1 files changed, 66 insertions, 10 deletions

diff --git a/synapse/rest/client/receipts.py b/synapse/rest/client/receipts.py
index f3ff156abe..18a282b22c 100644
--- a/synapse/rest/client/receipts.py
+++ b/synapse/rest/client/receipts.py
@@ -15,8 +15,8 @@
 import logging
 from typing import TYPE_CHECKING, Tuple
 
-from synapse.api.constants import ReceiptTypes
-from synapse.api.errors import SynapseError
+from synapse.api.constants import MAIN_TIMELINE, ReceiptTypes
+from synapse.api.errors import Codes, SynapseError
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 from synapse.http.site import SynapseRequest
@@ -43,13 +43,13 @@ class ReceiptRestServlet(RestServlet):
         self.receipts_handler = hs.get_receipts_handler()
         self.read_marker_handler = hs.get_read_marker_handler()
         self.presence_handler = hs.get_presence_handler()
+        self._main_store = hs.get_datastores().main
 
         self._known_receipt_types = {
             ReceiptTypes.READ,
             ReceiptTypes.READ_PRIVATE,
             ReceiptTypes.FULLY_READ,
         }
-        self._msc3771_enabled = hs.config.experimental.msc3771_enabled
 
     async def on_POST(
         self, request: SynapseRequest, room_id: str, receipt_type: str, event_id: str
@@ -66,13 +66,29 @@ class ReceiptRestServlet(RestServlet):
 
         # Pull the thread ID, if one exists.
         thread_id = None
-        if self._msc3771_enabled:
-            if "thread_id" in body:
-                thread_id = body.get("thread_id")
-                if not thread_id or not isinstance(thread_id, str):
-                    raise SynapseError(
-                        400, "thread_id field must be a non-empty string"
-                    )
+        if "thread_id" in body:
+            thread_id = body.get("thread_id")
+            if not thread_id or not isinstance(thread_id, str):
+                raise SynapseError(
+                    400,
+                    "thread_id field must be a non-empty string",
+                    Codes.INVALID_PARAM,
+                )
+
+            if receipt_type == ReceiptTypes.FULLY_READ:
+                raise SynapseError(
+                    400,
+                    f"thread_id is not compatible with {ReceiptTypes.FULLY_READ} receipts.",
+                    Codes.INVALID_PARAM,
+                )
+
+            # Ensure the event ID roughly correlates to the thread ID.
+            if not await self._is_event_in_thread(event_id, thread_id):
+                raise SynapseError(
+                    400,
+                    f"event_id {event_id} is not related to thread {thread_id}",
+                    Codes.INVALID_PARAM,
+                )
 
         await self.presence_handler.bump_presence_active_time(requester.user)
 
@@ -93,6 +109,46 @@ class ReceiptRestServlet(RestServlet):
 
         return 200, {}
 
+    async def _is_event_in_thread(self, event_id: str, thread_id: str) -> bool:
+        """
+        The event must be related to the thread ID (in a vague sense) to ensure
+        clients aren't sending bogus receipts.
+
+        A thread ID is considered valid for a given event E if:
+
+        1. E has a thread relation which matches the thread ID;
+        2. E has another event which has a thread relation to E matching the
+           thread ID; or
+        3. E is recursively related (via any rel_type) to an event which
+           satisfies 1 or 2.
+
+        Given the following DAG:
+
+            A <---[m.thread]-- B <--[m.annotation]-- C
+            ^
+            |--[m.reference]-- D <--[m.annotation]-- E
+
+        It is valid to send a receipt for thread A on A, B, C, D, or E.
+
+        It is valid to send a receipt for the main timeline on A, D, and E.
+
+        Args:
+            event_id: The event ID to check.
+            thread_id: The thread ID the event is potentially part of.
+
+        Returns:
+            True if the event belongs to the given thread, otherwise False.
+        """
+
+        # If the receipt is on the main timeline, it is enough to check whether
+        # the event is directly related to a thread.
+        if thread_id == MAIN_TIMELINE:
+            return MAIN_TIMELINE == await self._main_store.get_thread_id(event_id)
+
+        # Otherwise, check if the event is directly part of a thread, or is the
+        # root message (or related to the root message) of a thread.
+        return thread_id == await self._main_store.get_thread_id_for_receipts(event_id)
+
 
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ReceiptRestServlet(hs).register(http_server)