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This is a denial-of-service (DoS) vulnerability in the Microsoft Defender Antimalware Platform, identified as CVE-2026-45498 with a CVSS 3.x score of 4.0 (Medium) according to Microsoft.
How CVE-2026-45498 Works
The vulnerability stems from insufficient validation of user-supplied input within the Microsoft Defender Antimalware Platform (CWE-20). Specifically, when Defender’s scanning engine processes a specially crafted file, an input validation error occurs. A remote attacker can craft and pass such a malicious file to the target system via common vectors like email attachments, web downloads, or network shares. Upon the Defender’s real-time protection scanning this file, the flaw triggers a system condition that leads to a denial-of-service state. As a result, the Microsoft Defender Antimalware Platform can be forced to stop functioning entirely, becoming unresponsive or partially degraded. This renders the antimalware protection effectively disabled, leaving the host unprotected against subsequent malware infections. Crucially, exploitation does not require any user interaction or elevated privileges on the target. Microsoft has confirmed that this vulnerability has been actively exploited in the wild.
DailyCVE Form
Platform: Microsoft Defender
Version: < 4.18.26040.7
Vulnerability : Input Validation Error
Severity: Medium (CVSS 4.0)
date: 2026-05-20
Prediction: Patch released 2026-05-20
Analytics
What Undercode Say:
This vulnerability is significant as it disables the first line of defense on Windows systems. The immediate availability of a patch (version 4.18.26040.7) and its inclusion in CISA’s KEV catalog, with a remediation deadline of June 3, 2026, indicate a high operational risk. Administrators should prioritize updating all Windows endpoints, as attackers are actively exploiting this flaw to shut down security controls before deploying further payloads.
Bash Commands and Codes to Check Vulnerability:
To check if a system is vulnerable, you can query the installed platform version using PowerShell:
Get-MpComputerStatus | Select-Object AntimalwarePlatformVersion
You can also use a vulnerability scanner script:
git clone https://github.com/ridhinva/defender-vulnerability-scanner.git cd defender-vulnerability-scanner python3 defender_scanner.py
For remote scanning:
python3 defender_scanner.py --remote <target_host>
Exploit:
A remote attacker can exploit this vulnerability by sending a specially crafted file to the target user via email or a web download. When Microsoft Defender automatically scans the file upon download or access, the insufficient input validation triggers the DoS condition, temporarily disabling antimalware protection without any user interaction. Proof-of-concept code was previously leaked under the name “UnDefend”.
Protection from this CVE:
The primary protection is to update Microsoft Defender Antimalware Platform to version 4.18.26040.7 or later. Most systems receive this update automatically through Windows Update and definition updates. Organizations should verify their version and ensure the update is applied before the CISA-mandated deadline of June 3, 2026.
Impact:
A successful attack leads to a denial-of-service condition where the Microsoft Defender Antimalware Platform stops functioning. This effectively disables real-time protection, leaving the host vulnerable to other malware and attacks. The impact is limited to availability (A:L), with no effect on confidentiality or integrity of system data.
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Sources:
Reported By: nvd.nist.gov
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