CriticalCVE-2026-45498

    Microsoft Defender Denial of Service Vulnerability (CVE-2026-45498)

    Published Last modified In CISA KEV since Sources: BSI, Msrc Update Guide, CVE.org, EUVD, pwn.watch

    What is affected, what to do

    Microsoft Defender Denial of Service Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Defender Antimalware Platform to version 4.18.26040.7. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Defender Antimalware Platform lt 4.0.0.0..4.18.26040.7
    Fixed in
    • Microsoft Defender Antimalware Platform 4.18.26040.7

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    4.0

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    4.0

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: BSI, Msrc Update Guide, CVE.org, EUVD, pwn.watch

    63.1 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.20

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    216.52 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: VulnCheck KEV (opens in a new tab) How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

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    4.0 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

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    4.0
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

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    63.1 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

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    99.20
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

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    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-400

    Response and mitigation

    Why act now?

    Prioritisation rationale

    On paper this vulnerability looks minor: it is local-only, has limited availability impact and does not directly affect confidentiality or integrity. In practice, a denial of service against the antivirus engine is a textbook defence-evasion primitive. An attacker who can already land code or a crafted file on a host can blind the scanner before staging the real payload, which explains its KEV listing despite the modest base severity. CISA does not flag known ransomware use, and exploitation likelihood is assessed as moderate. Because an official fix exists and platform updates carry little operational risk, treat this as "high, remediate within 7 days". Shorten that to 72 h for multi-user systems (VDI, terminal servers, developer workstations) and for critical-infrastructure servers where Defender is the only endpoint control.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory the platform version: Run Get-MpComputerStatus | Select AMProductVersion, AMServiceEnabled, RealTimeProtectionEnabled fleet-wide, or pull "Antimalware platform version" from Defender for Endpoint/Intune device inventory. Anything below 4.18.26040.7 is vulnerable.
      • Deploy platform update 4.18.26040.7: Approve the Defender platform update (product category "Microsoft Defender Antivirus") in WSUS/MECM, or make sure Intune/Windows Update for Business rings for the antimalware platform are not pinned to an older channel. Air-gapped and OT segments need the platform package staged manually.
      • Find hosts without an update path: Check servers running Defender in passive mode, golden images, VDI masters and devices with frozen update rings separately. The platform is usually still installed on these systems and is therefore exposed.
      • Review current protection state: Hunt for endpoints where MsMpEng.exe crashed or real-time protection dropped in the last 14 days (see detection rules). Treat those hosts as potentially compromised and triage them before trusting them as "protected" again.
      • Enforce Tamper Protection: Enable it tenant-wide via Microsoft Defender XDR or Intune and document any exclusions. This stops an attacker from following a crash by disabling the service through configuration.

    2. Runbook step 2

      Mitigation layers

      • Configuration / device compliance: Tie an Intune compliance policy requiring real-time protection and antimalware to Conditional Access. A device whose Defender is knocked over loses access to M365 and SSO-backed resources until protection is restored.
      • Endpoint hardening / application control: The attack vector is local, so enforce WDAC or AppLocker for user-writable paths (%USERPROFILE%\Downloads, %TEMP%, %APPDATA%). Also enable the ASR rules "Block executable files unless they meet a prevalence, age, or trusted list criterion" and "Block executable content from email client and webmail".
      • IAM / least privilege: Restrict interactive and RDP logon on servers to admin-tier accounts (SeRemoteInteractiveLogonRight, SeInteractiveLogonRight). Reduce standing local admin rights with LAPS and just-in-time elevation to shrink the foothold needed for local exploitation.
      • Network containment: When the Defender sensor goes silent, trigger automatic device isolation through MDE (automated investigation or an isolation playbook). Alternatively, move the device to a quarantine VLAN via NAC.
      • Detection redundancy: For hosts that cannot be updated quickly, tighten EDR heartbeat monitoring (alert after more than 30 min without telemetry). Prioritise network-side detection (proxy, DNS and Zeek telemetry) for those assets.

    3. Runbook step 3

      Detection rules

      • Scanner crash (Windows Application log): Event ID 1000/1001 with faulting application MsMpEng.exe or MpDefenderCoreService.exe. Example SPL: index=wineventlog EventCode=1000 Faulting_Application_Name="MsMpEng.exe" | stats count by host, Faulting_Module_Name
      • Service termination (System log, Service Control Manager): Event ID 7031/7034 for "Microsoft Defender Antivirus Service" (WinDefend). Example KQL: Event | where Source=="Service Control Manager" and EventID in (7031,7034) and RenderedDescription has "Microsoft Defender Antivirus"
      • Real-time protection failure (Microsoft-Windows-Windows Defender/Operational): Event IDs 5001 (RTP disabled), 3002 (RTP feature failure), 5010/5012 (scanning disabled) and 5007 (configuration change). The Sigma rule should alert on 3002 or 5001 when no authorised change (change ticket or maintenance window) precedes it.
      • Activity around the crash (Sysmon/EDR): Correlate Sysmon EID 11 (FileCreate) or MDE DeviceFileEvents in the 5 minutes before an MsMpEng.exe crash. Freshly dropped archives, scripts or binaries in user paths, followed by process creation (EID 1) from the same paths right after the outage, strongly indicate deliberate evasion.
      • Sensor silence (MDE): DeviceInfo | summarize LastSeen=max(Timestamp) by DeviceName | where LastSeen < ago(30m), filtered to devices that are still active according to DHCP or proxy telemetry.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Microsoft Defender Denial of Service Vulnerability

    Source: BSI CSAF, BSI WID Portal, Microsoft Security Update Guide, CISA KEV Catalog, cvelistv5, NVD (NIST), ENISA EUVD, CVE.org (MITRE), pwn.watch

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    MicrosoftMicrosoft Defender Antimalware Platform
    • 4.0.0.0 – < 4.18.26040.7
    • 4.18.26040.7
    microsoftdefender_antimalware_platform
    • < 4.18.26040.7
    • 4.18.26040.7
    MicrosoftDefender Antimalware Platform
    • < 4.18.26040.7
    • 4.18.26040.7

    Vendors and products

    Weakness classes (CWE)

    • CWE-400Uncontrolled Resource Consumption

      Class

      The product does not properly control the allocation and maintenance of a limited resource.

      Entry at MITRE CWE-400

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    PoC-in-GitHub (nomi-sec)Vulnerability-scanner (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    One NVD translation into another language is hidden.

    Linked advisories

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