[SECURITY] Fix customer session fixation on cross-domain login (CVE-2026-101268, Z#23247268)

src/pretix/presale/templates/pretixpresale/fragment_login_status.html

@@ -14,10 +14,23 @@
                 {% trans "Log out" %}
             </a>
         {% else %}
-            <a href="{% abseventurl request.organizer "presale:organizer.customer.login" %}{% if request.resolver_match.url_name != "organizer.customer.login" %}?next={% if request.event_domain %}{{ request.scheme }}://{{ request.get_host }}{% endif %}{{ request.path|urlencode }}%3F{{ request.META.QUERY_STRING|urlencode }}{% endif %}{% if request.event_domain %}&request_cross_domain_customer_auth=true{% endif %}">
-                <span class="fa fa-sign-in" aria-hidden="true"></span>
-                {% trans "Log in" %}</a>
-
+            <form
+                    {% if request.event_domain %}
+                        action="{% abseventurl request.event "presale:event.customer.loginstart" %}" method="post"
+                    {% else %}
+                        action="{% abseventurl request.organizer "presale:organizer.customer.login" %}" method="get"
+                    {% endif %}
+                    class="helper-display-inline">
+                {% if request.event_domain %}
+                    {% csrf_token %}
+                {% endif %}
+                {% if request.resolver_match.url_name != "organizer.customer.login" %}
+                    <input type="hidden" name="next" value="{% if request.event_domain %}{{ request.scheme }}://{{ request.get_host }}{% endif %}{{ request.path }}?{{ request.META.QUERY_STRING }}">
+                {% endif %}
+                <button class="btn btn-link" type="submit">
+                    <span class="fa fa-sign-in" aria-hidden="true"></span>
+                    {% trans "Log in" %}</button>
+            </form>
         {% endif %}
     </nav>
 {% endif %}

src/pretix/presale/urls.py

@@ -98,6 +98,7 @@ event_patterns = [
     re_path(r'unlock/(?P<hash>[a-z0-9]{64})/$', pretix.presale.views.user.UnlockHashView.as_view(),
             name='event.payment.unlock'),
     re_path(r'resend/$', pretix.presale.views.user.ResendLinkView.as_view(), name='event.resend_link'),
+    re_path(r'^account/loginstart$', pretix.presale.views.customer.LoginStartView.as_view(), name='event.customer.loginstart'),
 
     re_path(r'^favicon.ico/?$',
             pretix.presale.views.organizer.OrganizerFavicon.as_view(),

src/pretix/presale/utils.py

@@ -151,7 +151,9 @@ def add_customer_to_request(request):
         else:
             parent_session_key = otpstore.get(f'customer_cross_domain_auth_{request.organizer.pk}')
 
-            if parent_session_key:  # not already invalidated, expired, …
+            expected_nonce = request.session.pop('cross_domain_customer_auth_nonce', None)
+            found_nonce = request.GET.get("cross_domain_customer_auth_nonce")
+            if parent_session_key and expected_nonce and expected_nonce == found_nonce:  # not already invalidated, expired, …
                 # Make sure the OTP can't be used again
                 otpstore.delete()
 

src/pretix/presale/views/customer.py

@@ -146,6 +146,7 @@ class LoginView(RedirectBackMixin, FormView):
             u = urlparse(url)
             qsl = parse_qs(u.query)
             qsl['cross_domain_customer_auth'] = otp
+            qsl['cross_domain_customer_auth_nonce'] = self.request.GET.get("request_cross_domain_customer_auth_nonce", "")
             url = urlunparse((u.scheme, u.netloc, u.path, u.params, urlencode(qsl, doseq=True), u.fragment))
 
         return url
@@ -703,6 +704,7 @@ class SSOLoginView(RedirectBackMixin, View):
         request.session[f'pretix_customerauth_{self.provider.pk}_nonce'] = nonce
         request.session[f'pretix_customerauth_{self.provider.pk}_popup_origin'] = popup_origin
         request.session[f'pretix_customerauth_{self.provider.pk}_cross_domain_requested'] = self.request.GET.get("request_cross_domain_customer_auth") == "true"
+        request.session[f'pretix_customerauth_{self.provider.pk}_cross_domain_nonce'] = self.request.GET.get("request_cross_domain_customer_auth_nonce")
         redirect_uri = build_absolute_uri(self.request.organizer, 'presale:organizer.customer.login.return', kwargs={
             'provider': self.provider.pk
         })
@@ -953,6 +955,28 @@ class SSOLoginReturnView(RedirectBackMixin, View):
                 u = urlparse(url)
                 qsl = parse_qs(u.query)
                 qsl['cross_domain_customer_auth'] = otp
+                qsl['cross_domain_customer_auth_nonce'] = self.request.session.get(f'pretix_customerauth_{self.provider.pk}_cross_domain_nonce', '')
                 url = urlunparse((u.scheme, u.netloc, u.path, u.params, urlencode(qsl, doseq=True), u.fragment))
 
         return url
+
+
+class LoginStartView(View):
+    # When a login is initiated on a event-domain-level view, we need to carry the user to the organizer domain through
+    # this POST request to be able to set a nonce on their current session. We can't just use a link, since then we'd
+    # need to create sessions for every anonymous user of the ticketshop, which is too expensive.
+
+    def post(self, request, *args, **kwargs):
+        if getattr(self.request, 'domain_mode', 'system') not in (KnownDomain.MODE_ORG_ALT_DOMAIN, KnownDomain.MODE_EVENT_DOMAIN):
+            raise Http404("Only active on event-level domains")
+
+        nonce = get_random_string(32)
+        request.session['cross_domain_customer_auth_nonce'] = nonce
+        query = {
+            "next": request.POST.get("next", ""),
+            "request_cross_domain_customer_auth_nonce": nonce,
+            "request_cross_domain_customer_auth": "true",
+        }
+        return redirect_to_url(
+            build_absolute_uri(self.request.organizer, "presale:organizer.customer.login") + "?" + urlencode(query)
+        )

src/pretix/presale/views/order.py

@@ -42,7 +42,7 @@ import os
 import re
 from collections import Counter, OrderedDict, defaultdict
 from decimal import Decimal
-from urllib.parse import quote
+from urllib.parse import quote, urlencode
 
 from django import forms
 from django.conf import settings
@@ -55,6 +55,7 @@ from django.http import (
     FileResponse, Http404, HttpResponseRedirect, JsonResponse,
 )
 from django.shortcuts import get_object_or_404, redirect, render
+from django.utils.crypto import get_random_string
 from django.utils.decorators import method_decorator
 from django.utils.functional import cached_property
 from django.utils.timezone import now
@@ -117,8 +118,14 @@ class OrderDetailMixin(NoSearchIndexViewMixin):
             login_url = eventreverse(self.request.organizer, 'presale:organizer.customer.login', kwargs={})
 
             if hasattr(self.request, "event_domain") and self.request.event_domain:
-                next_url = quote(self.request.scheme + "://" + self.request.get_host() + self.request.get_full_path())
-                return redirect_to_url(f'{login_url}?next={next_url}&request_cross_domain_customer_auth=true')
+                nonce = get_random_string(32)
+                self.request.session['cross_domain_customer_auth_nonce'] = nonce
+                query = {
+                    "next": self.request.scheme + "://" + self.request.get_host() + self.request.get_full_path(),
+                    "request_cross_domain_customer_auth_nonce": nonce,
+                    "request_cross_domain_customer_auth": "true",
+                }
+                return redirect_to_url(f'{login_url}?{urlencode(query)}')
 
             else:
                 next_url = quote(self.request.get_full_path())

src/pretix/static/pretixpresale/scss/main.scss

@@ -221,6 +221,11 @@ footer nav .btn-link {
         /*border-bottom: 2px solid $brand-primary;*/
         font-weight: bold;
     }
+    .btn-link {
+        padding: 0;
+        margin-left: 5px;
+        vertical-align: top;
+    }
     img {
         vertical-align: baseline;
     }
@@ -653,6 +658,35 @@ h2 .label {
 }
 
 
+.helper-position-relative {
+    position: relative;
+}
+.helper-display-block {
+    display: block !important;
+}
+.helper-display-inline {
+    display: inline !important;
+}
+.helper-display-inline-block {
+    display: inline-block !important;
+}
+.helper-display-none-soft {
+    display: none;
+}
+.helper-display-none {
+    display: none !important;
+}
+.helper-width-auto {
+    width: auto;
+}
+.helper-width-100 {
+    width: 100%;
+}
+.helper-space-below {
+    margin-bottom: 10px;
+}
+
+
 @import "_iframe.scss";
 @import "_a11y.scss";
 @import "_print.scss";

src/tests/presale/test_customer.py

@@ -721,9 +721,21 @@ def _cross_domain_login(env, client, client2, org_alt=False):
         else:
             KnownDomain.objects.create(domainname='event.test', organizer=env[0], event=env[1])
 
-    # Log in on org domain
+    # Start session on event domain
     path = '/conf/' if org_alt else '/'
-    r = client.post(f'/account/login?next=https://event.test{path}redeem&request_cross_domain_customer_auth=true', {
+    r = client2.post(f'{path}account/loginstart', {
+        'next': f'https://event.test{path}redeem',
+    }, HTTP_HOST='event.test')
+    assert r.status_code == 302
+    u = urlparse(r.headers['Location'])
+    assert u.netloc == 'org.test'
+    assert u.path == '/account/login'
+    assert 'request_cross_domain_customer_auth=' in u.query
+    assert 'request_cross_domain_customer_auth_nonce=' in u.query
+    assert 'next=' in u.query
+
+    # Log in on org domain
+    r = client.post(f'{u.path}?{u.query}', {
         'email': 'john@example.org',
         'password': 'foo',
     }, HTTP_HOST='org.test')
@@ -734,6 +746,7 @@ def _cross_domain_login(env, client, client2, org_alt=False):
     assert u.path == path + 'redeem'
     q = parse_qs(u.query)
     assert 'cross_domain_customer_auth' in q
+    assert 'cross_domain_customer_auth_nonce' in q
 
     # Take session over to event domain
     r = client2.get(f'{path}?{u.query}', HTTP_HOST='event.test')
@@ -745,7 +758,7 @@ def _cross_domain_login(env, client, client2, org_alt=False):
 def test_cross_domain_login(env, client, client2):
     _cross_domain_login(env, client, client2)
 
-    # Logged in on evnet domain
+    # Logged in on event domain
     r = client.get('/', HTTP_HOST='event.test')
     assert r.status_code == 200
     assert b'john@example.org' in r.content
@@ -896,7 +909,14 @@ def test_cross_domain_login_with_sso(env, client, client2, provider):
         },
     )
 
-    url = f'/account/login/{provider.pk}/?next=https://event.test/redeem&request_cross_domain_customer_auth=true'
+    r = client2.post('/account/loginstart', {"next": "https://event.test/redeem"}, follow=False, HTTP_HOST='event.test')
+    assert r.status_code == 302
+    assert "/account/login" in r['Location']
+
+    u = urlparse(r.headers['Location'])
+    nonce = parse_qs(u.query)['request_cross_domain_customer_auth_nonce'][0]
+    url = (f'/account/login/{provider.pk}/?next=https://event.test/redeem&request_cross_domain_customer_auth=true&'
+           f'request_cross_domain_customer_auth_nonce={nonce}')
     r = client.get(url, follow=False, HTTP_HOST='org.test')
     assert r.status_code == 302
     assert "/authorize" in r['Location']
@@ -910,6 +930,7 @@ def test_cross_domain_login_with_sso(env, client, client2, provider):
     assert u.path == '/redeem'
     q = parse_qs(u.query)
     assert 'cross_domain_customer_auth' in q
+    assert 'cross_domain_customer_auth_nonce' in q
 
     # Take session over to event domain
     r = client2.get(f'/?{u.query}', HTTP_HOST='event.test')