[SECURITY] Fix checkout validation bypass (CVE-2026-101266, Z#23245008)

src/pretix/presale/views/checkout.py

@@ -21,6 +21,7 @@
 #
 from urllib.parse import quote, urlencode
 
+from django.conf import settings
 from django.contrib import messages
 from django.http import Http404
 from django.utils.decorators import method_decorator
@@ -29,6 +30,7 @@ from django.views.generic import View
 
 from pretix.base.services.cart import CartError
 from pretix.base.signals import validate_cart
+from pretix.base.views.tasks import AsyncAction
 from pretix.helpers.http import redirect_to_url
 from pretix.multidomain.urlreverse import eventreverse
 from pretix.presale.checkoutflow import get_checkout_flow
@@ -51,7 +53,12 @@ class CheckoutView(View):
     def dispatch(self, request, *args, **kwargs):
         self.request = request
 
-        if not cart_exists(request) and "async_id" not in request.GET:
+        is_asyncaction_call = (
+            request.method == "GET" and
+            'async_id' in request.GET and
+            settings.HAS_CELERY
+        )
+        if not cart_exists(request) and not is_asyncaction_call:
             messages.error(request, _("Your cart is empty"))
             return self.redirect(self.get_index_url(self.request))
 
@@ -78,7 +85,9 @@ class CheckoutView(View):
                 utm_params = {k: v for k, v in request.GET.items() if k.startswith("utm_")}
                 return self.redirect(step.get_step_url(request) + '?' + urlencode(utm_params))
             is_selected = (step.identifier == kwargs.get('step', ''))
-            if "async_id" not in request.GET and not is_selected and not step.is_completed(request, warn=not is_selected):
+
+            is_valid_asyncaction_call = is_asyncaction_call and isinstance(step, AsyncAction)
+            if not is_valid_asyncaction_call and not is_selected and not step.is_completed(request, warn=not is_selected):
                 return self.redirect(step.get_step_url(request))
             if is_selected:
                 if request.method.lower() in self.http_method_names:

src/tests/presale/test_checkout.py

@@ -5959,3 +5959,14 @@ class CustomerCheckoutTestCase(BaseCheckoutTestCase, TestCase):
         response = self.client.get('/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug), follow=True)
         assert 'Gift card' in response.content.decode()
         assert '(1 available)' in response.content.decode()
+
+    def test_validation_bypass_error_async_id_is_fixed(self):
+        with scopes_disabled():
+            CartPosition.objects.create(
+                event=self.event, cart_id=self.session_key, item=self.ticket,
+                price=0, listed_price=0, price_after_voucher=0, expires=now() + timedelta(minutes=10)
+            )
+
+        response = self.client.post('/%s/%s/checkout/confirm/?async_id=1' % (self.orga.slug, self.event.slug), follow=False)
+        self.assertRedirects(response, '/%s/%s/checkout/customer/' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)