Security Experts:

Microsoft Fixes Antimalware Engine Flaw Found by Google Experts

It took Microsoft less than three days to patch a critical remote code execution vulnerability found by Google Project Zero researchers in the company’s Malware Protection Engine. Most users don’t need to take any action as the affected products should be updated automatically.

The existence of the flaw was disclosed by Google Project Zero researchers Tavis Ormandy and Natalie Silvanovich on Friday. Ormandy described the weakness as “the worst Windows remote code exec [vulnerability] in recent memory.”

The expert said the vulnerability was “wormable,” it affected default Windows installations, and exploitation did not require access to the victim’s network.

Microsoft announced a patch for the vulnerability and published an advisory on Monday. While Ormandy initially described it as a Windows vulnerability, the security hole actually affects the Microsoft Malware Protection Engine, which is shipped with several of the company’s Windows antimalware products.

The list of impacted software includes Forefront Endpoint Protection, Endpoint Protection, Forefront Security for SharePoint Service Pack 3, System Center Endpoint Protection, Security Essentials, Windows Defender, and Windows Intune Endpoint Protection.

According to Ormandy and Silvanovich, the vulnerability, tracked as CVE-2017-0290, affects the “MsMpEng” service, which runs unsandboxed with SYSTEM privileges and is accessible without authentication via Windows services such as Exchange and IIS.

The Google Project Zero researchers said the vulnerability can be exploited via email – opening the malicious email is not necessary for exploitation – or by getting the targeted user to access a specially crafted link.

“This level of accessibility is possible because MsMpEng uses a filesystem minifilter to intercept and inspect all system filesystem activity, so 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,” the experts said in their advisory.

The researchers believe that vulnerabilities in the Microsoft Malware Protection Engine “are among the most severe possible in Windows, due to the privilege, accessibility, and ubiquity of the service.”

Google Project Zero has made public technical details and proof-of-concept (PoC) exploit code.

Microsoft pointed out that the patch should be sent out automatically to antimalware products that use the Malware Protection Engine within 48 hours. However, the company has advised users to check their software to ensure that the automatic deployment of updates and malware definitions is working properly.

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Eduard Kovacs is an international correspondent for SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.