Cybersecurity has come to the fore again. A mysterious hacker has seemingly waged a personal war against Microsoft. According to information from The Register Another dangerous zero-day exploit has been made public. This time, the target is Microsoft Defender. Surprisingly, this exploit works even on fully updated, i.e., fully patched, Windows systems. The way this hacker is consistently targeting Microsoft makes it clear that it’s no longer just a case of bug hunting.
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mage Source / Credit: securityweek.com
Who is this hacker and what is the whole matter?
The name of this serial zero-day hunter isNightmare Eclipse।Which is described by securityweek.This is the person who has been publicly revealing flaws in Microsoft products since April 2026. Sources suggest this individual may be a former Microsoft employee who is deeply displeased with the company. ShieldBreak is Nightmare Eclipse’s tenth zero-day as part of this “scorched-earth” strategy against Microsoft since early April.
This statement is embarrassing for Microsoft. Nightmare Eclipse always releases its exploits on the same day Microsoft releases its monthly Patch Tuesday. The same thing happened this time. This new zero-day appeared just hours after Redmond’s monthly Patch Tuesday, in which the company fixed 421 security flaws in its products, but ShieldBreak was not among them.
What is ShieldBreak and how dangerous is it?
ShieldBreak is actually aLocal Privilege-Escalation Exploit Is Simply put, if an attacker gains access to the target system through any means, even through a limited user account, they can exploit this vulnerability to take control of the entire system.System-Level Control SYSTEM privileges are considered the highest access level in Windows. Those above this level have the power to bypass any security.
Some time ago, Microsoft RoguePlanet (CVE-2026-50656), a file system race-condition bug in Defender. But Nightmare Eclipse claims that ShieldBreak completely bypasses that patch. Cybersecurity expert and former Microsoft employee Kevin Beaumont reported that the two exploits differ significantly in how they work while RoguePlanet used virtual disks and NT native file manipulation, ShieldBreak relies on user-mode callback hooks to alter file contents during Defender’s cloud-hydration scans via the Cloud Filter API (cfapi).
Which systems are at risk?
This is important to know, and it’s even more concerning to know how many systems this exploit affects. Below is the confirmation.
- Windows 11 (including 25H2 and Canary channels)
- Windows Server 2025
- Windows 10 Even though it’s no longer officially supported, it’s still vulnerable to this vulnerability.
According to Nightmare Eclipse, the proof-of-concept (PoC) test was a success.100% rate, which makes it even more dangerous. Usually, race-condition exploits sometimes work, sometimes not, but this is believed to be the case here.
This isn’t just a theory. Kevin Beaumont himself tested the exploit and confirmed that it works on the latest Windows 11. He’s also released three detection and hunting queries to help defenders, so security teams can catch suspicious activity before Microsoft’s official patch arrives.
Microsoft’s silence and the growing controversy
Microsoft hasn’t yet offered a concrete solution to this entire issue. In previous months, when Nightmare Eclipse exposed similar flaws, the company even threatened legal action, using terms like “malicious activity causing real harm.” But questions are bound to arise. When the flaws are real and users are vulnerable, warnings alone won’t suffice.
What should ordinary users and companies do?
Until Microsoft releases an official patch, it’s wise to follow the advice of security experts.
- Detection and Hunting Queries Issued by Kevin Beaumont Implement this in your Defender for Endpoint setup.
- Monitor for suspicious local privilege-escalation activity, especially on systems that are Internet-facing.
- Avoid running files and scripts downloaded from unknown sources, as this exploit does not work without local access.
- IT teams should immediately alert their Security Operations Center about this threat.
The ongoing dispute between Nightmare Eclipse and Microsoft is no longer just a technical issue, but rather shows how one angry individual can shake even the largest tech company. A new zero-day after Patch Tuesday every month has become almost a pattern, and this is a serious warning for Microsoft. Until a definitive solution is found, the best course of action for users and organizations is to remain vigilant.
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