diff --git a/src/pretix/presale/checkoutflow.py b/src/pretix/presale/checkoutflow.py
index b60a543ce7..ff2b2b8452 100644
--- a/src/pretix/presale/checkoutflow.py
+++ b/src/pretix/presale/checkoutflow.py
@@ -298,11 +298,13 @@ class CustomerStep(CartMixin, TemplateFlowStep):
             elif request.customer:
                 self.cart_session['customer_mode'] = 'login'
                 self.cart_session['customer'] = request.customer.pk
+                self.cart_session['customer_cart_tied_to_login'] = True
                 return redirect(self.get_next_url(request))
             elif self.login_form.is_valid():
                 customer_login(self.request, self.login_form.get_customer())
                 self.cart_session['customer_mode'] = 'login'
                 self.cart_session['customer'] = self.login_form.get_customer().pk
+                self.cart_session['customer_cart_tied_to_login'] = True
                 return redirect(self.get_next_url(request))
             else:
                 return self.render()
@@ -311,6 +313,7 @@ class CustomerStep(CartMixin, TemplateFlowStep):
                 customer = self.register_form.create()
                 self.cart_session['customer_mode'] = 'login'
                 self.cart_session['customer'] = customer.pk
+                self.cart_session['customer_cart_tied_to_login'] = False
                 return redirect(self.get_next_url(request))
             else:
                 return self.render()
diff --git a/src/pretix/presale/templates/pretixpresale/fragment_login_status.html b/src/pretix/presale/templates/pretixpresale/fragment_login_status.html
index 1e558804a8..d4c18ddf60 100644
--- a/src/pretix/presale/templates/pretixpresale/fragment_login_status.html
+++ b/src/pretix/presale/templates/pretixpresale/fragment_login_status.html
@@ -15,7 +15,7 @@
                 <span class="fa fa-sign-out" aria-hidden="true"></span>
             </a>
         {% else %}
-            <a href="{% abseventurl request.organizer "presale:organizer.customer.login" %}{% if not request.event_domain %}?next={{ request.path|urlencode }}%3F{{ request.META.QUERY_STRING|urlencode }}{% endif %}">
+            <a href="{% abseventurl request.organizer "presale:organizer.customer.login" %}?next={% if request.event_domain %}{{ request.scheme }}://{{ request.get_host }}{% endif %}{{ request.path|urlencode }}%3F{{ request.META.QUERY_STRING|urlencode }}{% if request.event_domain %}&request_cross_domain_customer_auth=true{% endif %}">
                 {% trans "Log in" %}</a>
 
         {% endif %}
diff --git a/src/pretix/presale/utils.py b/src/pretix/presale/utils.py
index 85393046d6..72ec113528 100644
--- a/src/pretix/presale/utils.py
+++ b/src/pretix/presale/utils.py
@@ -31,7 +31,7 @@
 # Unless required by applicable law or agreed to in writing, software distributed under the Apache License 2.0 is
 # distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations under the License.
-
+import re
 import warnings
 from importlib import import_module
 from urllib.parse import urljoin
@@ -64,17 +64,36 @@ def get_customer(request):
     if not hasattr(request, '_cached_customer'):
         session_key = f'customer_auth_id:{request.organizer.pk}'
         hash_session_key = f'customer_auth_hash:{request.organizer.pk}'
+        dependency_key = f'customer_auth_session_dependency:{request.organizer.pk}'
+
+        # By default, we look at the regular django session
+        session = request.session
+
+        # However, if an event uses a custom domain, the event is at a different domain
+        # than our actual session cookie. The login state is therefore not determined
+        # by our request session, but by the "parent session", the user's session on the
+        # organizer level. This approach guarantees e.g. a global logout feature.
+        if session.get(dependency_key):
+            sparent = SessionStore(session[dependency_key])
+            try:
+                sparent.load()
+            except:
+                # parent session no longer exists
+                request._cached_customer = None
+                return
+            else:
+                session = sparent
 
