[1.6.x] Fixed #22996 -- Prevented crash with unencoded query string

Thanks Jorge Carleitao for the report and Aymeric Augustin, Tim Graham
for the reviews.
Backport of fa02120d36 from master.

4 files changed, 33 insertions, 10 deletions

diff --git a/django/core/handlers/wsgi.py b/django/core/handlers/wsgi.py
index adc8804b80..f9b2633bb6 100644
--- a/django/core/handlers/wsgi.py
+++ b/django/core/handlers/wsgi.py
@@ -134,7 +134,7 @@ class WSGIRequest(http.HttpRequest):
             # The WSGI spec says 'QUERY_STRING' may be absent.
             raw_query_string = self.environ.get('QUERY_STRING', str(''))
             if six.PY3:
-                raw_query_string = raw_query_string.encode('iso-8859-1').decode('utf-8')
+                raw_query_string = raw_query_string.encode('iso-8859-1')
             self._get = http.QueryDict(raw_query_string, encoding=self._encoding)
         return self._get
 
diff --git a/django/http/request.py b/django/http/request.py
index deb0724fc7..1a9a038b7e 100644
--- a/django/http/request.py
+++ b/django/http/request.py
@@ -290,8 +290,12 @@ class QueryDict(MultiValueDict):
         self.encoding = encoding
         if six.PY3:
             if isinstance(query_string, bytes):
-                # query_string contains URL-encoded data, a subset of ASCII.
-                query_string = query_string.decode()
+                # query_string normally contains URL-encoded data, a subset of ASCII.
+                try:
+                    query_string = query_string.decode(encoding)
+                except UnicodeDecodeError:
+                    # ... but some user agents are misbehaving :-(
+                    query_string = query_string.decode('iso-8859-1')
             for key, value in parse_qsl(query_string or '',
                                         keep_blank_values=True,
                                         encoding=encoding):
diff --git a/docs/releases/1.6.6.txt b/docs/releases/1.6.6.txt
index f3ff77dca0..f98aaa4176 100644
--- a/docs/releases/1.6.6.txt
+++ b/docs/releases/1.6.6.txt
@@ -40,3 +40,6 @@ Bugfixes
 * Fixed JavaScript errors while editing multi-geometry objects in the OpenLayers
   widget (`#23137 <https://code.djangoproject.com/ticket/23137>`_,
   `#23293 <https://code.djangoproject.com/ticket/23293>`_).
+
+* Prevented a crash on Python 3 with query strings containing unencoded
+  non-ASCII characters (`#22996 <http://code.djangoproject.com/ticket/22996>`_).
diff --git a/tests/handlers/tests.py b/tests/handlers/tests.py
index f5dc7c8531..572192efe9 100644
--- a/tests/handlers/tests.py
+++ b/tests/handlers/tests.py
@@ -42,14 +42,30 @@ class HandlerTests(TestCase):
         self.assertEqual(response.status_code, 400)
 
     def test_non_ascii_query_string(self):
-        """Test that non-ASCII query strings are properly decoded (#20530)."""
+        """
+        Test that non-ASCII query strings are properly decoded (#20530, #22996).
+        """
         environ = RequestFactory().get('/').environ
-        raw_query_string = 'want=café'
-        if six.PY3:
-            raw_query_string = raw_query_string.encode('utf-8').decode('iso-8859-1')
-        environ['QUERY_STRING'] = raw_query_string
-        request = WSGIRequest(environ)
-        self.assertEqual(request.GET['want'], "café")
+        raw_query_strings = [
+            b'want=caf%C3%A9', # This is the proper way to encode 'café'
+            b'want=caf\xc3\xa9', # UA forgot to quote bytes
+            b'want=caf%E9', # UA quoted, but not in UTF-8
+            b'want=caf\xe9', # UA forgot to convert Latin-1 to UTF-8 and to quote (typical of MSIE)
+        ]
+        got = []
+        for raw_query_string in raw_query_strings:
+            if six.PY3:
+                # Simulate http.server.BaseHTTPRequestHandler.parse_request handling of raw request
+                environ['QUERY_STRING'] = str(raw_query_string, 'iso-8859-1')
+            else:
+                environ['QUERY_STRING'] = raw_query_string
+            request = WSGIRequest(environ)
+            got.append(request.GET['want'])
+        if six.PY2:
+            self.assertListEqual(got, ['café', 'café', 'caf\ufffd', 'caf\ufffd'])
+        else:
+            # On Python 3, %E9 is converted to the unicode replacement character by parse_qsl
+            self.assertListEqual(got, ['café', 'café', 'caf\ufffd', 'café'])
 
     def test_non_ascii_cookie(self):
         """Test that non-ASCII cookies set in JavaScript are properly decoded (#20557)."""