CVE-2023-43665 is a security issue found in Django, one of the most popular Python web frameworks. This vulnerability affects Django version 3.2 before 3.2.22, 4.1 before 4.1.12, and 4.2 before 4.2.6. The problem exists specifically in the django.utils.text.Truncator methods: chars() and words(), when used with the parameter html=True. In simple terms, this bug could let an attacker freeze your website if you use certain template filters to display HTML content.

Let’s break down what this means, see how it can be misused, and find out what you should do.

What Is the Problem?

The Truncator’s chars() and words() methods are usually used to shorten text for display, like making an excerpt. When these functions are called with html=True, they’re supposed to keep HTML tags from breaking, so the output stays valid HTML. That’s helpful if you want to safely limit rich text.

However: When these methods handle a very large or badly-formed HTML string, they can get stuck or take a super long time to process. This is a classic “Denial of Service” (DoS) scenario—a user can send input that hogs the server’s resources, making your website unresponsive to others.

Both the truncatechars_html and truncatewords_html template filters in Django rely on these Truncator methods, so if your templates use them, the risk is real.

> Note: This issue is actually a failed fix for an older Django flaw, CVE-2019-14232.

You use Django versions 3.2.x (before 3.2.22), 4.1.x (before 4.1.12), or 4.2.x (before 4.2.6).

- Your site uses the truncatechars_html or truncatewords_html template filters, or if Truncator.chars(html=True) or Truncator.words(html=True) are used directly.

Why Does It Happen?

Truncating words or characters in HTML is tricky—it requires parsing and reconstructing the HTML to avoid breaking tags. Django tries to do this safely, but its handling can get tripped up if someone provides an input like:

Trick text designed to create huge processing loops

This causes Django’s parser to use up a lot of CPU and memory, grinding the server to a halt.

Exploit Example

Let’s see how this issue can cause denial-of-service. Here’s a minimal Python code example, assuming you’re using a vulnerable version of Django:

from django.utils.text import Truncator

# This is a maliciously crafted huge and malformed HTML string
malicious_html = "<div>" + ("<b>" * 100000) + "Hello" + ("</b>" * 100000) + "</div>"

# This will hang or take very long in vulnerable Django, exhaust server resources
truncated = Truncator(malicious_html).chars(50, html=True)
print(truncated)

Or, in a Django template

{{ malicious_html|truncatechars_html:50 }}

If malicious_html is user-controlled, an attacker can submit input that ties up the server for a long time.

References

- Official Django Security Advisory
- NVD Vulnerability Entry
- Vulnerability detail and patch
- Previous issue: CVE-2019-14232

How Can I Fix This?

Solution: Upgrade your Django version immediately.

Do this using pip

pip install --upgrade "django>=3.2.22"    # or >=4.1.12, >=4.2.6 as appropriate

Mitigation: If you can’t upgrade right away, avoid using truncatechars_html and truncatewords_html, especially on any user-submitted content.

Conclusion

While this vulnerability requires a specific set of conditions, the risk of a denial-of-service outage is serious. Django’s popularity means lots of sites use its HTML-safe template filters, so it’s important to apply this security update—even if your site *might* not be affected.

Stay safe: upgrade as soon as possible.

*If you want to learn more, check the official Django release notes for details and other patches. If you run into this issue or have questions, consider posting on community forums or Django’s security mailing list.*

Timeline

Published on: 11/03/2023 05:15:30 UTC
Last modified on: 12/21/2023 22:15:14 UTC