (recognising then by some embedded CLSID or other magic number) rather than by name, so that the extension isn’t always important and can’t always save you.


“a range of different programming errors, namely: buffer overflow, type confusion, use after free, integer overflow and null pointer dereference.”
Oh, that’s it? Only five major issues?…some that have been known about for decades?


Not sure what you mean – that these types of error are not new in programming, or that these specific bugs are not new and have been in the code for ages? I assume these bugs are newly found because they have CVE numbers allocated this year.


Paul, does this affect Office for Mac, too?


I don’t know – the Microsoft advisory just says “Office 2019” and “Office 365 ProPlus” without specifying an OS. I assume that if the Mac version were affected (Autodesk’s FBX software development kit is available for Mac) then it would have been updated, so if you are up to date you should be OK.


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