message: Restrict sending DMs to inaccessible users.

This commit adds code to not allow guest users to send DMs
to users they cannot access.

zerver/actions/message_send.py

@@ -75,6 +75,7 @@ from zerver.lib.timestamp import timestamp_to_datetime
 from zerver.lib.topic import participants_for_topic
 from zerver.lib.url_preview.types import UrlEmbedData
 from zerver.lib.user_message import UserMessageLite, bulk_insert_ums
+from zerver.lib.users import check_user_can_access_all_users, get_accessible_user_ids
 from zerver.lib.validator import check_widget_content
 from zerver.lib.widget import do_widget_post_save_actions
 from zerver.models import (
@@ -1460,6 +1461,21 @@ def check_private_message_policy(
         raise JsonableError(_("Direct messages are disabled in this organization."))
 
 
+def check_sender_can_access_recipients(
+    realm: Realm, sender: UserProfile, user_profiles: Sequence[UserProfile]
+) -> None:
+    if check_user_can_access_all_users(sender):
+        return
+
+    users_accessible_to_sender = set(get_accessible_user_ids(realm, sender))
+    # Guest users can access all the bots (including cross-realm bots).
+    non_bot_recipient_user_ids = {user.id for user in user_profiles if not user.is_bot}
+
+    inaccessible_recipients = non_bot_recipient_user_ids - users_accessible_to_sender
+    if inaccessible_recipients:
+        raise JsonableError(_("You do not have permission to access some of the recipients."))
+
+
 # check_message:
 # Returns message ready for sending with do_send_message on success or the error message (string) on error.
 def check_message(
@@ -1545,6 +1561,8 @@ def check_message(
             "JabberMirror",
         ]
 
+        check_sender_can_access_recipients(realm, sender, user_profiles)
+
         check_private_message_policy(realm, sender, user_profiles)
 
         # API super-users who set the `forged` flag are allowed to

zerver/tests/test_message_send.py

@@ -777,6 +777,75 @@ class MessagePOSTTest(ZulipTestCase):
         )
         self.assert_json_error(result, f"'{othello.email}' is no longer using Zulip.")
 
+    def test_personal_message_to_inaccessible_users(self) -> None:
+        othello = self.example_user("othello")
+        cordelia = self.example_user("cordelia")
+        hamlet = self.example_user("hamlet")
+        iago = self.example_user("iago")
+
+        self.set_up_db_for_testing_user_access()
+        self.login("polonius")
+
+        result = self.client_post(
+            "/json/messages",
+            {
+                "type": "direct",
+                "content": "Test direct message",
+                "to": orjson.dumps([othello.id]).decode(),
+            },
+        )
+        self.assert_json_error(
+            result, "You do not have permission to access some of the recipients."
+        )
+
+        result = self.client_post(
+            "/json/messages",
+            {
+                "type": "direct",
+                "content": "Test direct message",
+                "to": orjson.dumps([hamlet.id]).decode(),
+            },
+        )
+        self.assert_json_success(result)
+        msg = self.get_last_message()
+        self.assertEqual(msg.content, "Test direct message")
+
+        result = self.client_post(
+            "/json/messages",
+            {
+                "type": "direct",
+                "content": "Test group direct message",
+                "to": orjson.dumps([othello.id, cordelia.id]).decode(),
+            },
+        )
+        self.assert_json_error(
+            result, "You do not have permission to access some of the recipients."
+        )
+
+        result = self.client_post(
+            "/json/messages",
+            {
+                "type": "direct",
+                "content": "Test group direct message",
+                "to": orjson.dumps([hamlet.id, cordelia.id]).decode(),
+            },
+        )
+        self.assert_json_error(
+            result, "You do not have permission to access some of the recipients."
+        )
+
+        result = self.client_post(
+            "/json/messages",
+            {
+                "type": "direct",
+                "content": "Test group direct message",
+                "to": orjson.dumps([hamlet.id, iago.id]).decode(),
+            },
+        )
+        self.assert_json_success(result)
+        msg = self.get_last_message()
+        self.assertEqual(msg.content, "Test group direct message")
+
     def test_invalid_type(self) -> None:
         """
         Sending a message of unknown type returns error JSON.