Patch Gap

Windows · updated Oct 27, 2026

Windows: three bugs in Microsoft Defender, the built-in antivirus, were used in real attacks

Patched: update nowBeing attacked nowHigh

Last checked · Check before installing; only use the official link.

Microsoft Defender is the antivirus built into Windows. Three bugs in it let a bad program that is already on your PC take more control, or stop Defender from working. Microsoft fixed them in April and May 2026. Defender normally updates itself in the background.

Status
Patched: update now
Being attacked?
Yes[4]
Severity
High
Updated

CISA lists all three as used in real attacks (added Apr 22 and May 20, 2026). Severity: Patch Gap rating: high, because CISA lists it as used in real attacks (we do not copy a score we have not checked). First reported .

Am I affected?

You are at risk only if Defender has not updated since May 2026. Open Windows Security > Virus & threat protection > Protection updates and click Check for updates. If it finds nothing new, you are covered.

AppDeviceAffectedFixed in
Windows 10 and 11WindowsMicrosoft Defender on Windows 10 and 11 without the April and May 2026 Defender updatesFixed by Microsoft Defender updates released in April 2026 (CVE-2026-33825) and May 2026 (the other two). Defender installs these by itself; Microsoft's pages list the fixed Defender versions.[1][2][3]

How to check your version

These checks only look. They do not change anything on your computer.

Windows 10 and 11 on Windows

Easy way: Start > Settings > Windows Update. Then click Update history to see recent updates.

Or type this in PowerShellRead-only. It only reads the version and changes nothing.

Get-CimInstance Win32_OperatingSystem | Select-Object Caption,Version,BuildNumber; Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5 HotFixID,InstalledOn

How to fix it

  1. Open Start and type Windows Security, then open it.
  2. Click Virus & threat protection.
  3. Under Virus & threat protection updates, click Protection updates.
  4. Click Check for updates.
  5. Also run Windows Update (Settings > Windows Update) and restart if asked.

More on each app: Is my Windows up to date?

Official links

Check before installing, and only use these official links or the app's own update button.

For AI agents

This page as data: /incidents/microsoft-defender-attacked-bugs-2026.json. All problems: /feed.json. Use read-only checks only, and ask before installing anything.

Tracking numbers (CVE): CVE-2026-33825, CVE-2026-41091, CVE-2026-45498

Sources

  1. MakerMicrosoft Security Update Guide: CVE-2026-33825 (April 2026 update) (Apr 14, 2026)
  2. MakerMicrosoft Security Update Guide: CVE-2026-41091 (May 2026 update) (May 12, 2026)
  3. MakerMicrosoft Security Update Guide: CVE-2026-45498 (May 2026 update) (May 12, 2026)
  4. CISACISA: Known Exploited Vulnerabilities catalog entry for CVE-2026-33825 (added 2026-04-22) (Apr 22, 2026)
  5. CISACISA: Known Exploited Vulnerabilities catalog entry for CVE-2026-41091, CVE-2026-45498 (added 2026-05-20) (May 20, 2026)

"Maker", "CISA", "NVD", "GitHub advisory" and "Official notice" are official sources. "Researcher" and "News" are extra context.