CVE-2026-45498: Microsoft Defender local denial of service
Tech GuruQuick Answer
CISA added CVE-2026-45498 to its Known Exploited Vulnerabilities catalog on 2026-05-20, confirming that exploitation has occurred. If you operate Microsoft Defender, identify the exact version and relevant configuration now, preserve security evidence, and follow the live vendor remediation. A KEV match does not prove your system was compromised, and installing an update does not rule out earlier exploitation.
Fix Or Immediate Action
What to do now: Verify that Microsoft Defender Antimalware Platform is version 4.18.26040.7 or later on every managed endpoint.
Last verified: 2026-09-17 UTC. CISA KEV snapshot version
2026.09.16. Recheck the live advisory before changing production.
Context
Scope And Impact In Plain Language
Who should check: Anyone who manages Microsoft Defender.
When this applies: Only when the exact product, version, and required configuration match the live vendor advisory. Owning the product family alone does not mean the device is vulnerable.
What could happen: An attacker may be able to crash, restart, or overload the affected service, causing an outage or loss of availability.
Not sure? Find the product name and version in the device or application, then compare them with the affected-products section of the linked vendor notice.
Status At A Glance
| Product scope | Microsoft — Defender |
|---|---|
| CVE | CVE-2026-45498 |
| TechGeeks response tier | P0 — confirm an exact match and act now |
| CISA KEV added | 2026-05-20 |
| Vendor severity | Low |
| CVSS | 4.0 (CVSS v3.1 base) |
| Ransomware field | Unknown — this is not the same as No; CISA has not marked the field Known. |
| Forensic-triage field | No in the current CISA field; organizations should still investigate when exposure or evidence warrants it. |
| CWE | Not listed in snapshot |
| Federal remediation date | 2026-06-03 — applies to covered federal agencies; other organizations should set a risk-based deadline. |
What Happened
Microsoft Defender Denial of Service Vulnerability. Microsoft Defender contains an unspecified vulnerability that allows for denial of service. CISA's KEV listing confirms exploitation evidence, but it does not state how widespread the activity is or prove that any particular installation was compromised.
Affected Scope
Microsoft Defender Antimalware Platform versions earlier than the fixed platform release identified by Microsoft.
Match the exact edition, hardware or virtual platform, release, patch level, enabled feature, and management exposure. A product-family name alone is not enough to establish applicability.
Steps
What To Do Now
- Confirm. Inventory every Defender asset and record its exact version, patch level, role, relevant configuration, and exposure.
- Preserve. Save UTC timestamps, off-device authentication, firewall, proxy, DNS, endpoint, application, and SIEM evidence before logs rotate or a disruptive change removes context.
- Contain. Restrict unnecessary internet and management-plane access using supported controls. Follow the live advisory: Open the live Microsoft advisory.
- Remediate. Verify that Microsoft Defender Antimalware Platform is version 4.18.26040.7 or later on every managed endpoint.
- Validate. Verify the resulting version or patch level, service health, logging, expected access, an expected denial, and representative dependent workflows.
- Escalate. If exposure overlapped the vulnerable period or evidence is suspicious, treat the system as a potential incident and follow vendor and incident-response recovery guidance.
Fixed Releases And Mitigation
Permanent Fix
Vendor-directed fix path: Verify that Microsoft Defender Antimalware Platform is version 4.18.26040.7 or later on every managed endpoint.
Verified fixed release, patch level, or update guidance captured at the last check:
- Microsoft Defender Antimalware Platform 4.18.26040.7 or later
Workaround Or Temporary Mitigation
Use the normal Microsoft Defender platform-update channel. No separate workaround is safely captured in the current research notes.
Important boundary: A workaround, access restriction, or provider-side control may reduce risk while the update is prepared, but it does not necessarily remove vulnerable code or erase an earlier compromise. Install the vendor-approved update when one is available.
Investigation And Recovery
CISA lists the vulnerability as known exploited. Review Defender health and service-availability telemetry for unexplained local denial-of-service events on hosts that ran an affected platform version.
When an attacker may have gained administrative, root, SYSTEM, identity, management-plane, or code-execution access, patching alone is not an eradication claim. Preserve evidence, scope affected accounts and downstream systems, rotate exposed credentials or certificates when justified, and rebuild from a trusted source when vendor or incident-response guidance requires it.
Notes
Exploitation Evidence
Vendor statement: Microsoft marked Exploited: Yes at original publication.
CISA statement: CVE-2026-45498 was added to KEV on 2026-05-20. The ransomware field is Unknown, and the forensic-triage field is No. These are separate signals.
Validation And Boundaries
This notice does not prove that every product carrying the family name is affected, that exploitation is widespread, that a clean indicator search rules out compromise, or that a successful update restored trust. TechGeeks has not reproduced the exploit and does not replace the vendor advisory, asset inventory, vulnerability scanner, or incident-response process.
Related TechGeeks Resources
- TechGeeks Security Notice Center
- All Security Notices
- What to Do When a Device Hits CISA KEV
- Router CVE Exposure, Version, and Mitigation Checklist
- IoT Firmware Update and End-of-Support Checklist
- Live Protect and Runtime Vulnerability Shielding
References
Wrap-Up
Record whether CVE-2026-45498 applies to your environment. If it does, assign an owner, preserve the change and investigation evidence, complete the vendor action, and document the validated result.
Correction policy: If a primary source changes affected versions, remediation, exploitation status, or recovery guidance, TechGeeks will update this page and its verification date.
