A disgruntled security researcher who's been feuding with Microsoft has released a newly discovered vulnerability in Windows Defender that could be used to help hackers attack PCs. To prove the threat is real, Nightmare Eclipse has released a proof-of-concept code that can exploit the bug in Windows Defender in Windows 11 25H2 and Windows Server 2025. Nightmare Eclipse appears to be a security researcher who formerly worked for Microsoft, uncovering software bugs.