CVE-2018-7537: Django Truncator Catastrophic Backtracking Vulnerability

Background

Django is a popular Python web framework that simplifies the development of web applications. It provides various utilities and features to handle common web development tasks. One of these utilities is the Truncator class in the django.utils.text module, which is responsible for truncating text in a safe and efficient manner. However, versions of Django prior to 2.0.3, 1.11.11, and 1.8.19 are vulnerable to a catastrophic backtracking vulnerability in the Truncator class when the html=True argument is passed to the chars() and words() methods. This vulnerability can lead to extremely slow evaluation of certain inputs, making the application susceptible to denial-of-service attacks.

Vulnerability Detail

The vulnerability arises from a regular expression used in the Truncator class. When the html=True argument is provided, the regular expression can exhibit catastrophic backtracking behavior, causing the evaluation to take an excessive amount of time for certain inputs. This can be exploited by an attacker to consume significant server resources and potentially render the application unresponsive. The vulnerable methods, chars() and words(), are used to implement the truncatechars_html and truncatewords_html template filters, which means that any usage of these filters in Django templates can trigger the vulnerability.

pip install --upgrade "django==6.1.2"

Workaround

If upgrading Django is not immediately feasible, a temporary workaround can be applied by avoiding the usage of the truncatechars_html and truncatewords_html template filters. Instead, developers can implement custom template filters or use alternative approaches to achieve the desired text truncation functionality. Here is an example of a custom template filter that truncates text without triggering the vulnerability:

from django import template
from django.utils.text import Truncator

register = template.Library()

@register.filter
def safe_truncatechars_html(value, arg):
    return Truncator(value).chars(arg)

Conclusion

The Django Truncator catastrophic backtracking vulnerability can lead to denial-of-service attacks and application unresponsiveness. It is crucial to upgrade Django to the fixed versions or apply the provided workaround to mitigate the risk. For further assistance and support in vulnerability remediation, please contact DevNack.com.

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