SSHClient: fix the host key test

Skip the host key check only, if the transport actually used
gssapi-keyex. Add tests for the missing-host-key RejectPolicy.

Before this change, a man-in-the-middle attack on the paramiko ssh
client with gss_kex=True was possible by having a server that does not
support gssapi-keyex and gives any or no host key.

paramiko/client.py

@@ -353,7 +353,7 @@ def connect(
         # If GSS-API Key Exchange is performed we are not required to check the
         # host key, because the host is authenticated via GSS-API / SSPI as
         # well as our client.
-        if not self._transport.use_gss_kex:
+        if not self._transport.gss_kex_used:
             our_server_key = self._system_host_keys.get(
                 server_hostkey_name, {}).get(keytype)
             if our_server_key is None:

tests/test_client.py

@@ -141,6 +141,7 @@ def _test_connection(self, **kwargs):
         self.assertTrue(self.ts.is_active())
         self.assertEqual('slowdive', self.ts.get_username())
         self.assertEqual(True, self.ts.is_authenticated())
+        self.assertEqual(False, self.tc.get_transport().gss_kex_used)
 
         # Command execution functions?
         stdin, stdout, stderr = self.tc.exec_command('yes')
@@ -366,3 +367,63 @@ def test_8_auth_trickledown(self):
             password='pygmalion',
         )
         self._test_connection(**kwargs)
+
+    def test_9_auth_trickledown_gsskex(self):
+        """
+        Failed gssapi-keyex auth doesn't prevent subsequent key auth from succeeding
+        """
+        if not paramiko.GSS_AUTH_AVAILABLE:
+            return  # for python 2.6 lacks skipTest
+        kwargs = dict(
+            gss_kex=True,
+            key_filename=[test_path('test_rsa.key')],
+        )
+        self._test_connection(**kwargs)
+
+    def test_10_auth_trickledown_gssauth(self):
+        """
+        Failed gssapi-with-mic auth doesn't prevent subsequent key auth from succeeding
+        """
+        if not paramiko.GSS_AUTH_AVAILABLE:
+            return  # for python 2.6 lacks skipTest
+        kwargs = dict(
+            gss_auth=True,
+            key_filename=[test_path('test_rsa.key')],
+        )
+        self._test_connection(**kwargs)
+
+    def test_11_reject_policy(self):
+        """
+        verify that SSHClient's RejectPolicy works.
+        """
+        threading.Thread(target=self._run).start()
+
+        self.tc = paramiko.SSHClient()
+        self.tc.set_missing_host_key_policy(paramiko.RejectPolicy())
+        self.assertEqual(0, len(self.tc.get_host_keys()))
+        self.assertRaises(
+            paramiko.SSHException,
+            self.tc.connect,
+            password='pygmalion', **self.connect_kwargs
+        )
+
+    def test_12_reject_policy_gsskex(self):
+        """
+        verify that SSHClient's RejectPolicy works,
+        even if gssapi-keyex was enabled but not used.
+        """
+        # Test for a bug present in paramiko versions released before 2017-08-01
+        if not paramiko.GSS_AUTH_AVAILABLE:
+            return  # for python 2.6 lacks skipTest
+        threading.Thread(target=self._run).start()
+
+        self.tc = paramiko.SSHClient()
+        self.tc.set_missing_host_key_policy(paramiko.RejectPolicy())
+        self.assertEqual(0, len(self.tc.get_host_keys()))
+        self.assertRaises(
+            paramiko.SSHException,
+            self.tc.connect,
+            password='pygmalion',
+            gss_kex=True,
+             **self.connect_kwargs
+        )