Microsoft Releases Emergency Patch For RCE Vuln

Late last night Microsoft released an emergency out-of-band patch to fix a vulnerability in Microsoft Malware Protection Engine (MsMpEng) that one of the researchers who found it called "the worst Windows remote code exec(ution) in recent memory," and for which US-CERT released an alert.

Announced over the weekend by a pair of researchers working for the Google Project Zero team, Tavis Ormandy and Natalie Silanovich, the vulnerability allows attackers to carry out remote code execution (RCE) by feeding MsMpEng a simple malicious file to trigger memory corruption. According to Silanovich, the vulnerability only requires a simple exploit to leverage, requiring so little code that it can fit in a single tweet. According to Microsoft Security Advisory 4022344, the affected version of the engine must scan the specially crafted file, but that can be easily achieved a number of ways.

"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," Microsoft advises. "In addition, an attacker could take advantage of websites that accept or host user-provided content, to upload a specially crafted file to a shared location that is scanned by the Malware Protection Engine running on the hosting server."

In fact, when Ormandy and Silanovich released their proof-of-concept code they warned anyone using Microsoft systems to take extra care with the file because simply downloading it could immediately crash MsMpEng in its default configuration. Given the triviality of exploit, the default-on status of MsMpEng in Windows systems from Windows 8 on up, and the permissions afforded to the service, Ormandy calls this particular hole "crazy bad."

"Vulnerabilities in MsMpEng are among the most severe possible in Windows, due to the privilege, accessibility, and ubiquity of the service," he wrote. "The core component of MsMpEng responsible for scanning and analysis is called mpengine. Mpengine is a vast and complex attack surface, comprising of handlers for dozens of esoteric archive formats, executable packers and cryptors, full system emulators and interpreters for various architectures and languages, and so on. All of this code is accessible to remote attackers."

Ormandy and Silanovich went through coordinated vulnerability disclosure procedures with Microsoft, which came up with this release within just a few days. Microsoft says that updates to the engine will be automatically installed with updated malware definitions for the affected products, such that the typical consumer end user should see the update applied within 48 hours. Consumers can speed up that timetable by manually updating their anti-malware software. Meanwhile, Microsoft advises enterprise customers to follow internal processes to confirm that their patch management software has approved and installed necessary definition and engine updates.

"Administrators of enterprise anti-malware deployments should ensure that their update management software is configured to automatically approve and distribute engine updates and new malware definitions. Enterprise administrators should also verify that the latest version of the Microsoft Malware Protection Engine and definition updates are being actively downloaded, approved and deployed in their environment," the advisory explains. 

The scramble to patch this vulnerability highlights the extreme sensitivity of flaws found within security products, say experts in the field. 

"The irony is that it is the product that is designed to protect these operating systems against malware which can now be targeted as a result of finding this issue," says Darron Gibbard, CTSO at Qualys. 

Steven Malone, director of security product management at email security company Mimecast agrees, explaining that the incident is also good lesson on why it's never good to depend on a single layer of security.

"Desktop security products often need high privileges in order to see everything and therefore any vulnerabilities can be particularly deadly," he says. "Incidents like this highlight that advanced security still requires a defense-in-depth strategy."

Related Content: