Add authentication to thirdparty bridge APIs

changelog.d/12746.bugfix

@@ -0,0 +1 @@
+Require an `access_token` in `/thirdparty/` requests to appservices, as required by the [Matrix specification](https://spec.matrix.org/v1.1/application-service-api/#third-party-networks).

synapse/appservice/api.py

@@ -14,7 +14,7 @@
 # limitations under the License.
 import logging
 import urllib.parse
-from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Tuple
+from typing import TYPE_CHECKING, Any, Dict, Iterable, List, Mapping, Optional, Tuple
 
 from prometheus_client import Counter
 from typing_extensions import TypeGuard
@@ -155,14 +155,21 @@ async def query_3pe(
         if service.url is None:
             return []
 
+        # This is required by the configuration.
+        assert service.hs_token is not None
+
         uri = "%s%s/thirdparty/%s/%s" % (
             service.url,
             APP_SERVICE_PREFIX,
             kind,
             urllib.parse.quote(protocol),
         )
         try:
-            response = await self.get_json(uri, fields)
+            args: Mapping[Any, Any] = {
+                **fields,
+                b"access_token": [service.hs_token],
+            }
+            response = await self.get_json(uri, args=args)
             if not isinstance(response, list):
                 logger.warning(
                     "query_3pe to %s returned an invalid response %r", uri, response
@@ -190,13 +197,15 @@ async def get_3pe_protocol(
             return {}
 
         async def _get() -> Optional[JsonDict]:
+            # This is required by the configuration.
+            assert service.hs_token is not None
             uri = "%s%s/thirdparty/protocol/%s" % (
                 service.url,
                 APP_SERVICE_PREFIX,
                 urllib.parse.quote(protocol),
             )
             try:
-                info = await self.get_json(uri)
+                info = await self.get_json(uri, {"access_token": service.hs_token})
 
                 if not _is_valid_3pe_metadata(info):
                     logger.warning(