Reject open redirection in the console proxy

Our console proxies (novnc, serial, spice) run in a websockify server
whose request handler inherits from the python standard
SimpleHTTPRequestHandler. There is a known issue [1] in the
SimpleHTTPRequestHandler which allows open redirects by way of URLs
in the following format:

  http://vncproxy.my.domain.com//example.com/%2F..

which if visited, will redirect a user to example.com.

We can intercept a request and reject requests that pass a redirection
URL beginning with "//" by implementing the
SimpleHTTPRequestHandler.send_head() method containing the
vulnerability to reject such requests with a 400 Bad Request.

This code is copied from a patch suggested in one of the issue comments
[2].

Closes-Bug: #1927677

[1] https://bugs.python.org/issue32084
[2] https://bugs.python.org/issue32084#msg306545

Change-Id: Ie36401c782f023d1d5f2623732619105dc2cfa24

nova/console/websocketproxy.py

@@ -20,6 +20,8 @@ Leverages websockify.py by Joel Martin
 
 import copy
 from http import cookies as Cookie
+from http import HTTPStatus
+import os
 import socket
 from urllib import parse as urlparse
 
@@ -271,6 +273,27 @@ class NovaProxyRequestHandler(websockify.ProxyRequestHandler):
     def socket(self, *args, **kwargs):
         return websockifyserver.WebSockifyServer.socket(*args, **kwargs)
 
+    def send_head(self):
+        # This code is copied from this example patch:
+        # https://bugs.python.org/issue32084#msg306545
+        path = self.translate_path(self.path)
+        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('//'):
+                    self.send_error(HTTPStatus.BAD_REQUEST,
+                                    "URI must not start with //")
+                    return None
+
+        return super(NovaProxyRequestHandler, self).send_head()
+
 
 class NovaWebSocketProxy(websockify.WebSocketProxy):
     def __init__(self, *args, **kwargs):

nova/tests/unit/console/test_websocketproxy.py

@@ -588,6 +588,40 @@ class NovaProxyRequestHandlerTestCase(test.NoDBTestCase):
         self.wh.socket.assert_called_with('node1', 10000, connect=True)
         self.wh.do_proxy.assert_called_with('<socket>')
 
+    def test_reject_open_redirect(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

releasenotes/notes/console-proxy-reject-open-redirect-4ac0a7895acca7eb.yaml

@@ -0,0 +1,19 @@
+---
+security:
+  - |
+    A vulnerability in the console proxies (novnc, serial, spice) that allowed
+    open redirection has been `patched`_. The novnc, serial, and spice console
+    proxies are implemented as websockify servers and the request handler
+    inherits from the python standard SimpleHTTPRequestHandler. There is a
+    `known issue`_ in the SimpleHTTPRequestHandler which allows open redirects
+    by way of URLs in the following format::
+
+      http://vncproxy.my.domain.com//example.com/%2F..
+
+    which if visited, will redirect a user to example.com.
+
+    The novnc, serial, and spice console proxies will now reject requests that
+    pass a redirection URL beginning with "//" with a 400 Bad Request.
+
+    .. _patched: https://bugs.launchpad.net/nova/+bug/1927677
+    .. _known issue: https://bugs.python.org/issue32084