Maliciously crafted files can disable Microsoft's antimalware products

A vulnerability in the engine used by many Microsoft antimalware products can lead to a persistent denial-of-service condition

A vulnerability allows attackers to disable Microsoft's antimalware products by sending specifically crafted files to users via websites, email or instant messaging applications.

The vulnerability is located in the Microsoft Malware Protection Engine, which sits at the core of many Microsoft security products for desktops and servers including Microsoft Forefront Client Security, Microsoft System Center 2012 Endpoint Protection, the Microsoft Malicious Software Removal Tool, Microsoft Security Essentials, Windows Intune Endpoint Protection and Windows Defender, which comes pre-installed in Windows Vista and later.

Microsoft fixed the vulnerability in Microsoft Malware Protection Engine 1.1.10701.0 released Tuesday. For home users, the new version should typically download and install automatically within 48 hours, but administrators in enterprise environments should make sure that their update management software is configured to approve the engine updates.

If left unpatched, the vulnerability can be exploited to force the Microsoft Malware Protection Engine into a scan timeout, essentially leading to a denial-of-service vulnerability. When this happens, the antimalware product will stop monitoring the system for threats until the rogue file is removed and the malware protection service is restarted.

"There are many ways that an attacker could place a specially crafted file in a location that is scanned by the Microsoft Malware Protection Engine," Microsoft said in a security advisory published Tuesday. "For example, an attacker could use a website to deliver a specially crafted file to the victim's system that is scanned when the website is viewed by the user. An attacker could also deliver a specially crafted file via an email message or in an Instant Messenger message that is scanned when the file is opened."

In addition, if the Malware Protection Engine runs on a Windows server hosting a website that accepts user-supplied content, an attacker could exploit the vulnerability by uploading a specially crafted file through the website.

Microsoft credited Tavis Ormandy, an information security engineer at Google, with discovering and reporting the vulnerability to the company.

Ormandy has found vulnerabilities in antimalware software in the past. In 2012 he found critical vulnerabilities in Sophos Antivirus.

Specific details about how to create the rogue files that prevent the Microsoft Malware Protection Engine from functioning properly were not released, but Ormandy hinted on Twitter that the problem involves JavaScript.

"This was an interesting bug in the JavaScript interpreter in Windows Defender," he said. "Yes, it has a JS interpreter."

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