feat(security): every page carries a Content-Security-Policy that needs no template change

A small middleware sends object-src 'none', base-uri 'self', form-action
'self' and frame-ancestors 'none': no plugin content, no injected <base>
redirecting relative URLs, no form posting to another origin, no framing.
script-src and style-src are left out: the templates still hold inline
scripts, event handlers and javascript: URLs, which an enforced script
policy would block.

The middleware is added to the settings template, which the update steps
and each Docker container rebuild settings.py from.

webvirtcloud/middleware.py

@@ -7,6 +7,8 @@ from operations import claims, jobs
 from operations.utils import busy_message, public_error_text, summary
 
 API_PREFIX = "/api/"
+# the directives that need no change to the templates' inline scripts and styles
+CONTENT_SECURITY_POLICY = "object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'"
 
 
 def error_text(request, exception):
@@ -21,6 +23,16 @@ def error_text(request, exception):
     return public_error_text(exception)
 
 
+class ContentSecurityPolicyMiddleware:
+    def __init__(self, get_response):
+        self.get_response = get_response
+
+    def __call__(self, request):
+        response = self.get_response(request)
+        response.setdefault("Content-Security-Policy", CONTENT_SECURITY_POLICY)
+        return response
+
+
 class ExceptionMiddleware:
     def __init__(self, get_response):
         self.get_response = get_response

webvirtcloud/settings.py.template

@@ -99,6 +99,7 @@ MIDDLEWARE = [
     "django.contrib.auth.middleware.RemoteUserMiddleware",
     "django.contrib.messages.middleware.MessageMiddleware",
     "django.middleware.clickjacking.XFrameOptionsMiddleware",
+    "webvirtcloud.middleware.ContentSecurityPolicyMiddleware",
     "appsettings.middleware.AppSettingsMiddleware",
     "webvirtcloud.middleware.ExceptionMiddleware",
     # last, as django-axes requires

webvirtcloud/tests/test_csp.py

@@ -0,0 +1,15 @@
+"""Every response carries the Content-Security-Policy directives that need no
+change to the templates' inline scripts and styles."""
+
+from django.test import TestCase
+from django.urls import reverse
+
+
+class ContentSecurityPolicyTests(TestCase):
+    def test_pages_and_redirects_carry_the_policy(self):
+        for url in (reverse("accounts:login"), reverse("index")):  # a page, and a redirect to the login
+            with self.subTest(url=url):
+                policy = self.client.get(url)["Content-Security-Policy"]
+                self.assertEqual(
+                    policy, "object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'"
+                )