Check Chosen Version matches the version in use by the connection

When processing the remote Version Information transport parameter,
check the Chosen Version matches the version in use by the connection or
abort the connection with a VERSION_NEGOTIATION_ERROR.

We also fix a bug in our server implementation, the transport parameters
need to be updated to reflect the Chosen Version.

src/aioquic/quic/connection.py

@@ -1169,7 +1169,7 @@ def _alpn_handler(self, alpn_protocol: str) -> None:
             )
 
         # For servers, determine the Negotiated Version.
-        if not self._is_client:
+        if not self._is_client and not self._version_negotiated_compatible:
             if self._remote_version_information is not None:
                 # Pick the first version we support in the client's available versions,
                 # which is compatible with the current version.
@@ -1186,6 +1186,14 @@ def _alpn_handler(self, alpn_protocol: str) -> None:
                         self._cryptos[tls.Epoch.INITIAL] = self._cryptos_initial[
                             version
                         ]
+
+                        # Update our transport parameters to reflect the chosen version.
+                        self.tls.handshake_extensions = [
+                            (
+                                tls.ExtensionType.QUIC_TRANSPORT_PARAMETERS,
+                                self._serialize_transport_parameters(),
+                            )
+                        ]
                         break
             self._version_negotiated_compatible = True
             self._logger.info(
@@ -2697,25 +2705,6 @@ def _parse_transport_parameters(
                         reason_phrase="%s is not allowed for clients" % attr,
                     )
 
-            # If a server receives Version Information where the Chosen Version
-            # is not included in Available Versions, it MUST treat is as a
-            # parsing failure.
-            #
-            # https://datatracker.ietf.org/doc/html/rfc9368#section-4
-            if (
-                quic_transport_parameters.version_information is not None
-                and quic_transport_parameters.version_information.chosen_version
-                not in quic_transport_parameters.version_information.available_versions
-            ):
-                raise QuicConnectionError(
-                    error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
-                    frame_type=QuicFrameType.CRYPTO,
-                    reason_phrase=(
-                        "version_information's chosen_version is not included "
-                        "in available_versions"
-                    ),
-                )
-
         if not from_session_ticket:
             if (
                 quic_transport_parameters.initial_source_connection_id
@@ -2783,7 +2772,42 @@ def _parse_transport_parameters(
                     ),
                 )
 
-        # store remote parameters
+            # Validate Version Information extension.
+            #
+            # https://datatracker.ietf.org/doc/html/rfc9368#section-4
+            if quic_transport_parameters.version_information is not None:
+                version_information = quic_transport_parameters.version_information
+
+                # If a server receives Version Information where the Chosen Version
+                # is not included in Available Versions, it MUST treat is as a
+                # parsing failure.
+                if (
+                    not self._is_client
+                    and version_information.chosen_version
+                    not in version_information.available_versions
+                ):
+                    raise QuicConnectionError(
+                        error_code=QuicErrorCode.TRANSPORT_PARAMETER_ERROR,
+                        frame_type=QuicFrameType.CRYPTO,
+                        reason_phrase=(
+                            "version_information's chosen_version is not included "
+                            "in available_versions"
+                        ),
+                    )
+
+                # Validate that the Chosen Version matches the version in use for the
+                # connection.
+                if version_information.chosen_version != self._crypto_packet_version:
+                    raise QuicConnectionError(
+                        error_code=QuicErrorCode.VERSION_NEGOTIATION_ERROR,
+                        frame_type=QuicFrameType.CRYPTO,
+                        reason_phrase=(
+                            "version_information's chosen_version does not match "
+                            "the version in use"
+                        ),
+                    )
+
+        # Store remote parameters.
         if not from_session_ticket:
             if quic_transport_parameters.ack_delay_exponent is not None:
                 self._remote_ack_delay_exponent = self._remote_ack_delay_exponent

src/aioquic/quic/packet.py

@@ -41,6 +41,7 @@ class QuicErrorCode(IntEnum):
     CRYPTO_BUFFER_EXCEEDED = 0xD
     KEY_UPDATE_ERROR = 0xE
     AEAD_LIMIT_REACHED = 0xF
+    VERSION_NEGOTIATION_ERROR = 0x11
     CRYPTO_ERROR = 0x100
 
 

tests/test_connection.py

@@ -2652,7 +2652,7 @@ def test_parse_transport_parameters_with_bad_initial_source_connection_id(self):
             cm.exception.reason_phrase, "initial_source_connection_id does not match"
         )
 
-    def test_parse_transport_parameters_with_bad_version_information(self):
+    def test_parse_transport_parameters_with_bad_version_information_1(self):
         server = create_standalone_server(self)
         data = encode_transport_parameters(
             QuicTransportParameters(
@@ -2674,6 +2674,31 @@ def test_parse_transport_parameters_with_bad_version_information(self):
             "available_versions",
         )
 
+    def test_parse_transport_parameters_with_bad_version_information_2(self):
+        server = create_standalone_server(self)
+        data = encode_transport_parameters(
+            QuicTransportParameters(
+                version_information=QuicVersionInformation(
+                    chosen_version=QuicProtocolVersion.VERSION_1,
+                    available_versions=[
+                        QuicProtocolVersion.VERSION_1,
+                        QuicProtocolVersion.VERSION_2,
+                    ],
+                )
+            )
+        )
+        server._crypto_packet_version = QuicProtocolVersion.VERSION_2
+        with self.assertRaises(QuicConnectionError) as cm:
+            server._parse_transport_parameters(data)
+        self.assertEqual(
+            cm.exception.error_code, QuicErrorCode.VERSION_NEGOTIATION_ERROR
+        )
+        self.assertEqual(cm.exception.frame_type, QuicFrameType.CRYPTO)
+        self.assertEqual(
+            cm.exception.reason_phrase,
+            "version_information's chosen_version does not match the version in use",
+        )
+
     def test_parse_transport_parameters_with_server_only_parameter(self):
         server = create_standalone_server(self)
         for active_connection_id_limit in [0, 1]: