An issue was discovered in Django 5.1 before 5.1.8 and 5.0 before 5.0.14. The NFKC normalization is slow on Windows. As a consequence, django.contrib.auth.views.LoginView, django.contrib.auth.views.LogoutView, and django.views.i18n.set_language are subject to a potential denial-of-service attack via certain inputs with a very large number of Unicode characters.

References

• https://nvd.nist.gov/vuln/detail/CVE-2025-27556
• https://docs.djangoproject.com/en/dev/releases/security
• https://groups.google.com/g/django-announce
• https://www.djangoproject.com/weblog/2025/apr/02/security-releases
• django/django@2cb311f
• django/django@39e2297
• django/django@8c6871b
• django/django@edc2716
• http://www.openwall.com/lists/oss-security/2025/04/02/2