Unauthorized privilege escalation in Mod module

@@ -9,16 +9,26 @@ Redbot 3.4.1 (2020-10-27)
 Read before updating
 --------------------
 
-1. This Red update bumps discord.py to version 1.5.1, which explicitly requests Discord intents. Red requires all Prvileged Intents to be enabled. More information can be found at :ref:`enabling-privileged-intents`.
-2. Mutes functionality has been moved from the Mod cog to a new separate cog (Mutes) featuring timed and role-based mutes. If you were using it (or want to start now), you can load the new cog with ``[p]load mutes``. You can see the full `Mutes changelog below <important-341-1>`.
-3. Information for Audio users that are using an external Lavalink instance (if you don't know what that is, you should skip this point):
+1. This release fixes a security issue in Mod cog. See `Security changelog below <important-341-2>` for more information.
+2. This Red update bumps discord.py to version 1.5.1, which explicitly requests Discord intents. Red requires all Prvileged Intents to be enabled. More information can be found at :ref:`enabling-privileged-intents`.
+3. Mutes functionality has been moved from the Mod cog to a new separate cog (Mutes) featuring timed and role-based mutes. If you were using it (or want to start now), you can load the new cog with ``[p]load mutes``. You can see the full `Mutes changelog below <important-341-1>`.
+4. Information for Audio users that are using an external Lavalink instance (if you don't know what that is, you should skip this point):
 
    We've updated our `application.yml file <https://github.com/Cog-Creators/Red-DiscordBot/blob/3.4.1/redbot/cogs/audio/data/application.yml>`_ and you should update your instance's ``application.yml`` appropriately.
    Please ensure that the WS port in Audio's settings (``[p]llset wsport``) is set to the port from the ``application.yml``.
 
 End-user changelog
 ------------------
 
+.. _important-341-2:
+
+Security
+********
+
+**NOTE:** If you can't update immediately, we recommend globally disabling the affected command until you can.
+
+- **Mod** - Fixed unauthorized privilege escalation exploit in ``[p]massban`` (also called ``[p]hackban``) command. Full security advisory `can be found on our GitHub <https://github.com/Cog-Creators/Red-DiscordBot/security/advisories/GHSA-mp9m-g7qj-6vqr>`_.
+
 Core Bot
 ********
 

@@ -2,7 +2,7 @@
 import contextlib
 import logging
 from datetime import datetime, timedelta, timezone
-from typing import Optional, Tuple, Union
+from typing import Dict, List, Optional, Tuple, Union
 
 import discord
 from redbot.core import commands, i18n, checks, modlog
@@ -440,28 +440,41 @@ def remove_processed(ids):
             await show_results()
             return
 
+        # We need to check here, if any of the users isn't a member and if they are,
+        # we need to use our `ban_user()` method to do hierarchy checks.
+        members: Dict[int, discord.Member] = {}
+        to_query: List[int] = []
+
         for user_id in user_ids:
-            user = guild.get_member(user_id)
-            if user is not None:
-                if user_id in tempbans:
-                    # We need to check if a user is tempbanned here because otherwise they won't be processed later on.
-                    continue
+            member = guild.get_member(user_id)
+            if member is not None:
+                members[user_id] = member
+            elif not guild.chunked:
+                to_query.append(user_id)
+
+        # If guild isn't chunked, we might possibly be missing the member from cache,
+        # so we need to make sure that isn't the case by querying the user IDs for such guilds.
+        while to_query:
+            queried_members = await guild.query_members(user_ids=to_query[:100], limit=100)
+            members.update((member.id, member) for member in queried_members)
+            to_query = to_query[100:]
+
+        # Call `ban_user()` method for all users that turned out to be guild members.
+        for member in members:
+            try:
+                success, reason = await self.ban_user(
+                    user=member, ctx=ctx, days=days, reason=reason, create_modlog_case=True
+                )
+                if success:
+                    banned.append(user_id)
                 else:
-                    # Instead of replicating all that handling... gets attr from decorator
-                    try:
-                        success, reason = await self.ban_user(
-                            user=user, ctx=ctx, days=days, reason=reason, create_modlog_case=True
-                        )
-                        if success:
-                            banned.append(user_id)
-                        else:
-                            errors[user_id] = _("Failed to ban user {user_id}: {reason}").format(
-                                user_id=user_id, reason=reason
-                            )
-                    except Exception as e:
-                        errors[user_id] = _("Failed to ban user {user_id}: {reason}").format(
-                            user_id=user_id, reason=e
-                        )
+                    errors[user_id] = _("Failed to ban user {user_id}: {reason}").format(
+                        user_id=user_id, reason=reason
+                    )
+            except Exception as e:
+                errors[user_id] = _("Failed to ban user {user_id}: {reason}").format(
+                    user_id=user_id, reason=e
+                )
 
         user_ids = remove_processed(user_ids)