Microsoft Warns of Two Actively Exploited Defender Vulnerabilities

Microsoft Warns of Two Actively Exploited Defender Vulnerabilities

Microsoft Warns of Two Actively Exploited Defender Vulnerabilities

Ravie LakshmananMay 21, 2026Endpoint Security / Vulnerability

Microsoft has disclosed that a privilege escalation and a denial-of-service flaw in Defender has come under active exploitation in the wild.

The former, tracked as CVE-2026-41091, is rated 7.8 on the CVSS scoring system. Successful exploitation of the flaw could allow an attacker to gain SYSTEM privileges.

"Improper link resolution before file access ('link following') in Microsoft Defender allows an authorized attacker to elevate privileges locally," Microsoft said in an advisory.

The second vulnerability under exploitation is CVE-2026-45498 (CVSS score: 4.0), a denial-of-service bug impacting Defender. The two vulnerabilities have been addressed in Microsoft Defender Antimalware Platform versions 1.1.26040.8 and 4.18.26040.7, respectively.

Although Microsoft has not formally confirmed, the vulnerability descriptions for CVE-2026-41091 and CVE-2026-45498 overlap with that of RedSun and UnDefend, two Defender zero-days that were disclosed by Chaotic Eclipse (aka Nightmare-Eclipse) last month. Huntress has since observed exploitation of both the vulnerabilities, alongside BlueHammer (CVE-2026-33825).

Also addressed in version 1.1.26040.8 is a heap-based buffer overflow vulnerability in Defender (CVE-2026-45584, CVSS score: 8.1) that an unauthorized attacker could exploit to achieve remote code execution. There is no evidence the vulnerability has been exploited in the wild. The tech giant noted that systems that have disabled Microsoft Defender are not susceptible to the vulnerabilities, adding that no action is required to install the update since it automatically updates malware definitions and the Microsoft Malware Protection Engine for optimal protection.

Microsoft credited five different parties with discovering and reporting CVE-2026-41091, including Sibusiso, Diffract, Andrew C. Dorman (aka ACD421), Damir Moldovanov, and an anonymous researcher.

To ensure the latest version of the Microsoft Malware Protection Platform and definition updates are being actively downloaded and installed, users are recommended to follow the steps below:

Open the Windows Security program. In the navigation pane, select Virus & threat protection. Then click on Protection Updates in the Virus & threat protection section updates. Select Check for updates. In the navigation pane, select Settings, and then select About. Examine the Antimalware ClientVersion number.

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has added both CVE-2026-41091 and CVE-2026-45498 to its Known Exploited Vulnerabilities (KEV) catalog, requiring Federal Civilian Executive Branch (FCEB) agencies to apply the fixes by June 3, 2026.

With the latest development, a total of three Microsoft vulnerabilities have been flagged as exploited within a span of a week. Last week, Redmond disclosed that a cross-site scripting flaw impacting on-premise versions of Exchange Server (CVE-2026-42897, CVSS score: 8.1) had been weaponized in real-world attacks.

Also added to the KEV catalog on Wednesday are four other Microsoft flaws from 2008, 2009, and 2010 -

CVE-2010-0806 - Microsoft Internet Explorer contains a use-after-free vulnerability that could allow remote attackers to execute arbitrary code. CVE-2010-0249 - Microsoft Internet Explorer contains a use-after-free vulnerability that could allow remote attackers to execute arbitrary code. CVE-2009-1537 - Microsoft DirectX contains a NULL byte overwrite vulnerability in the QuickTime Movie Parser Filter in quartz.dll in DirectShow, which could allow remote attackers to execute arbitrary code via a crafted QuickTime media file. CVE-2008-4250 - Microsoft Windows contains a buffer overflow vulnerability in the Windows Server Service that allows remote attackers to execute arbitrary code via a crafted RPC request.

Another vulnerability that finds a mention in the list is CVE-2009-3459, a heap-based buffer overflow vulnerability in Adobe Acrobat and Reader that could allow remote attackers to execute arbitrary code via a crafted PDF file that triggers memory corruption.

Update

Security researcher Nightmare-Eclipse has moved to GitLab after their GitHub account was taken down. In a post dated May 23, 2026, they accuse Microsoft of flagging their GitHub account and wiping it clean. The researcher has also confirmed that CVE-2026-45498 is UnDefend and CVE-2026-41091 is RedSun.

The signed message ends with a direct message to the company: "Mark this date July 14th, I will make sure your bones are shattered that day. Nothing will be released this June (or maybe I will release [something], depending on circumstances)."

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