Vulnerability Note VU#421280
Microsoft Office Equation Editor stack buffer overflow
Microsoft Equation Editor contains a stack buffer overflow, which can allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable system.
Microsoft Equation Editor is a component that comes with Microsoft Office. It is an out-of-process COM server that is hosted by eqnedt32.exe. The Microsoft Equation Editor contains a stack buffer overflow vulnerability.
Memory corruption vulnerabilities in modern software are often mitigated by exploit protections, such as DEP and ASLR. More modern memory corruption protections include features like CFG. Even in a modern, fully-patched Microsoft Office 2016 system, the Microsoft Equation Editor lacks any exploit protections, however. This lack of exploit protections allows an attacker to achieve code execution more easily than if protections were in place. For example, because eqnedt32.exe was linked without the /DYNAMICBASE flag, it will not be loaded at a randomized location by default.
Because Equation Editor is an out-of-process COM server, this also means that protections specific to any Microsoft Office application may not have an effect on this vulnerability. For example, if the exploit document is an RTF document, the document will open in Microsoft Word. However, the COM server eqnedt32.exe is invoked by the Windows DCOM Server Process Launcher service, as opposed to Word itself. For this reason, EMET or Windows Defender Exploit Guard protections specific to the Microsoft Office programs themselves will not protect users. For this same reason, none of the Windows Defender Exploit Guard Attack Surface Reduction (ASR) protections will help either.
Windows 7 users who have EMET configured for ASLR to be "always on" at a system-wide level are protected against known exploitation techniques for this vulnerability. Starting with Windows 8.0, system-wide ASLR receives entropy for non-DYNAMICBASE code only if bottom-up ASLR is enabled on a system-wide level as well. Neither EMET nor Windows Defender Exploit Guard configures system-wide bottom-up ASLR though. Because of this, Windows 8.0 through Windows 10 systems must enable specific protections for this vulnerability.
By convincing a user to open a specially-crafted Office document, a remote, unauthenticated attacker may be able to execute arbitrary code with the privileges of the logged-on user.
Disable Microsoft Equation Editor in Office
The vulnerable Equation Editor component can be disabled in Microsoft Office by importing the following registry values:
Windows Registry Editor Version 5.00
Add EMET or Windows Defender Exploit Guard protections to eqnedt32.exe
Exploitation of the vulnerable Equation Editor can be prevented by applying exploit mitigations to the eqnedt32.exe executable. In particular, enabling ASLR for should be sufficient to block the code re-use attack that is outlined in the Embedi documentation.
Enable system-wide ASLR in Windows
Windows with properly-enabled system-wide ASLR (see VU#817544 for more details affecting Windows 8 and newer systems) will block known exploits for this vulnerability.
Vendor Information (Learn More)
If you are a vendor and your product is affected, let
|Vendor||Status||Date Notified||Date Updated|
|Microsoft Corporation||Affected||-||15 Nov 2017|
This issue was reported by Microsoft, who in turn credit Denis Selianin of Embedi with discovery.
This document was written by Will Dormann.
14 Nov 2017
Date First Published:
15 Nov 2017
Date Last Updated:
20 Nov 2017
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