How to update Windows Defender virus definitions

If Microsoft Defender says its virus protection is out of date, the usual fix is to force a manual security intelligence update. Start in Windows Security, then use Windows Update if that fails, and fall back to the PowerShell `Update-MpSignature` command if needed. If updates still won’t install, the usual causes are no internet access, a third-party antivirus conflict, or a disabled update service.

What it looks like when Defender definitions are behind

You’re probably here because Windows says virus protection is out of date, Microsoft Defender Antivirus says security intelligence is stale, or the Virus & threat protection page shows an old update time. You may also see an error after clicking an update button, or Defender may look fine until it tries to scan and then report that it can’t update its protection files.

The exact wording can vary by Windows version, but the pattern is the same: the definition date is old, automatic updates stopped working, or the update action fails right away. If that matches what you’re seeing, the steps below are the right place to start.

When this fix applies and when it doesn’t

These steps apply to Microsoft Defender Antivirus on supported versions of Windows, including Windows 10 and Windows 11. You’ll need an administrator account for the fallback command steps, and some actions may ask for permission before they run.

If you have a third-party antivirus installed and it has replaced Defender, Defender may be in passive mode or turned off. In that case, updating Defender definitions may not help until you remove or disable the other antivirus product. Before you change anything, make sure the PC is online and check whether Windows Update is paused, because both issues can stop definition updates.

A restart is usually not needed for a normal definition update, but it can help if Defender services are stuck.

Update the virus definitions in Windows Security

This is the first and safest fix. Open Windows Security and force Defender to check for new security intelligence.

1. Open Windows Security.
2. Go to Virus & threat protection.
3. Under Virus & threat protection updates, select Check for updates.

Windows Security > Virus & threat protection > Virus & threat protection updates > Check for updates

If the update succeeds, Defender should download the latest security intelligence and clear the out-of-date warning after a short delay. If nothing happens, or you get an error, move to Windows Update next. If Windows Security says it is already up to date but the warning still shows, refresh the page or wait a minute; sometimes the status lags behind the actual download.

Use Windows Update or Microsoft Update Catalog if the built-in update fails

If Windows Security won’t update Defender, the next place to check is Windows Update. Defender security intelligence can arrive through the normal update channel, and a pending Windows Update can block or delay it.

1. Open Settings.
2. Go to Windows Update.
3. Select Check for updates.

Settings > Windows Update > Check for updates

If Windows Update finds and installs something, check Defender again afterward. Sometimes the security intelligence update is bundled with another update or clears after the system finishes applying pending items.

If that still doesn’t fix it, use the Microsoft Defender update command from an elevated PowerShell window.

1. Right-click Start and open Windows Terminal (Admin) or PowerShell (Admin).
2. Run the update command.

Update-MpSignature

This tells Defender to pull the latest security intelligence directly. If it completes without errors, recheck Windows Security. If you get an error, the exact message varies, but it usually points to a connection problem, a disabled service, or another antivirus product interfering.

If the command also fails, the next fallback is Microsoft Update Catalog. Search there for the current Microsoft Defender Antivirus security intelligence update for your Windows version and install it manually. Use this only if the built-in update paths keep failing and you need to refresh definitions right away.

Confirm the definitions actually changed

Don’t stop at the update button. Check that the security intelligence version or last update time actually changed.

1. Open Windows Security.
2. Go to Virus & threat protection.
3. Open Protection updates or Virus & threat protection updates, depending on your version.
4. Confirm that the security intelligence date is current and the status no longer says out of date.

Security intelligence version: updated recently
Last updated: current date/time

If the date changed and the warning disappeared, the update worked. If the version still looks stale after a successful command or Windows Update run, close and reopen Windows Security once, then check again. A short delay is normal, but the information should eventually match the latest update time.

What to try if updates still won’t install

When Defender updates keep failing, work through the common blockers in a safe order.

• No internet: make sure the PC can reach the web, not just a local network. Try another site or switch from Wi-Fi to Ethernet if you can.
• Third-party antivirus: if another antivirus is installed, it may block Defender updates or keep Defender turned off. Update that product, or remove it if you want Defender to take over again.
• Paused or broken Windows Update: open Windows Update and make sure updates aren’t paused. If they are, resume them and try again.
• Stopped services: if Windows Update or Defender-related services are disabled, updates may fail until they are running again.

If you want to check the Windows Update service from an elevated PowerShell window, use this:

Get-Service wuauserv,bits,winmgmt,WinDefend

If one of those services is stopped, that helps explain why updates won’t install. If the service is disabled, set it back to its normal startup behavior before trying the Defender update again. If Windows Security still won’t update after the network, antivirus, and service checks, restart the PC and run the update command again, because a stuck service sometimes clears only after reboot.

Conclusion

Try the Windows Security update first, then Windows Update, then the PowerShell signature update if the first two fail. If the warning still won’t clear, fix the network, any third-party antivirus conflict, or the update services before trying again.

Frequently Asked Questions

Can I update Microsoft Defender definitions without restarting Windows?

Usually yes. A restart is not normally required for a definition update, though it can help if the update services are stuck.

Why does Defender say it is up to date but still shows an old date?

Sometimes the status screen lags behind the actual update. Refresh Windows Security, reopen it, or check the security intelligence version again after a short wait.

Does a third-party antivirus stop Defender updates?

It can. Some antivirus products disable Defender or interfere with its update path, so Defender may not update until the other product is removed or reconfigured.