Vulnerabilities Nightmare Eclipse Drops ‘LegacyHive’ Windows Zero-Day The researcher stripped the proof-of-concept (PoC) exploit to prevent immediate exploitation of the vulnerability. By Ionut Arghire | July 16, 2026 (2:48 AM ET) Flipboard Reddit Whatsapp Whatsapp Email Nightmare Eclipse, the disgruntled security researcher who has been dropping zero-day exploits targeting Microsoft products, released another unpatched Windows vulnerability this week, right on the July 2026 Patch Tuesday . The fresh exploit, named LegacyHive , is a local privilege escalation bug in the Windows User Profile Service that allows an attacker to load other users’ hives, including those of administrators. Also known as Chaotic Eclipse, Nightmare Eclipse released proof-of-concept (PoC) exploit code that works on systems running Microsoft’s July 2026 patches. “The PoC requires another standard user credentials and a third username (which can be an administrator account), if the PoC is successful, it will end up mounting the target user hive in current user classes root,” the researcher explains. Unlike previously dropped zero-day exploits from Nightmare Eclipse, LegacyHive was released with a stripped PoC to prevent the security defect’s in-the-wild exploitation. According to the researcher, the exploit originally did not require user credentials and allowed any hive to be loaded, not just the usrclass.dat hive. That is still possible, the researcher says, but would require some work. Advertisement. Scroll to continue reading. To date, Nightmare Eclipse released over half a dozen zero-days in Microsoft products, including BlueHammer , RedSun , and UnDefend , which have been exploited in attacks, along with GreenPlasma , RoguePlanet , YellowKey , and GreatXML . Microsoft has yet to acknowledge the LegacyHive exploit. SecurityWeek has emailed the company for a statement and will update this article if it responds. Related: Unpatched Cursor Vulnerability Exposes Users to Code Execution Related: CISA Urges Immediate Patching of Exploited SharePoint Vulnerabilities Related: Windows Bind Link Attacks Can Hide Malware From EDR Tools Related: Progress Confirms Zero-Day Vulnerability Behind ShareFile Disruption Written By Ionut Arghire Ionut Arghire is an international correspondent for SecurityWeek. Daily Briefing Newsletter Subscribe to the SecurityWeek Email Briefing for the latest cybersecurity threats, trends, and expert insights. 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