Flask Security Scanner
Flask gives you nothing secure by default and the deployment checklist is short - but skipping any item creates a real vulnerability. DEBUG=True in production turns your 500 error page into a remote code execution vector. Missing security headers leave every visitor exposed to clickjacking and XSS escalation. Our scanner tests each item from outside your server.
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What the scanner checks
DEBUG=True and Werkzeug debugger
Triggers a 404 on a non-existent path and inspects the error page for Werkzeug stack traces, environment variable dumps, and the interactive PIN-based debugger. The Werkzeug debugger allows arbitrary Python execution - it is RCE, not just information disclosure.
Session cookie security flags
Inspects the Flask session cookie for Secure (HTTPS-only), HttpOnly (no JavaScript access), and SameSite (CSRF mitigation) flags. Flask does not set Secure by default. Missing flags expose session tokens to network interception and JavaScript-based theft.
CSRF protection via flask-wtf
Tests form-accepting POST endpoints for CSRF token validation. Sends a request without a token and checks the response code. A 200 or 302 instead of 400/403 indicates CSRF protection is absent, meaning any website can submit forms on behalf of your authenticated users.
Security headers via flask-talisman
Flask ships with zero security headers. Checks for HSTS (prevents HTTP downgrade), Content-Security-Policy (blocks XSS escalation), X-Frame-Options (prevents clickjacking), X-Content-Type-Options (blocks MIME sniffing), and Referrer-Policy. All require flask-talisman or manual middleware.
Exposed sensitive files
Tests for publicly accessible .env, requirements.txt, config.py, /console, and /_debug URLs. Flask's development server serves from the project root by default. Moving to production with the wrong working directory or misconfigured WSGI config exposes these files.
SSL/TLS and HTTPS enforcement
Checks certificate validity, expiry, and whether HTTP requests are redirected to HTTPS. Flask itself handles neither - this comes from your WSGI host (gunicorn + nginx, Railway, Heroku). The scanner verifies the enforcement is actually in place.
Flask Is Secure-by-Configuration, Not Secure-by-Default
Flask's design philosophy is explicit over implicit. It ships with the minimum to route requests and render templates - which means security is entirely opt-in. Jinja2 auto-escapes HTML by default (XSS protection), and the WSGI layer handles TLS. Everything else - CSRF protection, security headers, secure cookie flags, rate limiting - requires a deliberate choice and usually a third-party extension.
The number-one Flask production mistake is deploying with DEBUG=True. Werkzeug's debug error page shows the full Python call stack, local variable values, and environment variables - including any database password or API key stored in os.environ. Worse, the interactive console is executable: an attacker who can trigger a 500 error can run arbitrary Python on your server. Set DEBUG=False in production and use a proper logging setup instead.
The Flask security stack for a production deployment looks like this: flask-talisman for security headers and HTTPS enforcement, flask-wtf for CSRF token generation and validation, SESSION_COOKIE_SECURE=True and SESSION_COOKIE_HTTPONLY=True for session security, and a strong random SECRET_KEY generated with secrets.token_hex(32). Our scanner tests each of these from the outside - no credentials needed.
Frequently Asked Questions
How do I check if DEBUG is False in my Flask app?
Trigger an error by visiting /nonexistent-path. If the response is an HTML page with a Python stack trace, file paths, and a console prompt, DEBUG=True. If it returns a plain 404 or your custom error template with no technical detail, DEBUG=False. Set DEBUG=False explicitly via app.config['DEBUG'] = False or FLASK_DEBUG=0 in your environment - never rely on defaults in production.
Is the Werkzeug debugger safe to run in production?
No. The Werkzeug interactive debugger is a remote code execution surface. It is PIN-protected, but the PIN is derived from predictable system values and has been bruteforced and bypassed in documented attacks (CVE-2016-10516 and variations). It is a development tool only. Set DEBUG=False and USE_DEBUGGER=False in production. Never expose port 5000 (Flask's default) to the internet.
How do I add security headers to Flask?
Install flask-talisman: pip install flask-talisman. Then wrap your app: from flask_talisman import Talisman; Talisman(app, content_security_policy={'default-src': ['self']}). This enables HSTS, forces HTTPS, and sets X-Content-Type-Options and X-Frame-Options automatically. Customize the CSP dict for your specific asset sources. Without talisman, Flask returns no security headers.
How do I add CSRF protection to Flask?
Install flask-wtf: pip install flask-wtf. Set WTF_CSRF_ENABLED=True and a strong SECRET_KEY. Add {{ form.hidden_tag() }} to every form template, or use CSRFProtect(app) for AJAX endpoints. Test it by sending a POST request without the csrf_token field - it should return 400 Bad Request. Without this, any website can silently submit forms on behalf of your logged-in users.
What SECRET_KEY should I use for Flask?
Generate it with Python: import secrets; print(secrets.token_hex(32)). This produces 64 hex characters (256 bits) of cryptographic randomness. Store it as an environment variable (SECRET_KEY in your .env or hosting config) - never hard-code it in source code. Flask uses SECRET_KEY to sign session cookies: a weak or default key ('dev', 'changeme') lets attackers forge session tokens and impersonate any user.
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