fix(admin): saving a user returns to ?next= only on this site

The user edit page redirected to its ?next= parameter after saving,
whatever it held: a link to the edit page with next=https://other.site
sent the administrator who saved the form to that site (CodeQL
py/url-redirection). The account page links there with a next of this
site.

next is now followed only when url_has_allowed_host_and_scheme accepts
it for this host, as user_instance_delete already does; otherwise the
user list. The test fails on the previous code for an absolute URL, a
scheme-relative one and a backslash variant.

admin/test_user_update_next.py

@@ -0,0 +1,26 @@
+"""After saving a user, the edit page goes back to ?next= only on this site."""
+
+from django.contrib.auth.models import User
+from django.test import TestCase
+from django.urls import reverse
+
+LIMITS = {"max_instances": 1, "max_cpus": 1, "max_memory": 1024, "max_disk_size": 4}
+
+
+class UserUpdateNextTests(TestCase):
+    def setUp(self):
+        self.client.force_login(User.objects.create_superuser("un-admin", "un@example.com", "pw"))
+        self.user = User.objects.create_user("un-user", password="pw")
+
+    def save(self, next_url):
+        url = reverse("admin:user_update", args=[self.user.id]) + "?next=" + next_url
+        return self.client.post(url, {"username": "un-user", "is_active": "on", **LIMITS})
+
+    def test_a_page_of_this_site(self):
+        account = reverse("accounts:account", args=[self.user.id])
+        self.assertRedirects(self.save(account), account, fetch_redirect_response=False)
+
+    def test_another_site_is_not_followed(self):
+        for next_url in ("https://evil.example/", "//evil.example/", "/\\evil.example/"):
+            with self.subTest(next_url=next_url):
+                self.assertRedirects(self.save(next_url), reverse("admin:user_list"), fetch_redirect_response=False)

admin/views.py

@@ -7,6 +7,7 @@ from django.contrib.auth.forms import AdminPasswordChangeForm
 from django.contrib.auth.models import Group, User
 from django.core.paginator import Paginator
 from django.shortcuts import get_object_or_404, redirect, render
+from django.utils.http import url_has_allowed_host_and_scheme
 from django.utils.translation import gettext_lazy as _
 from django.views.decorators.http import require_POST
 from logs.models import Logs
@@ -123,8 +124,12 @@ def user_update(request, pk):
     if user_form.is_valid() and attributes_form.is_valid():
         user_form.save()
         attributes_form.save()
-        next = request.GET.get("next")
-        return redirect(next or "admin:user_list")
+        next_url = request.GET.get("next")
+        if next_url and url_has_allowed_host_and_scheme(
+            url=next_url, allowed_hosts={request.get_host()}, require_https=request.is_secure()
+        ):
+            return redirect(next_url)
+        return redirect("admin:user_list")
 
     return render(
         request,