diff --git a/shared/auth/backends.py b/shared/auth/backends.py
index 9be8c7a..8ad1c4d 100644
--- a/shared/auth/backends.py
+++ b/shared/auth/backends.py
@@ -2,6 +2,7 @@ from authens.backends import ENSCASBackend
 
 from django.contrib.auth import get_user_model
 from django.contrib.auth.backends import ModelBackend
+from django.core.exceptions import PermissionDenied
 
 User = get_user_model()
 
@@ -18,6 +19,12 @@ class CASBackend(ENSCASBackend):
 
         return User.objects.create_user(username=username, email=email, full_name=name)
 
+    def _get_or_create(self, cas_login, attributes):
+        try:
+            return super()._get_or_create(cas_login, attributes)
+        except ValueError:
+            raise PermissionDenied
+
 
 class PwdBackend(ModelBackend):
     """Password authentication"""