Strip unauthorized fields from `unsigned` object in events received over federation (#11530)

* add some tests to verify we are stripping unauthorized fields out of unsigned

* add function to strip unauthorized fields from the unsigned object of event

* newsfragment

* update newsfragment number

* add check to on_send_membership_event

* refactor tests

* fix lint error

* slightly refactor tests and add some comments

* slight refactor

* refactor tests

* fix import error

* slight refactor

* remove unsigned filtration code from synapse/handlers/federation_event.py

* lint

* move unsigned filtering code to event base

* refactor tests

* update newsfragment

* requested changes

* remove unused retun values

3 files changed, 99 insertions, 0 deletions

diff --git a/changelog.d/11530.bugfix b/changelog.d/11530.bugfix
new file mode 100644
index 0000000000..7ea9ba4e49
--- /dev/null
+++ b/changelog.d/11530.bugfix
@@ -0,0 +1,2 @@
+Fix a long-standing issue which could cause Synapse to incorrectly accept data in the unsigned field of events
+received over federation.
\ No newline at end of file
diff --git a/synapse/federation/federation_base.py b/synapse/federation/federation_base.py
index addc0bf000..896168c05c 100644
--- a/synapse/federation/federation_base.py
+++ b/synapse/federation/federation_base.py
@@ -230,6 +230,10 @@ def event_from_pdu_json(pdu_json: JsonDict, room_version: RoomVersion) -> EventB
     # origin, etc etc)
     assert_params_in_dict(pdu_json, ("type", "depth"))
 
+    # Strip any unauthorized values from "unsigned" if they exist
+    if "unsigned" in pdu_json:
+        _strip_unsigned_values(pdu_json)
+
     depth = pdu_json["depth"]
     if not isinstance(depth, int):
         raise SynapseError(400, "Depth %r not an intger" % (depth,), Codes.BAD_JSON)
@@ -245,3 +249,24 @@ def event_from_pdu_json(pdu_json: JsonDict, room_version: RoomVersion) -> EventB
 
     event = make_event_from_dict(pdu_json, room_version)
     return event
+
+
+def _strip_unsigned_values(pdu_dict: JsonDict) -> None:
+    """
+    Strip any unsigned values unless specifically allowed, as defined by the whitelist.
+
+    pdu: the json dict to strip values from. Note that the dict is mutated by this
+    function
+    """
+    unsigned = pdu_dict["unsigned"]
+
+    if not isinstance(unsigned, dict):
+        pdu_dict["unsigned"] = {}
+
+    if pdu_dict["type"] == "m.room.member":
+        whitelist = ["knock_room_state", "invite_room_state", "age"]
+    else:
+        whitelist = ["age"]
+
+    filtered_unsigned = {k: v for k, v in unsigned.items() if k in whitelist}
+    pdu_dict["unsigned"] = filtered_unsigned
diff --git a/tests/test_federation.py b/tests/test_federation.py
index 3eef1c4c05..2b9804aba0 100644
--- a/tests/test_federation.py
+++ b/tests/test_federation.py
@@ -17,7 +17,9 @@ from unittest.mock import Mock
 from twisted.internet.defer import succeed
 
 from synapse.api.errors import FederationError
+from synapse.api.room_versions import RoomVersions
 from synapse.events import make_event_from_dict
+from synapse.federation.federation_base import event_from_pdu_json
 from synapse.logging.context import LoggingContext
 from synapse.types import UserID, create_requester
 from synapse.util import Clock
@@ -276,3 +278,73 @@ class MessageAcceptTests(unittest.HomeserverTestCase):
             "ed25519:" + remote_self_signing_key in self_signing_key["keys"].keys(),
         )
         self.assertTrue(remote_self_signing_key in self_signing_key["keys"].values())
+
+
+class StripUnsignedFromEventsTestCase(unittest.TestCase):
+    def test_strip_unauthorized_unsigned_values(self):
+        event1 = {
+            "sender": "@baduser:test.serv",
+            "state_key": "@baduser:test.serv",
+            "event_id": "$event1:test.serv",
+            "depth": 1000,
+            "origin_server_ts": 1,
+            "type": "m.room.member",
+            "origin": "test.servx",
+            "content": {"membership": "join"},
+            "auth_events": [],
+            "unsigned": {"malicious garbage": "hackz", "more warez": "more hackz"},
+        }
+        filtered_event = event_from_pdu_json(event1, RoomVersions.V1)
+        # Make sure unauthorized fields are stripped from unsigned
+        self.assertNotIn("more warez", filtered_event.unsigned)
+
+    def test_strip_event_maintains_allowed_fields(self):
+        event2 = {
+            "sender": "@baduser:test.serv",
+            "state_key": "@baduser:test.serv",
+            "event_id": "$event2:test.serv",
+            "depth": 1000,
+            "origin_server_ts": 1,
+            "type": "m.room.member",
+            "origin": "test.servx",
+            "auth_events": [],
+            "content": {"membership": "join"},
+            "unsigned": {
+                "malicious garbage": "hackz",
+                "more warez": "more hackz",
+                "age": 14,
+                "invite_room_state": [],
+            },
+        }
+
+        filtered_event2 = event_from_pdu_json(event2, RoomVersions.V1)
+        self.assertIn("age", filtered_event2.unsigned)
+        self.assertEqual(14, filtered_event2.unsigned["age"])
+        self.assertNotIn("more warez", filtered_event2.unsigned)
+        # Invite_room_state is allowed in events of type m.room.member
+        self.assertIn("invite_room_state", filtered_event2.unsigned)
+        self.assertEqual([], filtered_event2.unsigned["invite_room_state"])
+
+    def test_strip_event_removes_fields_based_on_event_type(self):
+        event3 = {
+            "sender": "@baduser:test.serv",
+            "state_key": "@baduser:test.serv",
+            "event_id": "$event3:test.serv",
+            "depth": 1000,
+            "origin_server_ts": 1,
+            "type": "m.room.power_levels",
+            "origin": "test.servx",
+            "content": {},
+            "auth_events": [],
+            "unsigned": {
+                "malicious garbage": "hackz",
+                "more warez": "more hackz",
+                "age": 14,
+                "invite_room_state": [],
+            },
+        }
+        filtered_event3 = event_from_pdu_json(event3, RoomVersions.V1)
+        self.assertIn("age", filtered_event3.unsigned)
+        # Invite_room_state field is only permitted in event type m.room.member
+        self.assertNotIn("invite_room_state", filtered_event3.unsigned)
+        self.assertNotIn("more warez", filtered_event3.unsigned)