registration: Check realm against PreregistrationUser realm.

We would allow a user with a valid invitation for one realm to use it
on a different realm instead.  On a server with multiple realms, an
authorized user of one realm could use this (by sending invites to
other email addresses they control) to create accounts on other
realms. (CVE-2017-0910)

With this commit, when sending an invitation, we record the inviting
user's realm on the PreregistrationUser row; and when registering a
user, we check that the PregistrationUser realm matches the realm the
user is trying to register on.  This resolves CVE-2017-0910 for
newly-sent invitations; the next commit completes the fix.

[greg: rewrote commit message]

zerver/lib/actions.py

@@ -4000,7 +4000,8 @@ def do_invite_users(user_profile, invitee_emails, streams, invite_as_admin=False
     for email in validated_emails:
         # The logged in user is the referrer.
         prereg_user = PreregistrationUser(email=email, referred_by=user_profile,
-                                          invited_as_admin=invite_as_admin)
+                                          invited_as_admin=invite_as_admin,
+                                          realm=user_profile.realm)
 
         prereg_user.save()
         stream_ids = [stream.id for stream in streams]

zerver/tests/test_signup.py

@@ -353,7 +353,7 @@ class LoginTest(ZulipTestCase):
         with queries_captured() as queries:
             self.register(self.nonreg_email('test'), "test")
         # Ensure the number of queries we make is not O(streams)
-        self.assert_length(queries, 65)
+        self.assert_length(queries, 67)
         user_profile = self.nonreg_user('test')
         self.assertEqual(get_session_dict_user(self.client.session), user_profile.id)
         self.assertFalse(user_profile.enable_stream_desktop_notifications)
@@ -1691,6 +1691,22 @@ class UserSignUpTest(ZulipTestCase):
         mock_error.assert_called_once()
         self.assertEqual(result.status_code, 302)
 
+    def test_replace_subdomain_in_confirmation_link(self) -> None:
+        """
+        Check that manually changing the subdomain in a registration
+        confirmation link doesn't allow you to register to a different realm.
+        """
+        email = "newuser@zulip.com"
+        self.client_post('/accounts/home/', {'email': email})
+        result = self.client_post(
+            '/accounts/register/',
+            {'password': "password",
+             'key': find_key_by_email(email),
+             'terms': True,
+             'full_name': "New User",
+             'from_confirmation': '1'},  subdomain="zephyr")
+        self.assert_in_success_response(["We couldn't find your confirmation link"], result)
+
     def test_failed_signup_due_to_restricted_domain(self) -> None:
         realm = get_realm('zulip')
         realm.invite_required = False

zerver/views/auth.py

@@ -48,9 +48,13 @@ import ujson
 def create_preregistration_user(email, request, realm_creation=False,
                                 password_required=True):
     # type: (Text, HttpRequest, bool, bool) -> HttpResponse
+    realm = None
+    if not realm_creation:
+        realm = get_realm(get_subdomain(request))
     return PreregistrationUser.objects.create(email=email,
                                               realm_creation=realm_creation,
-                                              password_required=password_required)
+                                              password_required=password_required,
+                                              realm=realm)
 
 def maybe_send_to_registration(request, email, full_name='', password_required=True):
     # type: (HttpRequest, Text, Text, bool) -> HttpResponse
@@ -74,7 +78,7 @@ def maybe_send_to_registration(request, email, full_name='', password_required=T
         prereg_user = None
         if settings.ONLY_SSO:
             try:
-                prereg_user = PreregistrationUser.objects.filter(email__iexact=email).latest("invited_at")
+                prereg_user = PreregistrationUser.objects.filter(email__iexact=email, realm=realm).latest("invited_at")
             except PreregistrationUser.DoesNotExist:
                 prereg_user = create_preregistration_user(email, request,
                                                           password_required=password_required)

zerver/views/registration.py

@@ -59,15 +59,13 @@ def accounts_register(request):
     is_realm_admin = prereg_user.invited_as_admin or realm_creation
 
     validators.validate_email(email)
-    if prereg_user.referred_by:
-        # If someone invited you, you are joining their realm regardless
-        # of your e-mail address.
-        realm = prereg_user.referred_by.realm
-    elif realm_creation:
+    if realm_creation:
         # For creating a new realm, there is no existing realm or domain
         realm = None
     else:
         realm = get_realm(get_subdomain(request))
+        if prereg_user.realm is not None and prereg_user.realm != realm:
+            return render(request, 'confirmation/link_does_not_exist.html')
 
     if realm and not email_allowed_for_realm(email, realm):
         return render(request, "zerver/closed_realm.html",