Nightmare Eclipse has released a new zero-day exploit called ShieldBreak, which can bypass Microsoft's RoguePlanet patch (CVE-2026-50656) and allow attackers to gain SYSTEM privileges on fully patched Windows 10, Windows 11, and Windows Server systems. The exploit has been confirmed by security expert Kevin Beaumont, who provided detection methods for it. ShieldBreak is the tenth zero-day from Nightmare Eclipse since April and was released shortly after Microsoft's monthly Patch Tuesday. The exploit allows local privilege escalation and has a 100% success rate on the latest version of Windows 11 and Windows Server 2025, while Windows 10 remains vulnerable. Microsoft is aware of the vulnerability and is investigating it, emphasizing the importance of coordinated vulnerability disclosure. Previous exploits from Nightmare Eclipse include LegacyHive and GreatXML, with earlier vulnerabilities having been patched but recent ones still unaddressed. Microsoft had threatened legal action against Nightmare Eclipse in May but later reconsidered its approach to vulnerability disclosure.