         with scope(organizer=request.organizer):
             try:
                 customer = request.organizer.customers.get(
                     is_active=True, is_verified=True,
-                    pk=request.session[session_key]
+                    pk=session[session_key]
                 )
             except (Customer.DoesNotExist, KeyError):
                 request._cached_customer = None
             else:
-                session_hash = request.session.get(hash_session_key)
+                session_hash = session.get(hash_session_key)
                 session_hash_verified = session_hash and constant_time_compare(
                     session_hash,
                     customer.get_session_auth_hash()
@@ -82,7 +101,7 @@ def get_customer(request):
                 if session_hash_verified:
                     request._cached_customer = customer
                 else:
-                    request.session.flush()
+                    session.flush()
                     request._cached_customer = None
 
     return request._cached_customer
@@ -96,12 +115,46 @@ def update_customer_session_auth_hash(request, customer):
 
 
 def add_customer_to_request(request):
+    if 'cross_domain_customer_auth' in request.GET and request.event_domain:
+        # The user is logged in on the main domain and now wants to take their session
+        # to a event-specific domain. We validate the one time token received via a
+        # query parameter and make sure we invalidate it right away. Then, we look up
+        # the users session on the main domain and store the dependency between the two
+        # sessions.
+        otp = re.sub('[^a-zA-Z0-9]', '', request.GET['cross_domain_customer_auth'])
+
+        otpstore = SessionStore(otp)
+        try:
+            otpstore.load()
+        except:
+            pass
+        else:
+            parent_session_key = otpstore.get(f'customer_cross_domain_auth_{request.organizer.pk}')
+
+            if parent_session_key:  # not already invalidated, expired, …
+                # Make sure the OTP can't be used again
+                otpstore.delete()
+
+                sparent = SessionStore(parent_session_key)
+                try:
+                    sparent.load()
+                except:
+                    # parent session no longer exists
+                    pass
+                else:
+                    dependency_key = f'customer_auth_session_dependency:{request.organizer.pk}'
+                    session_key = f'customer_auth_id:{request.organizer.pk}'
+                    request.session[dependency_key] = parent_session_key
+                    if session_key in request.session:
+                        del request.session[session_key]
+
     request.customer = SimpleLazyObject(lambda: get_customer(request))
 
 
 def customer_login(request, customer):
     session_key = f'customer_auth_id:{request.organizer.pk}'
     hash_session_key = f'customer_auth_hash:{request.organizer.pk}'
+    dependency_key = f'customer_auth_session_dependency:{request.organizer.pk}'
     session_auth_hash = customer.get_session_auth_hash()
 
     if session_key in request.session:
@@ -114,6 +167,7 @@ def customer_login(request, customer):
     else:
         request.session.cycle_key()
 
+    request.session.pop(dependency_key, None)
     request.session[session_key] = customer.pk
     request.session[hash_session_key] = session_auth_hash
     request.customer = customer
@@ -127,6 +181,12 @@ def customer_login(request, customer):
 def customer_logout(request):
     session_key = f'customer_auth_id:{request.organizer.pk}'
     hash_session_key = f'customer_auth_hash:{request.organizer.pk}'
+    dependency_key = f'customer_auth_session_dependency:{request.organizer.pk}'
+
+    # Remove dependency on parent session
+    request.session.pop(dependency_key, None)
+    # We do not remove the actual parent session as we have no way of e.g. cycling its ID.
+    # Instead, LogoutView will redirect the user to the logout of the parent session.
 
     # Remove user session
     customer_id = request.session.pop(session_key, None)
diff --git a/src/pretix/presale/views/cart.py b/src/pretix/presale/views/cart.py
index be3a652da7..31e9b9da8c 100644
--- a/src/pretix/presale/views/cart.py
+++ b/src/pretix/presale/views/cart.py
@@ -343,31 +343,45 @@ def get_or_create_cart_id(request, create=True):
     if current_id and current_id in request.session.get('carts', {}):
         if current_id != orig_current_id:
             request.session[session_keyname] = current_id
-        return current_id
-    else:
-        cart_data = {}
-        if prefix and 'take_cart_id' in request.GET and current_id:
-            new_id = current_id
-            cached_widget_data = widget_data_cache.get('widget_data_{}'.format(current_id))
-            if cached_widget_data:
-                cart_data['widget_data'] = cached_widget_data
+
+        cart_invalidated = (
+            request.session['carts'][current_id].get('customer_cart_tied_to_login', False) and
+            request.session['carts'][current_id].get('customer') and
+            (not request.customer or request.session['carts'][current_id].get('customer') != request.customer.pk)
+        )
+
+        if cart_invalidated:
+            # This cart was created with a login but the person is now logged out.
+            # Destroy the cart for privacy protection.
+            request.session['carts'][current_id] = {}
         else:
-            if not create:
-                return None
-            new_id = generate_cart_id(request, prefix=prefix)
+            return current_id
 
-        if 'widget_data' not in cart_data and 'widget_data' in request.GET:
-            try:
-                cart_data['widget_data'] = json.loads(request.GET.get('widget_data'))
-            except ValueError:
-                pass
+    cart_data = {}
+    if prefix and 'take_cart_id' in request.GET and current_id:
+        new_id = current_id
+        cached_widget_data = widget_data_cache.get('widget_data_{}'.format(current_id))
+        if cached_widget_data:
+            cart_data['widget_data'] = cached_widget_data
+    else:
+        if not create:
+            return None
+        new_id = generate_cart_id(request, prefix=prefix)
 
-        if 'carts' not in request.session:
-            request.session['carts'] = {}
-        if new_id not in request.session['carts']:
-            request.session['carts'][new_id] = cart_data
-        request.session[session_keyname] = new_id
-        return new_id
+    if 'widget_data' not in cart_data and 'widget_data' in request.GET:
+        try:
+            cart_data['widget_data'] = json.loads(request.GET.get('widget_data'))
+        except ValueError:
+            pass
+
+    if 'carts' not in request.session:
+        request.session['carts'] = {}
+
+    if new_id not in request.session['carts']:
+        request.session['carts'][new_id] = cart_data
+
+    request.session[session_keyname] = new_id
+    return new_id
 
 
 def cart_session(request):
diff --git a/src/pretix/presale/views/customer.py b/src/pretix/presale/views/customer.py
index 05c711a24c..bf29630168 100644
--- a/src/pretix/presale/views/customer.py
+++ b/src/pretix/presale/views/customer.py
@@ -19,8 +19,12 @@
 # You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
 # <https://www.gnu.org/licenses/>.
 #
-from urllib.parse import quote
+from importlib import import_module
+from urllib.parse import (
+    parse_qs, quote, urlencode, urljoin, urlparse, urlsplit, urlunparse,
+)
 
+from django.conf import settings
 from django.contrib import messages
 from django.core.signing import BadSignature, dumps, loads
 from django.db import transaction
@@ -38,6 +42,7 @@ from django.views.generic import DeleteView, FormView, ListView, View
 
 from pretix.base.models import Customer, InvoiceAddress, Order, OrderPosition
 from pretix.base.services.mail import mail
+from pretix.multidomain.models import KnownDomain
 from pretix.multidomain.urlreverse import build_absolute_uri, eventreverse
 from pretix.presale.forms.customer import (
     AuthenticationForm, ChangeInfoForm, ChangePasswordForm, RegistrationForm,
@@ -47,6 +52,8 @@ from pretix.presale.utils import (
     customer_login, customer_logout, update_customer_session_auth_hash,
 )
 
+SessionStore = import_module(settings.SESSION_ENGINE).SessionStore
+
 
 class RedirectBackMixin:
     redirect_field_name = 'next'
@@ -57,9 +64,14 @@ class RedirectBackMixin:
             self.redirect_field_name,
             self.request.GET.get(self.redirect_field_name, '')
         )
+        hosts = list(KnownDomain.objects.filter(event__organizer=self.request.organizer).values_list('domainname', flat=True))
+        siteurlsplit = urlsplit(settings.SITE_URL)
+        if siteurlsplit.port and siteurlsplit.port not in (80, 443):
+            hosts = ['%s:%d' % (h, siteurlsplit.port) for h in hosts]
+
         url_is_safe = url_has_allowed_host_and_scheme(
             url=redirect_to,
-            allowed_hosts=None,
+            allowed_hosts=hosts,
             require_https=self.request.is_secure(),
         )
         return redirect_to if url_is_safe else ''
@@ -96,11 +108,23 @@ class LoginView(RedirectBackMixin, FormView):
 
