Microsoft has released security patches to address a Windows zero-day vulnerability known as "LegacyHive," disclosed after the July 2026 Patch Tuesday.

The security flaw was disclosed by a security researcher who uses the "Nightmare Eclipse" handle in protest of Microsoft's bug bounty and vulnerability disclosure practices.

Nightmare Eclipse published a LegacyHive proof-of-concept (PoC) exploit hours after the July 2026 Patch Tuesday security updates were released, claiming it exploits a security vulnerability in the Windows User Profile Service.

However, unlike previous exploits they released, the LegacyHive PoC requires additional credentials, making it harder for threat actors to weaponize the vulnerability.

"Microsoft is aware of the reported vulnerability and is actively investigating the validity and potential applicability of these claims," a Microsoft spokesperson told BleepingComputer when asked for a statement regarding LegacyHive.

Vulnerability analyst Will Dormann explained that non-admin users can use Nightmare Eclipse's exploit to modify the classes registry hive and gain automatic code execution when the admin account logs in to a compromised system.

One day after the PoC was released, cybersecurity expert Kevin Beaumont also published LegacyHive exploitation detection queries for Microsoft Defender for Endpoint (MDE) and confirmed that the exploit worked.

Microsoft has now patched the vulnerability this week as part of its August Patch Tuesday updates and now tracks it as CVE-2026-62832. However, it has yet to acknowledge that Nightmare Eclipse discovered the flaw, instead tagging it as reported by an anonymous researcher.

The company says that LegacyHive stems from improper link resolution before file access ('link following') in the Windows User Profile Service, and successful exploitation allows local attackers to gain administrator privileges.

"An authenticated attacker who has credentials for another local account could run a specially crafted application to load another user's registry hive," Microsoft says. "Successful exploitation could allow the attacker to access or modify another user's data and gain administrator privileges. User interaction is not required."

ACROS Security, the company behind the 0Patch cybersecurity platform, also released free unofficial LegacyHive patches on July 20 for systems running Windows 10 2004 or later and Windows Server 2022 or later.

Nightmare Eclipse has disclosed multiple zero-day flaws since April 2026, including ShieldBreak, LegacyHive, RoguePlanet, YellowKey, BlueHammer, RedSun, GreenPlasma, MiniPlasma, and UnDefend in Microsoft Defender, BitLocker, and other Windows components.

Microsoft patched the YellowKey, GreenPlasma, and MiniPlasma flaws as part of the June 2026 Patch Tuesday, and the RoguePlanet vulnerability in July, but the other zero-days are still awaiting an official patch.