diff --git a/changelog.d/17839.bugfix b/changelog.d/17839.bugfix
new file mode 100644
index 0000000000..57667a6df5
--- /dev/null
+++ b/changelog.d/17839.bugfix
@@ -0,0 +1 @@
+Check if user has membership in a room before tagging it. Contributed by Lama Alosaimi.
\ No newline at end of file
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 75c60e3c34..70cbbc352b 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -1190,6 +1190,26 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
origin_server_ts=origin_server_ts,
)
+ async def check_for_any_membership_in_room(
+ self, *, user_id: str, room_id: str
+ ) -> None:
+ """
+ Check if the user has any membership in the room and raise error if not.
+
+ Args:
+ user_id: The user to check.
+ room_id: The room to check.
+
+ Raises:
+ AuthError if the user doesn't have any membership in the room.
+ """
+ result = await self.store.get_local_current_membership_for_user_in_room(
+ user_id=user_id, room_id=room_id
+ )
+
+ if result is None or result == (None, None):
+ raise AuthError(403, f"User {user_id} has no membership in room {room_id}")
+
async def _should_perform_remote_join(
self,
user_id: str,
diff --git a/synapse/rest/client/tags.py b/synapse/rest/client/tags.py
index 554bcb95dd..b6648f3499 100644
--- a/synapse/rest/client/tags.py
+++ b/synapse/rest/client/tags.py
@@ -78,6 +78,7 @@ class TagServlet(RestServlet):
super().__init__()
self.auth = hs.get_auth()
self.handler = hs.get_account_data_handler()
+ self.room_member_handler = hs.get_room_member_handler()
async def on_PUT(
self, request: SynapseRequest, user_id: str, room_id: str, tag: str
@@ -85,6 +86,12 @@ class TagServlet(RestServlet):
requester = await self.auth.get_user_by_req(request)
if user_id != requester.user.to_string():
raise AuthError(403, "Cannot add tags for other users.")
+ # Check if the user has any membership in the room and raise error if not.
+ # Although it's not harmful for users to tag random rooms, it's just superfluous
+ # data we don't need to track or allow.
+ await self.room_member_handler.check_for_any_membership_in_room(
+ user_id=user_id, room_id=room_id
+ )
body = parse_json_object_from_request(request)
diff --git a/tests/rest/client/test_tags.py b/tests/rest/client/test_tags.py
new file mode 100644
index 0000000000..5d596409e1
--- /dev/null
+++ b/tests/rest/client/test_tags.py
@@ -0,0 +1,95 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2024 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+
+"""Tests REST events for /tags paths."""
+
+from http import HTTPStatus
+
+import synapse.rest.admin
+from synapse.rest.client import login, room, tags
+
+from tests import unittest
+
+
+class RoomTaggingTestCase(unittest.HomeserverTestCase):
+ """Tests /user/$user_id/rooms/$room_id/tags/$tag REST API."""
+
+ servlets = [
+ room.register_servlets,
+ tags.register_servlets,
+ login.register_servlets,
+ synapse.rest.admin.register_servlets_for_client_rest_resource,
+ ]
+
+ def test_put_tag_checks_room_membership(self) -> None:
+ """
+ Test that a user can add a tag to a room if they have membership to the room.
+ """
+ user1_id = self.register_user("user1", "pass")
+ user1_tok = self.login(user1_id, "pass")
+ room_id = self.helper.create_room_as(user1_id, tok=user1_tok)
+ tag = "test_tag"
+
+ # Make the request
+ channel = self.make_request(
+ "PUT",
+ f"/user/{user1_id}/rooms/{room_id}/tags/{tag}",
+ content={"order": 0.5},
+ access_token=user1_tok,
+ )
+ # Check that the request was successful
+ self.assertEqual(channel.code, HTTPStatus.OK, channel.result)
+
+ def test_put_tag_fails_if_not_in_room(self) -> None:
+ """
+ Test that a user cannot add a tag to a room if they don't have membership to the
+ room.
+ """
+ user1_id = self.register_user("user1", "pass")
+ user1_tok = self.login(user1_id, "pass")
+ user2_id = self.register_user("user2", "pass")
+ user2_tok = self.login(user2_id, "pass")
+ # Create the room with user2 (user1 has no membership in the room)
+ room_id = self.helper.create_room_as(user2_id, tok=user2_tok)
+ tag = "test_tag"
+
+ # Make the request
+ channel = self.make_request(
+ "PUT",
+ f"/user/{user1_id}/rooms/{room_id}/tags/{tag}",
+ content={"order": 0.5},
+ access_token=user1_tok,
+ )
+ # Check that the request failed with the correct error
+ self.assertEqual(channel.code, HTTPStatus.FORBIDDEN, channel.result)
+
+ def test_put_tag_fails_if_room_does_not_exist(self) -> None:
+ """
+ Test that a user cannot add a tag to a room if the room doesn't exist (therefore
+ no membership in the room.)
+ """
+ user1_id = self.register_user("user1", "pass")
+ user1_tok = self.login(user1_id, "pass")
+ room_id = "!nonexistent:test"
+ tag = "test_tag"
+
+ # Make the request
+ channel = self.make_request(
+ "PUT",
+ f"/user/{user1_id}/rooms/{room_id}/tags/{tag}",
+ content={"order": 0.5},
+ access_token=user1_tok,
+ )
+ # Check that the request failed with the correct error
+ self.assertEqual(channel.code, HTTPStatus.FORBIDDEN, channel.result)
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