diff --git a/doc/admin/config.rst b/doc/admin/config.rst
index ed4b4748dc..75adf720a4 100644
--- a/doc/admin/config.rst
+++ b/doc/admin/config.rst
@@ -29,6 +29,7 @@ Example::
     currency=EUR
     datadir=/data
     plugins_default=pretix.plugins.sendmail,pretix.plugins.statistics
+    cookie_domain=.pretix.de
 
 ``instance_name``
     The name of this installation. Default: ``pretix.de``
@@ -47,6 +48,9 @@ Example::
     A comma-separated list of plugins that are enabled by default for all new events.
     Defaults to ``pretix.plugins.sendmail,pretix.plugins.statistics``.
 
+``cookie_domain``
+    The cookie domain to be set. Defaults to ``None``.
+
 
 Locale settings
 ---------------
diff --git a/src/pretix/multidomain/middlewares.py b/src/pretix/multidomain/middlewares.py
index 7d34b99bc5..df43f1097c 100644
--- a/src/pretix/multidomain/middlewares.py
+++ b/src/pretix/multidomain/middlewares.py
@@ -13,6 +13,8 @@ from django.utils.http import cookie_date
 
 from pretix.multidomain.models import KnownDomain
 
+LOCAL_HOST_NAMES = ('testserver', 'localhost')
+
 
 class MultiDomainMiddleware:
     def process_request(self, request):
@@ -37,7 +39,7 @@ class MultiDomainMiddleware:
                 kd = KnownDomain.objects.get(domainname=domain)  # noqa
                 request.domain = kd
             except:
-                if settings.DEBUG or domain in ('testserver', 'localhost') or domain == default_domain:
+                if settings.DEBUG or domain in LOCAL_HOST_NAMES or domain == default_domain:
                     request.urlconf = "pretix.multidomain.maindomain_urlconf"
                 else:
                     raise DisallowedHost("Unknown host: %r" % host)
@@ -87,7 +89,8 @@ class SessionMiddleware(BaseSessionMiddleware):
                         request.session.save()
                         response.set_cookie(settings.SESSION_COOKIE_NAME,
                                             request.session.session_key, max_age=max_age,
-                                            expires=expires, domain=request.host,
+                                            expires=expires,
+                                            domain=get_cookie_domain(request),
                                             path=settings.SESSION_COOKIE_PATH,
                                             secure=request.scheme == 'https',
                                             httponly=settings.SESSION_COOKIE_HTTPONLY or None)
@@ -113,7 +116,7 @@ class CsrfViewMiddleware(BaseCsrfMiddleware):
         response.set_cookie(settings.CSRF_COOKIE_NAME,
                             request.META["CSRF_COOKIE"],
                             max_age=settings.CSRF_COOKIE_AGE,
-                            domain=request.host,
+                            domain=get_cookie_domain(request),
                             path=settings.CSRF_COOKIE_PATH,
                             secure=request.scheme == 'https',
                             httponly=settings.CSRF_COOKIE_HTTPONLY
@@ -122,3 +125,19 @@ class CsrfViewMiddleware(BaseCsrfMiddleware):
         patch_vary_headers(response, ('Cookie',))
         response.csrf_processing_done = True
         return response
+
+
+def get_cookie_domain(request):
+    if '.' not in request.host:
+        # As per spec, browsers do not accept cookie domains without dots in it,
+        # e.g. "localhost", see http://curl.haxx.se/rfc/cookie_spec.html
+        return None
+    default_domain, default_port = split_domain_port(urlparse(settings.SITE_URL).netloc)
+    if request.host == default_domain:
+        # We are on our main domain, set the cookie domain the user has chosen
+        return settings.SESSION_COOKIE_DOMAIN
+    else:
+        # We are on an organizer's custom domain, set no cookie domain, as we do not want
+        # the cookies to be present on any other domain. Setting an explicit value can be
+        # dangerous, see http://erik.io/blog/2014/03/04/definitive-guide-to-cookie-domains/
+        return None
diff --git a/src/pretix/settings.py b/src/pretix/settings.py
index d952f14631..3666be2db7 100644
--- a/src/pretix/settings.py
+++ b/src/pretix/settings.py
@@ -118,6 +118,8 @@ if HAS_CELERY:
     CELERY_RESULT_BACKEND = config.get('celery', 'backend')
     CELERY_SEND_TASK_ERROR_EMAILS = bool(ADMINS)
 
+SESSION_COOKIE_DOMAIN = config.get('pretix', 'cookie_domain', fallback=None)
+
 # Internal settings
 
 STATIC_ROOT = '_static'