Pierluigi Paganini
April 18, 2026
Attackers are exploiting three recently disclosed zero-day flaws in Microsoft Defender to gain higher privileges on compromised systems. The vulnerabilities, called BlueHammer, RedSun, and UnDefend, were revealed by a researcher known as Chaotic Eclipse after criticizing Microsoft’s handling of the disclosure.
Chaotic Eclipse also published proof-of-concept code for the unpatched Windows bug.
BlueHammer and RedSun let attackers escalate privileges locally in Microsoft Defender. UnDefend instead triggers a denial-of-service, blocking security definition updates and weakening protection.
At this time, Microsoft has only fixed the BlueHammer flaw, tracked as CVE-2026-33825, but the others remain unpatched.
Huntress researchers reported attackers are exploiting the three Windows flaws to target systems, though the victims and attackers remain unknown.
Huntress said it saw real-world exploitation of all three flaws. Attackers used BlueHammer starting April 10, 2026, then followed with RedSun and UnDefend proof-of-concept exploits on April 16.
Researchers believe attackers are using public exploit code released online by Chaotic Eclipse.
Huntress said attackers started exploiting BlueHammer on April 10, 2026, then followed with RedSun and UnDefend proof-of-concept exploits on April 16.
When exploit code becomes publicly available, threat actors can quickly weaponize it in attacks in the wild.
Follow me on Twitter: @securityaffairs and Facebook and Mastodon
(SecurityAffairs – hacking, Microsoft defender)
