You don't even have to open a file: Windows Defender just scans an incoming e-mail.
Sebastian Anthony - 9/5/2017
A massive and rather embarrassing remote code execution vulnerability has been discovered in Microsoft's MsMpEng, the malware protection engine used by Windows Defender, Microsoft Security Essentials, Microsoft Forefront, and Microsoft Endpoint in almost every recent version of Windows (7, 8, 8.1, 10, and Server 2016). Notably, Windows Defender is installed by default on all consumer-oriented Windows PCs.
The exploit (officially dubbed CVE-2017-0290) allows for a remote attacker to take over a system without any interaction from the system owner: it's simply enough for the attacker to send an e-mail or instant message that is scanned by Windows Defender. Likewise, anything else that is automatically scanned by Microsoft's malware protection engine—websites, file shares—could be used as an attack vector.
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