submessages: Add verify_submessage_sender.

Before this change a rogue actor could try to
widgetize another person's message. (The
rogue actor would already have access to read
the message.)

zerver/lib/actions.py

@@ -2151,6 +2151,30 @@ def bulk_insert_ums(ums: List[UserMessageLite]) -> None:
         execute_values(cursor.cursor, query, vals)
 
 
+def verify_submessage_sender(
+    *,
+    message_id: int,
+    message_sender_id: int,
+    submessage_sender_id: int,
+) -> None:
+    """Even though our submessage architecture is geared toward
+    collaboration among all message readers, we still enforce
+    the the first person to attach a submessage to the message
+    must be the original sender of the message.
+    """
+
+    if message_sender_id == submessage_sender_id:
+        return
+
+    if SubMessage.objects.filter(
+        message_id=message_id,
+        sender_id=message_sender_id,
+    ).exists():
+        return
+
+    raise JsonableError(_("You cannot attach a submessage to this message."))
+
+
 def do_add_submessage(
     realm: Realm,
     sender_id: int,

zerver/tests/test_submessage.py

@@ -100,6 +100,42 @@ class TestBasics(ZulipTestCase):
         result = self.client_post("/json/submessage", payload)
         self.assert_json_error(result, "Invalid message(s)")
 
+    def test_original_sender_enforced(self) -> None:
+        cordelia = self.example_user("cordelia")
+        hamlet = self.example_user("hamlet")
+        stream_name = "Verona"
+
+        message_id = self.send_stream_message(
+            sender=cordelia,
+            stream_name=stream_name,
+        )
+        self.login_user(hamlet)
+
+        payload = dict(
+            message_id=message_id,
+            msg_type="whatever",
+            content="{}",
+        )
+
+        # Hamlet can't just go attaching submessages to Cordelia's
+        # message, even though he does have read access here to the
+        # message itself.
+        result = self.client_post("/json/submessage", payload)
+        self.assert_json_error(result, "You cannot attach a submessage to this message.")
+
+        # Since Hamlet is actually subcribed to the stream, he is welcome
+        # to send submessages to Cordelia once she initiates the "subconversation".
+        do_add_submessage(
+            realm=cordelia.realm,
+            sender_id=cordelia.id,
+            message_id=message_id,
+            msg_type="whatever",
+            content="whatever",
+        )
+
+        result = self.client_post("/json/submessage", payload)
+        self.assert_json_success(result)
+
     def test_endpoint_success(self) -> None:
         cordelia = self.example_user("cordelia")
         hamlet = self.example_user("hamlet")

zerver/views/submessage.py

@@ -4,7 +4,7 @@ from django.http import HttpRequest, HttpResponse
 from django.utils.translation import gettext as _
 
 from zerver.decorator import REQ, has_request_variables
-from zerver.lib.actions import do_add_submessage
+from zerver.lib.actions import do_add_submessage, verify_submessage_sender
 from zerver.lib.message import access_message
 from zerver.lib.response import json_error, json_success
 from zerver.lib.validator import check_int
@@ -23,6 +23,12 @@ def process_submessage(
 ) -> HttpResponse:
     message, user_message = access_message(user_profile, message_id, lock_message=True)
 
+    verify_submessage_sender(
+        message_id=message.id,
+        message_sender_id=message.sender_id,
+        submessage_sender_id=user_profile.id,
+    )
+
     try:
         orjson.loads(content)
     except orjson.JSONDecodeError: