Google Researchers Find Wormable 'Crazy Bad' Windows Exploit (bleepingcomputer.com)

An anonymous reader quotes a report from BleepingComputer: Two Google security experts have found a severe remote code execution (RCE) bug in the Windows OS, which they've described as "crazy bad." The two experts are Natalie Silvanovich and Tavis Ormandy, both working for Project Zero, a Google initiative for discovering and helping patch zero-days in third-party software products. The two didn't release in-depth details about the vulnerability, but only posted a few cryptic tweets regarding the issue. Drilled with questions by the Twitter's infosec community, Ormandy later revealed more details: the attacker and the victim don't necessarily need to be on the same LAN; the attack works on a default Windows install, meaning victims don't need to install extra software on their systems to become vulnerable; the attack is wormable (can self-replicate). The tweets came days before Microsoft's May 2017 Patch Tuesday, scheduled tomorrow, May 9. The researchers said a report is coming, alluding the vulnerability might be patched this month, and they'll be free to publish their findings.

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Windows Defender - CVE-2017-0290

[Etcetera]

Official announcement: https://technet.microsoft.com/en-us/library/security/4022344

More background / report: https://bugs.chromium.org/p/project-zero/issues/detail?id=1252&desc=5

On workstations, attackers can access mpengine by sending emails to users (reading the email or opening attachments is not necessary), visiting links in a web browser, instant messaging and so on. 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. MIME types and file extensions are not relevant to this vulnerability, as MsMpEng uses it's own content identification system.

Vulnerabilities in MsMpEng are among the most severe possible in Windows, due to the privilege, accessibility, and ubiquity of the service.

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.

tl;dr: The Javascript engine in Windows Defender (which tries to figure out if it's a virus) has a flaw. Exploit works and can be leveraged if you can force the victim to write something to disk (triggering a scan): eg, sending an email, viewing an image, writing a log entry, etc.

Not a Windows Update, the fix is coming as part of the Windows Defender definitions updates rollout process.