A security researcher known as Nightmare Eclipse has revealed a new unpatched vulnerability in Windows, named LegacyHive, which is a local privilege escalation flaw in the Windows User Profile Service. This vulnerability allows attackers to access and load user hives of other accounts, including administrators. Nightmare Eclipse has provided proof-of-concept exploit code that works on systems with Microsoft's July 2026 patches. The exploit initially did not require user credentials but now necessitates them for accessing other hives. Nightmare Eclipse has previously disclosed over half a dozen zero-day vulnerabilities affecting Microsoft products. Microsoft has not yet acknowledged the LegacyHive exploit.