views/switch.py in django-grappelli (aka Django Grappelli) before 2.15.2 attempts to prevent external redirection with startswith("/") but this does not consider a protocol-relative URL (e.g., //example.com) attack.
https://github.com/sehmaschine/django-grappelli/pull/976
https://github.com/sehmaschine/django-grappelli/issues/975
https://github.com/sehmaschine/django-grappelli/compare/2.15.1...2.15.2
https://github.com/sehmaschine/django-grappelli/commit/4ca94bcda0fa2720594506853d85e00c8212968f