Fix and changelog re #1283

paramiko/auth_handler.py

@@ -676,17 +676,39 @@ def _handle_local_gss_failure(self, e):
             self.auth_event.set()
         return
 
-    _handler_table = {
+    # TODO: do the same to the other tables, in Transport.
+    # TODO 3.0: MAY make sense to make these tables into actual
+    # classes/instances that can be fed a mode bool or whatever. Or,
+    # alternately (both?) make the message types small classes or enums that
+    # embed this info within themselves (which could also then tidy up the
+    # current 'integer -> human readable short string' stuff in common.py).
+    # TODO: if we do that, also expose 'em publicly.
+
+    # Messages which should be handled _by_ servers (sent by clients)
+    _server_handler_table = {
         MSG_SERVICE_REQUEST: _parse_service_request,
-        MSG_SERVICE_ACCEPT: _parse_service_accept,
         MSG_USERAUTH_REQUEST: _parse_userauth_request,
+        MSG_USERAUTH_INFO_RESPONSE: _parse_userauth_info_response,
+    }
+
+    # Messages which should be handled _by_ clients (sent by servers)
+    _client_handler_table = {
+        MSG_SERVICE_ACCEPT: _parse_service_accept,
         MSG_USERAUTH_SUCCESS: _parse_userauth_success,
         MSG_USERAUTH_FAILURE: _parse_userauth_failure,
         MSG_USERAUTH_BANNER: _parse_userauth_banner,
         MSG_USERAUTH_INFO_REQUEST: _parse_userauth_info_request,
-        MSG_USERAUTH_INFO_RESPONSE: _parse_userauth_info_response,
     }
 
+    # NOTE: prior to the fix for #1283, this was a static dict instead of a
+    # property. Should be backwards compatible in most/all cases.
+    @property
+    def _handler_table(self):
+        if self.transport.server_mode:
+            return self._server_handler_table
+        else:
+            return self._client_handler_table
+
 
 class GssapiWithMicAuthHandler(object):
     """A specialized Auth handler for gssapi-with-mic
@@ -782,9 +804,15 @@ def _parse_userauth_request(self, m):
         self._restore_delegate_auth_handler()
         return self._delegate._parse_userauth_request(m)
 
-    _handler_table = {
+    __handler_table = {
         MSG_SERVICE_REQUEST: _parse_service_request,
         MSG_USERAUTH_REQUEST: _parse_userauth_request,
         MSG_USERAUTH_GSSAPI_TOKEN: _parse_userauth_gssapi_token,
         MSG_USERAUTH_GSSAPI_MIC: _parse_userauth_gssapi_mic,
     }
+
+    @property
+    def _handler_table(self):
+        # TODO: determine if we can cut this up like we did for the primary
+        # AuthHandler class.
+        return self.__handler_table

sites/www/changelog.rst

@@ -7,6 +7,18 @@ Changelog
   This behavior probably didn't cause any outright errors, but it doesn't seem
   to conform to the RFCs and could cause (non-infinite) feedback loops in some
   scenarios (usually those involving Paramiko on both ends).
+- :bug:`1283 (1.17+)` Fix exploit (CVE pending) in Paramiko's server mode
+  (**not** client mode) where hostile clients could trick the server into
+  thinking they were authenticated without actually submitting valid
+  authentication.
+
+  Specifically, steps have been taken to start separating client and server
+  related message types in the message handling tables within ``Transport`` and
+  ``AuthHandler``; this work is not complete but enough has been performed to
+  close off this particular exploit (which was the only obvious such exploit
+  for this particular channel).
+
+  Thanks to Daniel Hoffman for the detailed report.
 - :support:`1292 backported` Backport changes from :issue:`979` (added in
   Paramiko 2.3) to Paramiko 2.0-2.2, using duck-typing to preserve backwards
   compatibility. This allows these older versions to use newer Cryptography

tests/test_transport.py

@@ -42,6 +42,7 @@
     ChannelException,
     Packetizer,
     Channel,
+    AuthHandler,
 )
 from paramiko import AUTH_FAILED, AUTH_SUCCESSFUL
 from paramiko import OPEN_SUCCEEDED, OPEN_FAILED_ADMINISTRATIVELY_PROHIBITED
@@ -54,8 +55,11 @@
     MAX_WINDOW_SIZE,
     DEFAULT_WINDOW_SIZE,
     DEFAULT_MAX_PACKET_SIZE,
+    MSG_NAMES,
+    MSG_UNIMPLEMENTED,
+    MSG_USERAUTH_SUCCESS,
 )
-from paramiko.py3compat import bytes
+from paramiko.py3compat import bytes, byte_chr
 from paramiko.message import Message
 
 from .util import needs_builtin, _support, slow
@@ -1052,3 +1056,48 @@ def test_server_does_not_respond_to_MSG_UNIMPLEMENTED(self):
 
     def test_client_does_not_respond_to_MSG_UNIMPLEMENTED(self):
         self._send_unimplemented(server_is_sender=True)
+
+    def _send_client_message(self, message_type):
+        self.setup_test_server(connect_kwargs={})
+        self.ts._send_message = Mock()
+        # NOTE: this isn't 100% realistic (most of these message types would
+        # have actual other fields in 'em) but it suffices to test the level of
+        # message dispatch we're interested in here.
+        msg = Message()
+        # TODO: really not liking the whole cMSG_XXX vs MSG_XXX duality right
+        # now, esp since the former is almost always just byte_chr(the
+        # latter)...but since that's the case...
+        msg.add_byte(byte_chr(message_type))
+        self.tc._send_message(msg)
+        # No good way to actually wait for server action (see above tests re:
+        # MSG_UNIMPLEMENTED). Grump.
+        time.sleep(0.1)
+
+    def _expect_unimplemented(self):
+        # Ensure MSG_UNIMPLEMENTED was sent (implies it hit end of loop instead
+        # of truly handling the given message).
+        # NOTE: When bug present, this will actually be the first thing that
+        # fails (since in many cases actual message handling doesn't involve
+        # sending a message back right away).
+        assert self.ts._send_message.call_count == 1
+        reply = self.ts._send_message.call_args[0][0]
+        reply.rewind()  # Because it's pre-send, not post-receive
+        assert reply.get_byte() == cMSG_UNIMPLEMENTED
+
+    def test_server_transports_reject_client_message_types(self):
+        # TODO: handle Transport's own tables too, not just its inner auth
+        # handler's table. See TODOs in auth_handler.py
+        for message_type in AuthHandler._client_handler_table:
+            self._send_client_message(message_type)
+            self._expect_unimplemented()
+            # Reset for rest of loop
+            self.tearDown()
+            self.setUp()
+
+    def test_server_rejects_client_MSG_USERAUTH_SUCCESS(self):
+        self._send_client_message(MSG_USERAUTH_SUCCESS)
+        # Sanity checks
+        assert not self.ts.authenticated
+        assert not self.ts.auth_handler.authenticated
+        # Real fix's behavior
+        self._expect_unimplemented()