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Microsoft disclosed active exploitation of two vulnerabilities in Microsoft Defender, CVE-2026-41091 and CVE-2026-45498, prompting CISA to add both flaws to its Known Exploited Vulnerabilities catalog. CVE-2026-41091 affects the Microsoft Malware Protection Engine and is a local privilege escalation issue tied to improper link resolution in Defender’s scanning logic, allowing an authenticated local attacker to obtain SYSTEM privileges. CVE-2026-45498 affects the Microsoft Defender Antimalware Platform and can cause a denial of service that impairs or crashes Defender protection capabilities.
Microsoft released fixes in Microsoft Malware Protection Engine 1.1.26040.8 and Microsoft Defender Antimalware Platform 4.18.26040.7, with updates delivered through normal Defender mechanisms rather than a separate manual patch. CISA ordered U.S. federal civilian agencies to remediate or discontinue affected products by June 3, 2026 under BOD 22-01. Reporting also linked the exploitation activity to public proof-of-concept releases by researcher Nightmare Eclipse, with Huntress observing attackers using Defender exploit code in the wild.
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4 events from the most recent confirmed update back to the earliest known activity.
Under Binding Operational Directive 22-01, CISA required U.S. Federal Civilian Executive Branch agencies to patch or discontinue affected Microsoft Defender products by 2026-06-03. Organizations were also advised to verify that Defender's automatic engine and platform updates had been applied successfully.
CISA added CVE-2026-41091 and CVE-2026-45498 to its Known Exploited Vulnerabilities catalog following Microsoft's disclosure of active exploitation. The action formally identified the flaws as exploited risks requiring prioritized remediation.
Microsoft acknowledged that CVE-2026-41091 and CVE-2026-45498 were publicly disclosed and are being exploited in the wild. Reporting linked the exploitation wave to public proof-of-concept releases, with Huntress observing attacker use of Defender exploit code.
Microsoft released fixes for CVE-2026-41091 and CVE-2026-45498 in Microsoft Malware Protection Engine version 1.1.26040.8 and Microsoft Defender Antimalware Platform version 4.18.26040.7. The flaws affect core Defender components, with one enabling local privilege escalation to SYSTEM and the other causing denial of service.
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Microsoft published a security advisory for an elevation of privilege vulnerability in the Microsoft Malware Protection Engine, tracked as **`CVE-2026-50656`** and described as a Microsoft Defender Elevation of Privilege Vulnerability. The issue affects a core Defender component used for malware scanning and protection, prompting immediate attention from defenders monitoring endpoint security exposure. The Canadian Centre for Cyber Security issued advisory **AV26-607**, urging organizations and administrators to review Microsoft’s guidance and apply the necessary updates as they become available. The notice reinforces that enterprises relying on Microsoft Defender should validate patch deployment and ensure affected systems receive the vendor’s remediation for **`CVE-2026-50656`**.
Jun 29, 2026
Microsoft said it is preparing a security update for **RoguePlanet**, a publicly disclosed Microsoft Defender zero-day tracked as `CVE-2026-50656`. The flaw affects fully patched **Windows 10** and **Windows 11** systems and allows local privilege escalation to **SYSTEM** by exploiting a race condition in the **Microsoft Malware Protection Engine**. Researcher **Nightmare Eclipse** released a public proof-of-concept exploit, and reports said the issue can be triggered whether Defender real-time protection is enabled or disabled, with possible impact in passive mode as well. The researcher said the bug may originally have supported remote code execution before Microsoft hardening changes in May blocked some attack paths, after which the exploit was reworked to bypass mitigations. Microsoft acknowledged the vulnerability and said it is investigating while developing a patch. The disclosure comes amid an ongoing dispute between Nightmare Eclipse and Microsoft over bug bounty and coordinated disclosure practices, and follows several other Windows flaws the researcher has publicly exposed, including **BlueHammer**, **RedSun**, **UnDefend**, **GreenPlasma**, and **YellowKey**.
Jun 29, 2026
A security researcher using the aliases **Chaotic Eclipse** and **Nightmare-Eclipse** publicly released working GitHub exploit code for **BlueHammer**, an unpatched Windows local privilege escalation flaw tied to Microsoft Defender’s signature update process. Independent testing by Will Dormann and others found the exploit worked, though reliability varied by platform and configuration. The attack abuses a **TOCTOU** race condition and path confusion in Defender’s update workflow, chaining components such as **VSS**, the **Cloud Files API**, and opportunistic locks to expose protected registry hives including `SAM`, `SYSTEM`, and `SECURITY`, extract NTLM password hashes, and potentially elevate a low-privileged user to **NT AUTHORITY\SYSTEM**. Microsoft initially responded with a Defender detection signature for the published proof of concept, but researchers warned that signature-based blocking did not address the underlying logic flaw. The issue was later tracked as **`CVE-2026-33825`** and patched by Microsoft on April 14, after which **CISA** ordered U.S. federal civilian agencies to remediate it within two weeks and added it to the **Known Exploited Vulnerabilities** catalog. Huntress reported the vulnerability was being used in zero-day intrusions involving hands-on-keyboard activity, and follow-on tooling such as **BlueSAM** showed how quickly the public disclosure was adapted for offensive use.
Jul 1, 2026