A serious MS Office remote code execution vulnerability has been patched by Microsoft – One that would allow malware to be installed remotely with no user interaction required. The flaw has been present in MS Office for the past 17 years.

The flaw, which was discovered by researchers at Embedi, is being tracked as CVE-2017-11882. The vulnerability is in the Microsoft Equation Editor, a part of MS Office that is used for inserting and editing equations – OLE objects – in documents: Specifically, the vulnerability is in the executable file EQNEDT32.exe.

The memory corruption vulnerability allows remote code execution on a targeted computer, and would allow an attacker to take full control of the system, if used with Windows Kernel privilege exploits. The flaw can be exploited on all Windows operating systems, including unpatched systems with the Windows 10 Creators Update.

Microsoft addressed the vulnerability in its November round of security updates. Any unpatched system is vulnerable to attack, so it is strongly advisable to apply the patch promptly. While the vulnerability could potentially have been exploited at any point in the past 17 years, attacks exploiting this MS Office remote code execution vulnerability are much more likely now that a patch has been released.

The flaw does not require the use of macros, only for the victim to open a specially crafted malicious Office document. Malicious documents designed to exploit the vulnerability would likely arrive via spam email, highlighting the importance of implementing a spam filtering solution such as SpamTitan to block the threat.

End users who are fooled into opening a malicious document can prevent infection by closing the document without enabling macros. In this case, malware would be installed simply by opening the document.

Microsoft has rated the vulnerability as important, rather than critical, although researchers at Embedi say this flaw is “extremely dangerous.” Embedi has developed a proof of concept attack that allowed them to successfully exploit the vulnerability. The researchers said, “By inserting several OLEs that exploited the described vulnerability, it was possible to execute an arbitrary sequence of commands (e.g. to download an arbitrary file from the Internet and execute it),”

EQNEDT32.exe is run outside of the Microsoft Office environment, so it is therefore not subject to Office and many Windows 10 protections. In addition to applying the patch, security researchers at Embedi recommend disabling EQNEDT32.EXE in the registry, as even with the patch applied, the executable still has a number of other vulnerabilities. Disabling the executable will not impact users since this is a feature of Office that is never needed by most users.