Huntress is warning that threat actors are exploiting three recently disclosed security flaws in Microsoft Defender to gain elevated privileges in compromised systems.
The activity involves the exploitation of three vulnerabilities that are codenamed BlueHammer (requires GitHub sign-in), RedSun, and UnDefend, all of which were released as zero-days by a researcher known as Chaotic Eclipse (aka Nightmare-Eclipse) in response to Microsoft's handling of the vulnerability disclosure process.
While both BlueHammer and RedSun are local privilege escalation (LPE) flaws impacting Microsoft Defender, UnDefend can be used to trigger a denial-of-service (DoS) condition and effectively block definition updates.
Microsoft moved to address BlueHammer as part of its Patch Tuesday updates released earlier this week. The vulnerability is being tracked under the CVE identifier CVE-2026-33825. However, the other flaws do not have a fix as of writing.
In a series of posts shared on X, Huntress said it observed all three flaws being exploited in the wild, with BlueHammer being weaponized since April 10, 2026, followed by the use of RedSun and UnDefend proof-of-concept (PoC) exploits on April 16.
"These invocations followed after typical enumeration commands: whoami /priv, cmdkey /list, net group, and others that indicate hands-on-keyboard threat actor activity," it added.
The cybersecurity vendor said it has taken steps to isolate the affected organization to prevent further post-exploitation. When reached for comment, Microsoft confirmed that the BlueHammer exploit has been addressed via CVE-2026-33825.
"Microsoft has a customer commitment to investigate reported security issues and update impacted devices to protect customers as soon as possible," a Microsoft spokesperson said. "We also support coordinated vulnerability disclosure, a widely adopted industry practice that helps ensure issues are carefully investigated and addressed before public disclosure, supporting both customer protection and the security research community."
Update
The U.S. Cybersecurity and Infrastructure Security Agency (CISA), on April 22, 2026, added CVE-2026-33825 to its Known Exploited Vulnerabilities (KEV) Catalog, requiring Federal Civilian Executive Branch (FCEB) agencies to apply the fixes by May 6, 2026.
In a related development, various independent researchers and security vendors have published their analyses of RedSun and UnDefend exploits, detailing how the PoCs turn Microsoft Defender against organizations it's designed to protect by enabling SYSTEM-level access and disrupting its update mechanism.
"The combination matters operationally," Vectra said. "An attacker uses BlueHammer or RedSun to achieve SYSTEM, then deploys UnDefend to ensure the endpoint protection layer becomes progressively less capable of catching follow-on activity. It is a layered degradation strategy, not a one-shot exploit."
(The story was updated after publication to include a response from Microsoft.)
Found this article interesting? Follow us on Google News, Twitter and LinkedIn to read more exclusive content we post.