Security researcher Abdelhamid Naceri, known as Nightmare Eclipse, released a zero-day exploit called BigDiskBuster that targets Microsoft Defender, preventing antivirus updates and leaving systems vulnerable. BigDiskBuster operates across all supported Windows versions and must run in the background to block updates. Naceri has previously released a similar exploit called UnDefend and has a history of releasing multiple zero-day exploits since April 2026 amid a dispute with Microsoft. Two weeks before BigDiskBuster, he introduced another exploit named ShieldCrash, which grants SYSTEM access and circumvents a patched flaw. Naceri's recent exploits include tools like LegacyHive, BlueHammer, RedSun, YellowKey, GreenPlasma, and MiniPlasma, all targeting Microsoft Defender and other Windows components. Microsoft has warned of potential legal action against malicious activities but has not commented on BigDiskBuster.