Microsoft Warns of Two Actively Exploited Defender Vulnerabilities

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.

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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.

The tech giant noted that systems that have disabled Microsoft Defender are not susceptible to the vulnerability, 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 the flaw, 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.

There are currently no details on how the vulnerabilities are being exploited in the wild. The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has added both of them to its Known Exploited Vulnerabilities (KEV) catalog, requiring Federal Civilian Executive Branch (FCEB) agencies to apply the fixes by June 3, 2026.

Last week, Microsoft also disclosed that a cross-site scripting flaw impacting on-premise versions of Exchange Server (CVE-2026-42897, CVSS score: 8.1) has 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.

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