Micropatching Brings the Abandoned Equation Editor Back to Life (2018)

Microsoft's removal of Equation Editor due to security flaws left many users stranded. The 0patch team explains how micropatching can safely bring this essential tool back to life.

How We Security-Adopted a Terminated Software Product

by Mitja Kolsek, the 0patch Team

Intro

A few days ago Microsoft's update removed Equation Editor from Microsoft Office, the official reason being

*"security issues with its implementation."*Most Office users couldn't care less about this removal, but if you've been happily using Equation Editor to edit Word documents with mathematical formulas just days ago, you suddenly can't do that anymore. You will still see your formulas in the document but you won't be able to edit them. Instead you'll get this:

We have no idea how many users are affected, but Twitter user @glyph raises an interesting point that those who work with Equation Editor may be tempted to forego this Office update - and by extension all future Office updates -, which will leave them vulnerable to exploits published in the future.

So um. My cousin, a high school math teacher, wrote all his lesson plans using Equation Editor. Help him out, tweeps: is there a migration path here? I promise you ten thousand math teachers will just run unpatched Word forever, macros enabled and all, if not… https://t.co/iuhXeoJhWv— glyph (@glyph) January 12, 2018

Worse even, affected users may decide to migrate back to unsupported versions of Office that don't receive security updates at all. This user, for instance, reports going back to Office 2000 on his Windows 10 computer. Office 2000 stopped receiving security updates in 2009.

Microsoft suggested affected users can

*"edit Equation Editor 3.0 equations without security issues"*with Wiris Suite's MathType, a commercial application that costs $97 ($57 academic). They did not specify the basis upon which the phrase

*"without security issues"*was provided, but MathType seems to have a clean public security record so far. Which doesn't say much as that was also true for Equation Editor until someone opened its hood.

We haven't tested MathType and can't tell how easy it is to start using it instead of Equation Editor with existing Word documents, but we don't particularly like the idea of suddenly deleting from users' computers a tool they might be using, and sending them to a store to buy a replacement.

Microsoft's unwillingness to continue supporting Equation Editor is understandable. Their manual patching of its recently discovered vulnerability reveals that, for whatever reason, their standard patching process cannot be applied to Equation Editor, and a deviation like that can be expensive. Furthermore, while they aren't new to manually patching executables, such patching can sometimes be fairly difficult to do. When you patch executable files directly, you may have to come up with a different clever space-saving hack for each patch, which can sometimes be very difficult and time-consuming. For instance, Microsoft's manual patches of Equation Editor required the patch author to invent a way to get some free space in the code for additional patch logic by de-optimizing a memory-copying routine.

So when Microsoft was faced with 8 (eight!)* new vulnerabilities in Equation Editor reported after their manual patch (one also reported by us), they gave up on the idea of continuing manual support for it.

We, on the other hand, haven't.

You see, it's much easier for us to create and support binary patches for a given executable module than it is for Microsoft. Why? Because we have a micropatch delivery agent (0patch Agent) that not only instantly downloads micropatches, but also injects them into running processes on the computer while automatically making room for the added code. So we don't have to invent a new way of making room for every micropatch we make, and can therefore focus on the patch itself. We also deliver our micropatches to agents every hour, and they are as trivial to revoke and un-apply as they are to apply. As much as we hate to repeat ourselves, this is how we believe security patching should look like in this century.

That said, we've already issued our micropatch for CVE-2018-0802, and it's been applied to all computers running 0patch Agent where the latest version of Equation Editor is still present. We're also teaming up with other security researchers who have found vulnerabilities in Equation Editor to micropatch those issues too. We urge everyone who finds additional security issues in Equation Editor to share their findings with us and help up create micropatches for them.

[Update 2/20/2018: We've just issued a micropatch for another Equation Editor vulnerability, CVE-2018-0798. Big thanks to the 360 Vulcan Team for their help with that!]

Bringing Equation Editor Back To Life

So you've installed Office Updates from January 9th 2018 and Equation Editor got removed from your computer. Specifically, the update deleted five files (including EQNEDT32.EXE) from the EQUATION folder, leaving the 1033 subfolder and EEINT.DLL inside it intact. It also unregistered Equation Editor as a local COM server by deleting CLSID {0002CE02-0000-0000-C000-000000000046} from registry. Note that Office 2016 still has several files in the EQUATION folder after the update, and in some cases, a 0-byte EQNEDT32.EXE file is left on the system.

The location of the EQUATION folder depends on both the Office version and whether it's 32-bit or 64-bit Office. These are the default locations:

32-bit Office 2007, 2010 and 2013 on 32-bit Windows: C:\Program Files\Common Files\microsoft shared\EQUATION
32-bit Office 2007, 2010 and 2013 on 64-bit Windows: C:\Program Files (x86)\Common Files\microsoft shared\EQUATION
64-bit Office 2007, 2010 and 2013: C:\Program Files\Common Files\Microsoft Shared\EQUATION
32-bit Office 2016 and 365 on 32-bit Windows: C:\Program Files\Microsoft Office\root\VFS\ProgramFilesCommonX86\Microsoft Shared\EQUATION
32-bit Office 2016 and 365 on 64-bit Windows: C:\Program Files (x86)\Microsoft Office\root\VFS\ProgramFilesCommonX86\Microsoft Shared\EQUATION
64-bit Office 2016 and 365: C:\Program Files\Microsoft Office\root\VFS\ProgramFilesCommonX64\Microsoft Shared\EQUATION

In order to get Equation Editor back while continuing to receive future Office security updates you need to do two things:

Restore deleted Equation Editor files.
Re-register Equation Editor as a local COM server.
Install free 0patch Agent to keep Equation Editor patched against known vulnerabilities.

Obviously, you don't technically need the last step to get Equation Editor working again, but you don't want to be vulnerable to trivial inexpensive attacks that can be delivered in any Word document you ever receive. So just to be clear, we don't recommend performing steps A and B if you don't also perform step C.

A. Restore deleted Equation Editor files

(Disclaimer: The following is not an official Microsoft-supported procedure and is not guaranteed to work or to not have unwanted side effects. In addition, future Office updates may bring additional blocking of Equation Editor and disable its use. You're doing this at your own risk.)

Unfortunately uninstalling the Office security update that removed Equation Editor doesn't bring the deleted files back. This leaves you with two options: (a) find a copy of Office you haven't updated yet, or (b) reinstall Office from your original media and apply all updates up to and including November 2017 updates. Once you do any of these, you will find these files in the EQUATION folder (possibly along some other files in Office 2016):