Nightmare Eclipse, the serial zero-day hunter who has an axe to grind with Microsoft, published a new Defender zero-day, ShieldBreak, that apparently bypasses Redmond’s RoguePlanet patch (CVE-2026-50656), allowing attackers to gain SYSTEM privileges on fully patched Windows 10, Windows 11, and Windows Server systems.

According to at least one other researcher, the exploit works. “I've tried it, it works on latest Windows 11,” former Microsoft employee and security expert Kevin Beaumont said. Beaumont also published three detections and hunting queries for ShieldBreak to help defenders rapidly find any stealthy threats. So until Microsoft fixes this latest zero-day, we’d highly suggest using these queries.

ShieldBreak is the 10th zero-day from Nightmare Eclipse since they began their scorched-earth strategy against Microsoft in early April. The prolific bug finder and exploit developer is suspected to be a former, very disgruntled, Microsoft employee. And in typical fashion, this latest zero-day drop occurred just hours after Redmond’s monthly Patch Tuesday that fixed 421 security problems in its products - but ShieldBreak isn't one of them.

It’s a local privilege-escalation exploit that, according to Nightmare, allows attackers to gain SYSTEM-level privileges. 

“The PoC was tested in the latest version of windows 11 25h2 (+Canary channel) and windows server 2025, the PoC also have a 100% success rate. Please note that Windows 10 (and respective server editions) are not currently supported, they are however vulnerable to ShieldBreak as well,” they said.

While Nightmare claims that the new exploit is a patch bypass for the earlier RoguePlanet vulnerability, CVE-2026-50656, which Microsoft quietly fixed in July, Beaumont pointed out that the two flaws operate very differently.

“RoguePlanet was a filesystem race condition vuln that uses virtual disks and NT native file manipulation to trick quarantine process into overwriting system files,” he posted. “ShieldBreak user-mode callback hook to change file contents during a Defender cloud-hydration scan via cfapi (Cloud Filter API).”

A Microsoft spokesperson told us the company "is aware of the reported vulnerability and is actively investigating the validity and potential applicability of these claims."

The spokesperson added: "Microsoft is committed to investigating security issues and updating impacted products to protect customers as soon as possible. Importantly, we support coordinated vulnerability disclosure, an industry standard that protects customers and supports the research community by ensuring their findings are thoroughly investigated and addressed before being made public." 

This latest zero-day comes a month after Nightmare Eclipse published its previous vulnerability along with partial exploit code. Nightmare’s July drop, called LegacyHive, is a local privilege escalation flaw that targets Windows’ user hives - the section of the Windows Registry that stores a user's specific desktop settings, application preferences, and environment configurations. It doesn’t have an official patch.

Neither does a June zero-day called GreatXML that Nightmare developed. The researcher claims the flaw allows a local attacker with administrator rights to bypass BitLocker encryption by manipulating the Windows Recovery Environment.

The prolific zero-day hunter’s earlier seven Windows bugs do have patches. These include BlueHammer (CVE-2026-33825), RedSun (CVE-2026-41091), UnDefend (CVE-2026-45498), YellowKey (CVE-2026-45585), GreenPlasma (CVE-2026-45586), MiniPlasma (CVE-2020-17103), and RoguePlanet (CVE-2026-50656).

After threatening legal action against Nightmare Eclipse in May, and then facing rapid backlash from just about every other security researcher on the planet, Microsoft walked back its talk of siccing its Digital Crimes Unit on people who don’t follow its vulnerability disclosure rules.®