URL redirect max length check should check %-encoded value

[Jacob Walls]

In #36767 we made configurable the length check on redirect URLs we added for CVE-2025-64458. "Cowork" sent us a flurry of nuisance security reports last night, but among them was a reasonable suggestion that we apply the length check against the percent-encoded URI:

django/http/response.py

@@ -641 +641 @@
         **kwargs,
     ):
         super().__init__(*args, **kwargs)
-        self["Location"] = iri_to_uri(redirect_to)
         redirect_to_str = str(redirect_to)
-        if max_length is not None and len(redirect_to_str) > max_length:
+        location = iri_to_uri(redirect_to_str)
+        if max_length is not None and len(location) > max_length:
             raise DisallowedRedirect(
                 f"Unsafe redirect exceeding {max_length} characters"
             )
+        self["Location"] = location
         parsed = urlsplit(redirect_to_str)
         if preserve_request:
             self.status_code = self.status_code_preserve_request

tests/httpwrappers/tests.py

@@ -24 +24 @@
     parse_cookie,
 )
 from django.test import SimpleTestCase
+from django.utils.encoding import iri_to_uri
 from django.utils.functional import lazystr
 from django.utils.http import MAX_URL_REDIRECT_LENGTH
 
@@ -498 +499 @@
                 response = response_class(long_url)
                 self.assertEqual(response.url, long_url)
 
+    def test_redirect_url_max_length_checks_encoded_location(self):
+        long_url = "/" + "é" * (MAX_URL_REDIRECT_LENGTH - 1)
+        self.assertLessEqual(len(long_url), MAX_URL_REDIRECT_LENGTH)
+        self.assertGreater(len(iri_to_uri(long_url)), MAX_URL_REDIRECT_LENGTH)
+        for response_class in (HttpResponseRedirect, HttpResponsePermanentRedirect):
+            with (
+                self.subTest(response_class=response_class),
+                self.assertRaisesMessage(
+                    DisallowedRedirect,
+                    f"Unsafe redirect exceeding {MAX_URL_REDIRECT_LENGTH} characters",
+                ),
+            ):
+                response_class(long_url)
+
+    def test_redirect_url_max_length_allows_encoded_location_at_limit(self):
+        redirect_to = "/" + "é" * ((MAX_URL_REDIRECT_LENGTH - 1) // 6)
+        location = iri_to_uri(redirect_to)
+        self.assertLessEqual(len(location), MAX_URL_REDIRECT_LENGTH)
+        for response_class in (HttpResponseRedirect, HttpResponsePermanentRedirect):
+            with self.subTest(response_class=response_class):
+                response = response_class(redirect_to)
+                self.assertEqual(response.url, location)
+
     def test_redirect_url_max_length_override_via_param(self):
         base_url = "https://example.com/"
         for (max_length, length), response_class in itertools.product(

Although we didn't take this as a security issue (the limit we apply on the raw value is good enough for DoS), this seems like a functionality bug to fix before the release.

[Jacob Walls]

​PR

[Jacob Walls <jacobtylerwalls@…>]

In 335c6d01:

Fixed #37095 -- Checked maximum redirect lengths against percent-encoded URLs.