address open redirect with 3 forward slashes

Ie36401c782f023d1d5f2623732619105dc2cfa24 was intended
to address OSSA-2021-002 (CVE-2021-3654) however after its
release it was discovered that the fix only worked
for urls with 2 leading slashes or more then 4.

This change adresses the missing edgecase for 3 leading slashes
and also maintian support for rejecting 2+.

Change-Id: I95f68be76330ff09e5eabb5ef8dd9a18f5547866
co-authored-by: Matteo Pozza
Closes-Bug: #1927677

nova/console/websocketproxy.py

@@ -280,14 +280,9 @@ def send_head(self):
         if os.path.isdir(path):
             parts = urlparse.urlsplit(self.path)
             if not parts.path.endswith('/'):
-                # redirect browser - doing basically what apache does
-                new_parts = (parts[0], parts[1], parts[2] + '/',
-                              parts[3], parts[4])
-                new_url = urlparse.urlunsplit(new_parts)
-
                 # Browsers interpret "Location: //uri" as an absolute URI
                 # like "http://URI"
-                if new_url.startswith('//'):
+                if self.path.startswith('//'):
                     self.send_error(HTTPStatus.BAD_REQUEST,
                                     "URI must not start with //")
                     return None

nova/tests/unit/console/test_websocketproxy.py

@@ -622,6 +622,40 @@ def fake_sendall(data):
         self.assertIn('Error code: 400', self.data)
         self.assertIn('Message: URI must not start with //', self.data)
 
+    def test_reject_open_redirect_3_slashes(self):
+        # This will test the behavior when an attempt is made to cause an open
+        # redirect. It should be rejected.
+        mock_req = mock.MagicMock()
+        mock_req.makefile().readline.side_effect = [
+            b'GET ///example.com/%2F.. HTTP/1.1\r\n',
+            b''
+        ]
+
+        # Collect the response data to verify at the end. The
+        # SimpleHTTPRequestHandler writes the response data by calling the
+        # request socket sendall() method.
+        self.data = b''
+
+        def fake_sendall(data):
+            self.data += data
+
+        mock_req.sendall.side_effect = fake_sendall
+
+        client_addr = ('8.8.8.8', 54321)
+        mock_server = mock.MagicMock()
+        # This specifies that the server will be able to handle requests other
+        # than only websockets.
+        mock_server.only_upgrade = False
+
+        # Constructing a handler will process the mock_req request passed in.
+        websocketproxy.NovaProxyRequestHandler(
+            mock_req, client_addr, mock_server)
+
+        # Verify no redirect happens and instead a 400 Bad Request is returned.
+        self.data = self.data.decode()
+        self.assertIn('Error code: 400', self.data)
+        self.assertIn('Message: URI must not start with //', self.data)
+
     @mock.patch('nova.objects.ConsoleAuthToken.validate')
     def test_no_compute_rpcapi_with_invalid_token(self, mock_validate):
         """Tests that we don't create a ComputeAPI object until we actually