Security researcher Abdelhamid Naceri, also known as Nightmare Eclipse, has disclosed a zero-day vulnerability named BigDiskBuster. The report, published on September 22, highlights that the flaw affects all supported versions of Microsoft Windows, including Windows 10, Windows 11, and Windows Server.
The vulnerability functions as a denial-of-service exploit that prevents Microsoft Defender from downloading virus definition updates and security signatures. If the tool runs in the background, the system remains stuck on its current version, unable to receive further security patches.
The researcher noted that the mechanism of BigDiskBuster is similar to the UnDefend vulnerability, which was publicly disclosed in April 2026. The exploit effectively disables the update process, leaving the antivirus software unable to maintain its protection against new threats.
