#36503: NFKC normalization DoS vulnerability in Django 4.2 LTS - needs backport
protection
-------------------------------------+-------------------------------------
Reporter: Nauman Tariq | Type: Bug
Status: new | Component:
| contrib.auth
Version: 4.2 | Severity: Normal
Keywords: security, | Triage Stage:
normalization, dos, | Unreviewed
authentication, LTS, backport |
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Django 4.2 LTS is vulnerable to denial-of-service attacks via slow NFKC
normalization on Windows systems. This vulnerability has been fixed in
Django 5.0.14+ and 5.1.8+ but needs to be backported to Django 4.2 LTS for
comprehensive protection.
## Problem
Python's `unicodedata.normalize("NFKC", ...)` is slow on Windows with long
Unicode strings. Django 4.2 uses NFKC normalization in several functions
without length validation:
1. `django.contrib.auth.forms._unicode_ci_compare`
2. `django.contrib.auth.base_user.AbstractBaseUser.normalize_username`
3. `django.utils.text.slugify` (when `allow_unicode=True`)
## Impact
- **Views affected:** LoginView, LogoutView, set_language
- **Attack vector:** Malicious Unicode input (10,000+ characters)
- **Result:** Server threads hang for seconds/minutes on Windows
- **Scope:** All Django 4.2 LTS applications on Windows
## Proposed Solution
Backport the same length checks used in Django 5.x:
- `_unicode_ci_compare`: 320 character limit (RFC 5321 email standard)
- `normalize_username`: 150 character limit (common username limits)
- `slugify`: 200 character truncation (reasonable slug length)
## Why Django 4.2 LTS Needs This
Django 4.2 is LTS (supported until April 2026). Production applications
using LTS releases should receive the same security protections available
in newer versions.
## Implementation
I have prepared a complete backport with:
- ✅ Length validation before NFKC normalization
- ✅ Full backward compatibility maintained
- ✅ Comprehensive test suite
- ✅ Consistent with Django 5.x approach
Repository: https://github.com/nauman3128/django/tree/27556
Commit: 01873a0847
## Testing
Comprehensive test coverage in
`tests/auth_tests/test_nfkc_vulnerability.py` verifies:
- Normal inputs work correctly
- Length limits enforced properly
- No performance degradation
- Backward compatibility preserved
This backport ensures Django 4.2 LTS users get the same protection as
Django 5.x users.
--
Ticket URL: <https://code.djangoproject.com/ticket/36503>
Django <https://code.djangoproject.com/>
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