A patch has been rushed and released to address a serious Microsoft Malware Protection Engine bug, termed ‘Crazy Bad’ by the researchers who discovered the flaw. If exploited, the vulnerability would allow threat actors to turn the malware protection software against itself.

If the Microsoft Malware Protection Engine bug is exploited, Microsoft’s malware protection engine could be used to install malware rather than remove it. Instead of searching for infected files that have been downloaded, the system would be downloading malware and infecting end users.

The Microsoft Malware Protection Engine bug affects a number of anti-malware software products including Windows Defender, Microsoft Security Essentials, Microsoft System Center Endpoint Protection, Microsoft Forefront Security for SharePoint, Microsoft Endpoint Protection, Windows Intune Endpoint Protection and Microsoft Forefront Endpoint Protection.

The remotely exploitable bug could allow a system to be completely compromised, giving attackers full access to an infected computer or server, since the software and all associated processes run at LocalSystem privilege level.

The flaw was discovered by Natalie Silvanovich and Tavis Ormandy of Google Project Zero who alerted Microsoft three days ago. Ormandy said the flaw was “The worst in recent memory.” Microsoft worked fast to patch the flaw and an update was pushed out yesterday.

While extremely serious, Microsoft does not believe any malicious actors have taken advantage of the flaw, although all unpatched systems are at risk.  Threat actors could take advantage of the Microsoft Malware Protection Engine bug in a number of ways, including sending specially crafted email messages. The Project Zero researchers note that simply sending a malicious email would be enough to allow the bug to be exploited. It would not be necessary for the user to open the email or an infected email attachment.  The researchers explained that “writing controlled contents to anywhere on disk (e.g. caches, temporary internet files, downloads (even unconfirmed downloads), attachments, etc) is enough to access functionality in mpengine.” Alternatively, the flaw could be exploited by visiting a malicious website if a link was sent via email or through instant messaging.

The patch for the vulnerability (CVE-2017-0290) will be installed automatically if users have auto-update turned on. System administrators who have set updates to manual should ensure the patch is applied as soon as possible to prevent the flaw from being exploited. The current, patched Malware Protection Engine is version 1.1.13704.0.