     def get_success_url(self):
         url = self.get_redirect_url()
-        if getattr(self.request, 'event_domain', False):
-            default_url = '/'
-        else:
-            default_url = eventreverse(self.request.organizer, 'presale:organizer.customer.profile', kwargs={})
-        return url or default_url
+
+        if not url:
+            return eventreverse(self.request.organizer, 'presale:organizer.customer.profile', kwargs={})
+
+        if self.request.GET.get("request_cross_domain_customer_auth") == "true":
+            otpstore = SessionStore()
+            otpstore[f'customer_cross_domain_auth_{self.request.organizer.pk}'] = self.request.session.session_key
+            otpstore.set_expiry(60)
+            otpstore.save(must_create=True)
+            otp = otpstore.session_key
+
+            u = urlparse(url)
+            qsl = parse_qs(u.query)
+            qsl['cross_domain_customer_auth'] = otp
+            url = urlunparse((u.scheme, u.netloc, u.path, u.params, urlencode(qsl, doseq=True), u.fragment))
+
+        return url
 
     def form_valid(self, form):
         """Security check complete. Log the user in."""
@@ -119,25 +143,39 @@ class LogoutView(View):
 
     def get_next_page(self):
         if getattr(self.request, 'event_domain', False):
-            next_page = '/'
+            # After we cleared the cookies on this domain, redirect to the parent domain to clear cookies as well
+            next_page = eventreverse(self.request.organizer, 'presale:organizer.customer.logout', kwargs={})
+            if self.redirect_field_name in self.request.POST or self.redirect_field_name in self.request.GET:
+                after_next_page = self.request.POST.get(
+                    self.redirect_field_name,
+                    self.request.GET.get(self.redirect_field_name)
+                )
+                next_page += '?' + urlencode({
+                    'next': urljoin(f'{self.request.scheme}://{self.request.get_host()}', after_next_page)
+                })
         else:
             next_page = eventreverse(self.request.organizer, 'presale:organizer.index', kwargs={})
 
-        if (self.redirect_field_name in self.request.POST or
-                self.redirect_field_name in self.request.GET):
-            next_page = self.request.POST.get(
-                self.redirect_field_name,
-                self.request.GET.get(self.redirect_field_name)
-            )
-            url_is_safe = url_has_allowed_host_and_scheme(
-                url=next_page,
-                allowed_hosts=None,
-                require_https=self.request.is_secure(),
-            )
-            # Security check -- Ensure the user-originating redirection URL is
-            # safe.
-            if not url_is_safe:
-                next_page = self.request.path
+            if (self.redirect_field_name in self.request.POST or
+                    self.redirect_field_name in self.request.GET):
+                next_page = self.request.POST.get(
+                    self.redirect_field_name,
+                    self.request.GET.get(self.redirect_field_name)
+                )
+                hosts = list(KnownDomain.objects.filter(event__organizer=self.request.organizer).values_list('domainname', flat=True))
+                siteurlsplit = urlsplit(settings.SITE_URL)
+                if siteurlsplit.port and siteurlsplit.port not in (80, 443):
+                    hosts = ['%s:%d' % (h, siteurlsplit.port) for h in hosts]
+                url_is_safe = url_has_allowed_host_and_scheme(
+                    url=next_page,
+                    allowed_hosts=hosts,
+                    require_https=self.request.is_secure(),
+                )
+                # Security check -- Ensure the user-originating redirection URL is
+                # safe.
+                if not url_is_safe:
+                    next_page = self.request.path
+
         return next_page
 
 
diff --git a/src/tests/presale/test_checkout.py b/src/tests/presale/test_checkout.py
index 183f621a10..310f0fbc47 100644
--- a/src/tests/presale/test_checkout.py
+++ b/src/tests/presale/test_checkout.py
@@ -3815,6 +3815,26 @@ class CustomerCheckoutTestCase(BaseCheckoutTestCase, TestCase):
         order = self._finish()
         assert order.customer == self.customer
 
+    def test_login_valid_but_removed_after_logout(self):
+        response = self.client.get('/%s/%s/checkout/start' % (self.orga.slug, self.event.slug), follow=True)
+        self.assertRedirects(response, '/%s/%s/checkout/customer/' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+
+        response = self.client.post('/%s/%s/checkout/customer/' % (self.orga.slug, self.event.slug), {
+            'customer_mode': 'login',
+            'login-email': 'john@example.org',
+            'login-password': 'foo',
+        }, follow=True)
+        self.assertRedirects(response, '/%s/%s/checkout/questions/' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+
+        self.client.get('/%s/account/logout' % (self.orga.slug,), follow=True)
+
+        response = self.client.get('/%s/%s/checkout/questions/' % (self.orga.slug, self.event.slug), follow=True)
+        self.assertRedirects(response, '/%s/%s/?require_cookie=true' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+        assert response.status_code == 200
+
     def test_login_invalid(self):
         response = self.client.get('/%s/%s/checkout/start' % (self.orga.slug, self.event.slug), follow=True)
         self.assertRedirects(response, '/%s/%s/checkout/customer/' % (self.orga.slug, self.event.slug),
diff --git a/src/tests/presale/test_customer.py b/src/tests/presale/test_customer.py
index 11b17ecf19..3555e25167 100644
--- a/src/tests/presale/test_customer.py
+++ b/src/tests/presale/test_customer.py
@@ -22,14 +22,17 @@
 import datetime
 from datetime import timedelta
 from decimal import Decimal
+from urllib.parse import parse_qs, urlparse
 
 import pytest
 from django.core import mail as djmail
 from django.core.signing import dumps
+from django.test import Client
 from django.utils.timezone import now
 from django_scopes import scopes_disabled
 
 from pretix.base.models import Event, Item, Order, OrderPosition, Organizer
+from pretix.multidomain.models import KnownDomain
 from pretix.presale.forms.customer import TokenGenerator
 
 
@@ -412,3 +415,152 @@ def test_login_per_org(env, client):
     })
     assert client.get('/bigevents/account/').status_code == 200
     assert client.get('/demo/account/').status_code == 302
+
+
+@pytest.fixture
+def client2():
+    # We need a second test client instance for cross domain stuff since the test client
+    # does not isolate sessions per-domain like browsers do
+    return Client()
+
+
+def _cross_domain_login(env, client, client2):
+    with scopes_disabled():
+        customer = env[0].customers.create(email='john@example.org', is_verified=True)
+        customer.set_password('foo')
+        customer.save()
+        KnownDomain.objects.create(domainname='org.test', organizer=env[0])
+        KnownDomain.objects.create(domainname='event.test', organizer=env[0], event=env[1])
+
+    # Log in on org domain
+    r = client.post('/account/login?next=https://event.test/redeem&request_cross_domain_customer_auth=true', {
+        'email': 'john@example.org',
+        'password': 'foo',
+    }, HTTP_HOST='org.test')
+    assert r.status_code == 302
+
+    u = urlparse(r.headers['Location'])
+    assert u.netloc == 'event.test'
+    assert u.path == '/redeem'
+    q = parse_qs(u.query)
+    assert 'cross_domain_customer_auth' in q
+
+    # Take session over to event domain
+    r = client2.get(f'/?{u.query}', HTTP_HOST='event.test')
+    assert r.status_code == 200
+    assert b'john@example.org' in r.content
+
+
+@pytest.mark.django_db
+def test_cross_domain_login(env, client, client2):
+    _cross_domain_login(env, client, client2)
+
+    # Logged in on org domain
+    r = client.get('/', HTTP_HOST='event.test')
+    assert r.status_code == 200
+    assert b'john@example.org' in r.content
+
+    # Logged in on event domain
+    r = client2.get('/', HTTP_HOST='org.test')
+    assert r.status_code == 200
+    assert b'john@example.org' in r.content
+
+
+@pytest.mark.django_db
+def test_cross_domain_logout_on_org_domain(env, client, client2):
+    _cross_domain_login(env, client, client2)
+
+    r = client.get('/account/logout', HTTP_HOST='org.test')
+    assert r.status_code == 302
+
+    # Logged out on org domain
+    r = client.get('/', HTTP_HOST='event.test')
+    assert r.status_code == 200
+    assert b'john@example.org' not in r.content
+
+    # Logged out on event domain
+    r = client2.get('/', HTTP_HOST='org.test')
+    assert r.status_code == 200
+    assert b'john@example.org' not in r.content
+
+
+@pytest.mark.django_db
+def test_cross_domain_logout_on_event_domain(env, client, client2):
+    _cross_domain_login(env, client, client2)
+
+    r = client2.get('/account/logout?next=/redeem', HTTP_HOST='event.test')
+    assert r.status_code == 302
+
+    u = urlparse(r.headers['Location'])
+    assert u.netloc == 'org.test'
+    assert u.path == '/account/logout'
+
+    r = client.get(f'{u.path}?{u.query}', HTTP_HOST='org.test')
+    assert r.status_code == 302
+    assert r.headers['Location'] == 'http://event.test/redeem'
+
+    # Logged out on org domain
+    r = client.get('/', HTTP_HOST='event.test')
+    assert r.status_code == 200
+    assert b'john@example.org' not in r.content
+
+    # Logged out on event domain
+    r = client2.get('/', HTTP_HOST='org.test')
+    assert r.status_code == 200
+    assert b'john@example.org' not in r.content
+
+
+@pytest.mark.django_db
+def test_cross_domain_login_otp_only_valid_once(env, client, client2):
+    with scopes_disabled():
+        customer = env[0].customers.create(email='john@example.org', is_verified=True)
+        customer.set_password('foo')
+        customer.save()
+        KnownDomain.objects.create(domainname='org.test', organizer=env[0])
+        KnownDomain.objects.create(domainname='event.test', organizer=env[0], event=env[1])
+
+    # Log in on org domain
+    r = client.post('/account/login?next=https://event.test/redeem&request_cross_domain_customer_auth=true', {
+        'email': 'john@example.org',
+        'password': 'foo',
+    }, HTTP_HOST='org.test')
+    assert r.status_code == 302
+
+    u = urlparse(r.headers['Location'])
+    assert u.netloc == 'event.test'
+    assert u.path == '/redeem'
+    q = parse_qs(u.query)
+    assert 'cross_domain_customer_auth' in q
+
+    # Take session over to event domain
+    r = client.get(f'/?{u.query}', HTTP_HOST='event.test')
+    assert r.status_code == 200
+    assert b'john@example.org' in r.content
+
+    # Try to use again
+    r = client2.get(f'/?{u.query}', HTTP_HOST='event.test')
+    assert r.status_code == 200
+    assert b'john@example.org' not in r.content
+
+
+@pytest.mark.django_db
+def test_cross_domain_login_validate_redirect_url(env, client, client2):
+    with scopes_disabled():
+        customer = env[0].customers.create(email='john@example.org', is_verified=True)
+        customer.set_password('foo')
+        customer.save()
+        KnownDomain.objects.create(domainname='org.test', organizer=env[0])
+        KnownDomain.objects.create(domainname='event.test', organizer=env[0], event=env[1])
+
+    # Log in on org domain
+    r = client.post('/account/login?next=https://evilcorp.test/redeem&request_cross_domain_customer_auth=true', {
+        'email': 'john@example.org',
+        'password': 'foo',
+    }, HTTP_HOST='org.test')
+    assert r.status_code == 302
+
+    u = urlparse(r.headers['Location'])
+    assert u.netloc == 'org.test'
+    assert u.path == '/account/'
+    q = parse_qs(u.query)
+    assert 'cross_domain_customer_auth' not in q