How to open an app that Windows security balks at

Windows Security (Microsoft Defender Antivirus and related protections) is designed to block apps that appear untrusted or potentially unsafe. When an installer from a site such as thonny.com is blocked, the safest approach is to:

1. Confirm the source is trusted
   • Only download installers from the official site or a reputable store.
   • On Windows, prefer Microsoft Store apps when available, because they are vetted and install without SmartScreen warnings.
2. Download the installer correctly
   • In a browser, select the download link.
   • Choose Save or Save as so the file is stored locally (for example, in Downloads). During download, Microsoft Defender Antivirus typically scans the file for known threats.
3. Let Microsoft Defender Antivirus scan the file
   • In File Explorer, right‑click the downloaded installer.
   • Select Scan with Microsoft Defender.
   • If the scan reports no threats, the file is considered clean by Defender’s current signatures.
4. Handle Windows Security/SmartScreen prompts
   • When running the installer, Windows may show a warning because the app is new or not widely recognized.
   • If the warning is from SmartScreen but Defender has not detected malware, and the app is from a trusted source, use the on‑screen option (such as More info → Run anyway) to proceed one time.
5. Keep protections enabled
   • Do not permanently disable Microsoft Defender Antivirus or Windows Security features just to run an installer.
   • If another antivirus is installed and active, it may also block the app; in that case, use its allow‑list/exception mechanism for a one‑time install instead of turning protection off.

These steps allow a one‑time install while still using Windows Security to verify that the file is not known malware.

References:

• Windows Security App Overview
• Stay Protected With the Windows Security App
• How to install programs from online sources on Windows