Set security headers on all responses

The app set no security headers at all, and the reverse proxy sets none
either.

- Content-Security-Policy: default-src 'none' with 'self' only for
  styles, images and fonts (MAX has no JavaScript), form-action 'self'
  (also blocks the http:// form action downgrade), frame-ancestors
  'none' and base-uri 'none'.
- X-Frame-Options: DENY, X-Content-Type-Options: nosniff,
  Referrer-Policy: same-origin.
- Strict-Transport-Security with max-age of 7 days, not in debug mode.

The headers are set in after_request, so they also apply to static
files, 404 and error responses.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

max/app.py

@@ -16,6 +16,7 @@ from .middlewares import (
     add_middleware_stack,
 )
 from .routes import init_routes
+from .security_headers import init_security_headers
 from .sessions import SessionInterface
 from .translations import TranslationHandler
 
@@ -64,6 +65,7 @@ def main():
     register_cli(app)
     init_auth(app)
     init_routes(app)
+    init_security_headers(app)
     return app
 
 

max/security_headers.py

@@ -0,0 +1,29 @@
+CONTENT_SECURITY_POLICY = "; ".join(
+    (
+        "default-src 'none'",
+        "style-src 'self'",
+        "img-src 'self'",
+        "font-src 'self'",
+        "form-action 'self'",
+        "frame-ancestors 'none'",
+        "base-uri 'none'",
+    )
+)
+HSTS_MAX_AGE = 7 * 24 * 60 * 60  # in seconds
+
+
+def init_security_headers(app):
+    headers = {
+        "Content-Security-Policy": CONTENT_SECURITY_POLICY,
+        "X-Frame-Options": "DENY",
+        "X-Content-Type-Options": "nosniff",
+        "Referrer-Policy": "same-origin",
+    }
+    if not app.debug:
+        headers["Strict-Transport-Security"] = f"max-age={HSTS_MAX_AGE}"
+
+    # after_request also covers static files, 404 and error responses
+    @app.after_request
+    def set_security_headers(response):
+        response.headers.update(headers)
+        return response