diff --git a/src/pretix/api/auth/utils.py b/src/pretix/api/auth/utils.py
new file mode 100644
index 0000000000..0d6e574e8c
--- /dev/null
+++ b/src/pretix/api/auth/utils.py
@@ -0,0 +1,9 @@
+def get_session_key_for_api_auth(user, auth):
+    if user.is_authenticated:
+        return f'api-upload-User-{user.pk}'
+    else:
+        return f'api-upload-{str(type(auth))}-{auth.pk}'
+
+
+def get_session_key_for_api_request(request):
+    return get_session_key_for_api_auth(request.user, request.auth)
diff --git a/src/pretix/api/serializers/fields.py b/src/pretix/api/serializers/fields.py
index d877028ab7..dce9fc0e69 100644
--- a/src/pretix/api/serializers/fields.py
+++ b/src/pretix/api/serializers/fields.py
@@ -37,6 +37,8 @@ from collections import OrderedDict
 from django.core.exceptions import ValidationError
 from rest_framework import serializers
 
+from pretix.api.auth.utils import get_session_key_for_api_request
+
 
 def remove_duplicates_from_list(data):
     return list(OrderedDict.fromkeys(data))
@@ -83,10 +85,16 @@ class UploadedFileField(serializers.Field):
         request = self.context.get('request', None)
         try:
             cf = CachedFile.objects.get(
-                session_key=f'api-upload-{str(type(request.user or request.auth))}-{(request.user or request.auth).pk}',
                 file__isnull=False,
                 pk=data[len("file:"):],
             )
+            if cf.session_key == "api-upload-<class 'django.contrib.auth.models.AnonymousUser'>-None":
+                # OK, backwards-compatibility of a security bug fixed 2026-09, delete this at some point, but should
+                # also be harmless because all files with this key are expired one day after deployment of this fix
+                # and no new files with this key are created
+                pass
+            elif cf.session_key != get_session_key_for_api_request(request):
+                self.fail('not_found')
         except (ValidationError, IndexError):  # invalid uuid
             self.fail('not_found')
         except CachedFile.DoesNotExist:
diff --git a/src/pretix/api/serializers/order.py b/src/pretix/api/serializers/order.py
index 9fa6b5ce92..404526e8b4 100644
--- a/src/pretix/api/serializers/order.py
+++ b/src/pretix/api/serializers/order.py
@@ -41,6 +41,7 @@ from rest_framework.exceptions import ValidationError
 from rest_framework.relations import SlugRelatedField
 from rest_framework.reverse import reverse
 
+from pretix.api.auth.utils import get_session_key_for_api_request
 from pretix.api.serializers import CompatibleJSONField
 from pretix.api.serializers.event import SubEventSerializer
 from pretix.api.serializers.forms import form_field_to_serializer_field
@@ -257,16 +258,21 @@ class AnswerSerializer(I18nAwareModelSerializer):
         if data['answer'] == 'file:keep':
             return data
         try:
-            ao = self.context["request"].user or self.context["request"].auth
             cf = CachedFile.objects.get(
-                session_key=f'api-upload-{str(type(ao))}-{ao.pk}',
                 file__isnull=False,
                 pk=data['answer'][len("file:"):],
             )
+            if cf.session_key == "api-upload-<class 'django.contrib.auth.models.AnonymousUser'>-None":
+                # OK, backwards-compatibility of a security bug fixed 2026-09, delete this at some point, but should
+                # also be harmless because all files with this key are expired one day after deployment of this fix
+                # and no new files with this key are created
+                pass
+            elif cf.session_key != get_session_key_for_api_request(self.context["request"]):
+                raise ValidationError('The submitted file ID "{fid}" was not found.'.format(fid=data['answer']))
         except (ValidationError, IndexError):  # invalid uuid
-            raise ValidationError('The submitted file ID "{fid}" was not found.'.format(fid=data))
+            raise ValidationError('The submitted file ID "{fid}" was not found.'.format(fid=data['answer']))
         except CachedFile.DoesNotExist:
-            raise ValidationError('The submitted file ID "{fid}" was not found.'.format(fid=data))
+            raise ValidationError('The submitted file ID "{fid}" was not found.'.format(fid=data['answer']))
 
         allowed_types = (
             'image/png', 'image/jpeg', 'image/gif', 'application/pdf'
diff --git a/src/pretix/api/views/checkin.py b/src/pretix/api/views/checkin.py
index c6567a4d5c..706a0b3c6e 100644
--- a/src/pretix/api/views/checkin.py
+++ b/src/pretix/api/views/checkin.py
@@ -50,6 +50,7 @@ from rest_framework.generics import ListAPIView
 from rest_framework.permissions import SAFE_METHODS
 from rest_framework.response import Response
 
+from pretix.api.auth.utils import get_session_key_for_api_auth
 from pretix.api.serializers.checkin import (
     CheckinListSerializer, CheckinRPCAnnulInputSerializer,
     CheckinRPCRedeemInputSerializer, MiniCheckinListSerializer,
@@ -327,10 +328,16 @@ with scopes_disabled():
 def _handle_file_upload(data, user, auth):
     try:
         cf = CachedFile.objects.get(
-            session_key=f'api-upload-{str(type(user or auth))}-{(user or auth).pk}',
             file__isnull=False,
             pk=data[len("file:"):],
         )
+        if cf.session_key == "api-upload-<class 'django.contrib.auth.models.AnonymousUser'>-None":
+            # OK, backwards-compatibility of a security bug fixed 2026-09, delete this at some point, but should
+            # also be harmless because all files with this key are expired one day after deployment of this fix
+            # and no new files with this key are created
+            pass
+        elif cf.session_key != get_session_key_for_api_auth(user, auth):
+            raise ValidationError('The submitted file ID "{fid}" was not found.'.format(fid=data))
     except (ValidationError, BaseValidationError, IndexError):  # invalid uuid
         raise ValidationError('The submitted file ID "{fid}" was not found.'.format(fid=data))
     except CachedFile.DoesNotExist:
diff --git a/src/pretix/api/views/upload.py b/src/pretix/api/views/upload.py
index e6426de647..bad09c4f3b 100644
--- a/src/pretix/api/views/upload.py
+++ b/src/pretix/api/views/upload.py
@@ -33,6 +33,7 @@ from rest_framework.views import APIView
 from pretix.api.auth.device import DeviceTokenAuthentication
 from pretix.api.auth.permission import AnyAuthenticatedClientPermission
 from pretix.api.auth.token import TeamTokenAuthentication
+from pretix.api.auth.utils import get_session_key_for_api_request
 from pretix.base.models import CachedFile
 from pretix.helpers.images import (
     IMAGE_TYPES, validate_uploaded_file_for_valid_image,
@@ -78,7 +79,7 @@ class UploadView(APIView):
             web_download=False,
             filename=file_obj.name,
             type=content_type,
-            session_key=f'api-upload-{str(type(request.user or request.auth))}-{(request.user or request.auth).pk}'
+            session_key=get_session_key_for_api_request(request)
         )
         cf.file.save(file_obj.name, file_obj)
         cf.save()