Vulnerability Note VU#421280
Microsoft Office Equation Editor stack buffer overflow
Original Release date: 15 Nov 2017 | Last revised: 20 Nov 2017

Overview
Microsoft Equation Editor contains a stack buffer overflow, which can allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable system.

Description
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 onquot; 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.

Impact
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.

Solution
Apply an update

This issue is addressed in CVE-2017-11882 | Microsoft Office Memory Corruption Vulnerability.
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

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Common\COM Compatibility\{0002CE02-0000-0000-C000-000000000046}]
"Compatibility Flagsquot;=dword:00000400

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office\Common\COM Compatibility\{0002CE02-0000-0000-C000-000000000046}]
"Compatibility Flagsquot;=dword:00000400

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)

VendorStatusDate NotifiedDate UpdatedMicrosoft CorporationAffected-15 Nov 2017If you are a vendor and your product is affected, let
us know.

CVSS Metrics (Learn More)

Group
Score
Vector

Base
7.5
AV:N/AC:L/Au:N/C:P/I:P/A:P

Temporal
5.5
E:U/RL:OF/RC:C

Environmental
5.5
CDP:ND/TD:H/CR:ND/IR:ND/AR:ND

References

https://www.kb.cert.org/vuls/id/817544

https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2017-11882
https://embedi.com/blog/skeleton-closet-ms-office-vulnerability-you-didnt-know-about

Moving Beyond EMET II – Windows Defender Exploit Guard


https://msdn.microsoft.com/en-us/library/bb430720.aspx?f=255MSPPError=-2147217396
https://msdn.microsoft.com/en-us/library/windows/desktop/mt637065(v=vs.85).aspx
https://msdn.microsoft.com/en-us/library/windows/desktop/ms683835(v=vs.85).aspx
https://www.microsoft.com/en-us/download/details.aspx?id=54264
https://docs.microsoft.com/en-us/windows/threat-protection/windows-defender-exploit-guard/attack-surface-reduction-exploit-guard
https://docs.microsoft.com/en-us/windows/threat-protection/windows-defender-exploit-guard/enable-attack-surface-reduction

Credit

This issue was reported by Microsoft, who in turn credit Denis Selianin of Embedi with discovery.
This document was written by Will Dormann.

Other Information

CVE IDs:
CVE-2017-11882

Date Public:
14 Nov 2017

Date First Published:
15 Nov 2017

Date Last Updated:
20 Nov 2017

Document Revision:
24

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