A vulnerability was recently discovered in Django, a popular Python web framework, which has the potential to cause a denial-of-service (DoS) attack through excessive memory usage. The vulnerability, assigned the Common Vulnerabilities and Exposures (CVE) identifier CVE-2023-23969, affects Django 3.2 before 3.2.17, 4. before 4..9, and 4.1 before 4.1.6. This post aims to provide an in-depth understanding of the vulnerability, how to exploit the vulnerability, and how to protect your applications from this specific attack vector.

Vulnerability details

The vulnerability arises from Django's handling of the Accept-Language header, which is used by web browsers to indicate the preferred language for the displayed content to the server. Django caches parsed values of the Accept-Language header to avoid repetitive parsing and reduce overall processing time. However, this caching behavior can lead to excessive memory usage if the raw value of the Accept-Language header is very large. An attacker can send requests with large Accept-Language headers repeatedly to consume server memory, ultimately causing a DoS.

Exploiting the vulnerability

To understand how an attacker can exploit this vulnerability, let's look at a code snippet representing the vulnerable caching behavior in Django:

:::python
# django/utils/translation/trans_real.py

return AcceptLangHeaderCache[lang_string]

:::

Here, the parse_accept_lang_header function takes the raw lang_string value of the Accept-Language header and checks if it exists in the AcceptLangHeaderCache. If it does not, the value is parsed and stored in the cache. However, this cache will continue to grow, as large and unique Accept-Language header values are added, consuming the server's memory.

An attacker can exploit this vulnerability with a simple Python script to generate HTTP requests with large and unique Accept-Language header values, like in the following example:

:::python

attacker.py

import os
import string
import random
import requests

url = 'http://example.com/'

print(response.status_code)

:::

Here, the generate_random_lang_header function generates random lengthy Accept-Language headers which are then sent in a loop with requests to the target server. As the script runs, it will consume the server's memory with each request, eventually causing a DoS.

Resolution

The developers of Django have provided patches for the vulnerability, fixing the excessive memory usage in Django 3.2.17, 4..9, and 4.1.6. Upgrading to these versions or newer will protect your application from this specific vulnerability. You can find more information about the patches and upgrade instructions in Django's official security release notes:

- Django 3.2.17 release notes
- Django 4..9 release notes
- Django 4.1.6 release notes

Conclusion

The discovery of CVE-2023-23969 highlights the importance of staying up-to-date with software updates and testing for potential security vulnerabilities. Always monitor security announcements and patch your applications regularly to prevent exploitation by attackers.

Timeline

Published on: 02/01/2023 19:15:00 UTC
Last modified on: 03/02/2023 16:15:00 UTC