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CVE-2026-45498 is a Medium‑severity Denial‑of‑Service vulnerability in the Microsoft Defender Antimalware Platform. The root cause lies in insufficient input validation within the core scanning engine. By crafting a malicious payload (e.g., a specially‑formed email attachment or network packet), an unauthenticated attacker can force the Defender component to enter a resource‑exhaustion condition.
When the engine parses certain file formats or network protocols, it fails to properly validate the structure and content of the incoming data. This leads to unbounded resource allocation (CWE‑770) and a lack of proper bounds checking (CWE‑129). As a result, the scanning process may:
Allocate excessive memory, leading to a system slowdown or crash.
Enter an infinite loop, effectively freezing the service.
Trigger a stack overflow, causing the Defender service to terminate entirely.
The attack vector is local (AV:L) and requires no authentication (PR:N) nor user interaction (UI:N). Once triggered, the Defender Antimalware Platform becomes unavailable, exposing the host to other threats while the service recovers or is manually restarted. Network administrators may observe excessive system load, performance degradation, and possible cascading failures in dependent security services.
This flaw is actively exploited in the wild, as confirmed by multiple government CERT advisories. Microsoft has addressed the issue by releasing updated version 4.18.26030.3011 and recommends that all users update immediately.
dailycve form
Platform: Microsoft Defender Antimalware
Version: 4.18.26030.3011 (vulnerable)
Vulnerability : Denial of Service (DoS)
Severity: MEDIUM (CVSS 4.0)
date: 2026-05-20
Prediction: 2026-05-20 (patch available)
Analytics under What Undercode Say:
Check current Defender antimalware platform version
Get-MpComputerStatus | Select-Object AMProductVersion
Verify the installed version is >= 4.18.26030.3011
$version = Get-MpComputerStatus | Select-Object -ExpandProperty AMProductVersion
if ($version -ge "4.18.26030.3011") {
Write-Host "Vulnerable version detected - update required"
} else {
Write-Host "Protected against CVE-2026-45498"
}
Force update of Defender definitions and platform
Update-MpSignature -UpdateSource MicrosoftUpdateServer
Start-MpScan -ScanType FullScan
Monitor Defender service health and restart if hung
$defenderStatus = Get-Service -Name WinDefend
if ($defenderStatus.Status -ne "Running") {
Write-EventLog -LogName "System" -Source "Microsoft-Windows-Windows Defender" -EventId 1001 -Message "Defender service down - potential DoS attack (CVE-2026-45498)"
Restart-Service -Name WinDefend
}
Exploit:
The vulnerability is triggered by submitting a malformed payload (e.g., a crafted .eml file or a specially‑constructed network packet) to a Microsoft Defender component that scans the content. Because of insufficient input validation, the scanning engine enters an infinite loop or allocates an uncontrolled amount of memory, leading to a Denial‑of‑Service condition. No authentication or user interaction is required, making the attack simple to execute locally.
Protection from this CVE
- Immediately update to Microsoft Defender Antimalware Platform version 4.18.26030.3011 or later.
- Enable automatic updates for Microsoft Malware Protection Platform and definition updates.
- Implement network segmentation and monitor for abnormal resource consumption by Defender services.
- Use application whitelisting to limit the execution of potentially malicious payloads.
- Deploy intrusion detection systems (IDS) to detect known exploit patterns.
Impact
Successful exploitation causes a complete or partial unavailability of Microsoft Defender Antimalware Platform, disrupting threat detection and protection. While the service is down, the host becomes vulnerable to other malware and attacks. The flaw directly impacts the availability component of the CIA triad and can be mapped to MITRE ATT&CK T1499 (Network Denial of Service).
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Sources:
Reported By: www.cve.org
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