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OSError: [Errno 13] Permission denied when binding to port

7 Dec 2025

1 min read

python flask errors

OSError: Permission denied on port

$ flask --app app.py run -h 0.0.0.0 -p 80
OSError: [Errno 13] Permission denied

Why this happens

On Unix-like systems, ports below 1024 require elevated privileges. Running Flask on port 80 or 443 without appropriate permissions triggers Permission denied.

Fix

Use a high port (e.g., 8000) during development, or run behind a reverse proxy (nginx) that listens on 80/443 and forwards to your Flask app. Avoid running your app with sudo—it’s unsafe.

Wrong code

# run.sh
flask --app app.py run -p 80

Fixed code

# run.sh
flask --app app.py run -p 8000
# In production, use a real WSGI server (gunicorn/uwsgi) behind nginx.

Notes

  • For containers, expose 80 externally while your app listens on an unprivileged internal port.
  • Security first: don’t grant root privileges to your dev server.