invites: Limit invites per day as a function of current users.

zerver/actions/invites.py

@@ -1,4 +1,5 @@
 import datetime
+import logging
 from typing import Any, Collection, Dict, List, Optional, Sequence, Set, Tuple, Union
 
 from django.conf import settings
@@ -7,6 +8,7 @@ from django.db import transaction
 from django.db.models import Q, Sum
 from django.utils.timezone import now as timezone_now
 from django.utils.translation import gettext as _
+from zxcvbn import zxcvbn
 
 from analytics.lib.counts import COUNT_STATS, do_increment_logging_stat
 from analytics.models import RealmCount
@@ -97,6 +99,63 @@ def check_invite_limit(realm: Realm, num_invitees: int) -> None:
             daily_limit_reached=True,
         )
 
+    if realm.plan_type == Realm.PLAN_TYPE_LIMITED and realm._max_invites is None:
+        # If they're a non-paid plan with default invitation limits,
+        # we further limit how many invitations can be sent in a day
+        # as a function of how many current users they have. The
+        # allowed ratio has some heuristics to lock down likely-spammy
+        # realms.  This ratio likely only matters for the first
+        # handful of invites; if those users accept, then the realm is
+        # unlikely to hit these limits.  If a real realm hits them,
+        # the resulting message suggests that they contact support if
+        # they have a real use case.
+        suspicion_score = 0
+        if zxcvbn(realm.string_id)["score"] == 4:
+            # Very high entropy realm names are suspicious
+            suspicion_score += 1
+
+        if not realm.description:
+            suspicion_score += 1
+
+        if realm.icon_source == Realm.ICON_FROM_GRAVATAR:
+            suspicion_score += 1
+
+        if realm.date_created >= timezone_now() - datetime.timedelta(hours=1):
+            suspicion_score += 1
+
+        current_user_count = len(
+            UserProfile.objects.filter(realm=realm, is_bot=False, is_active=True)
+        )
+        if current_user_count == 1:
+            suspicion_score += 1
+
+        estimated_sent = RealmCount.objects.filter(
+            realm=realm, property="messages_sent:message_type:day"
+        ).aggregate(messages=Sum("value"))
+        if estimated_sent["messages"] == 0:
+            suspicion_score += 1
+
+        if suspicion_score == 6:
+            permitted_ratio = 2
+        elif suspicion_score >= 3:
+            permitted_ratio = 3
+        else:
+            permitted_ratio = 5
+
+        # For now, simply log the data; this will change to a raise of
+        # InvitationError once we've done some auditing.
+        logging.warning(
+            "%s (suspicion %d/6) inviting %d more, have %d recent, %d max, but only %d current users.  Ratio %.1f, %d allowed",
+            realm.string_id,
+            suspicion_score,
+            num_invitees,
+            recent_invites,
+            realm.max_invites,
+            current_user_count,
+            (num_invitees + recent_invites) / current_user_count,
+            permitted_ratio,
+        )
+
     default_max = settings.INVITES_DEFAULT_REALM_DAILY_MAX
     newrealm_age = datetime.timedelta(days=settings.INVITES_NEW_REALM_DAYS)
     if realm.date_created <= timezone_now() - newrealm_age: