CVE-2022-23833: In-Depth Analysis of Django MultiPartParser Infinite Loop Vulnerability

A recent vulnerability in Django's MultiPartParser caught the attention of security professionals and developers. This vulnerability, represented by the ID CVE-2022-23833, affects Django 2.2 before version 2.2.27, 3.2 before version 3.2.12, and 4. before version 4..2. In particular, this vulnerability causes an infinite loop when parsing files passed through multipart forms with certain inputs.

In this post, we will delve into the details of this vulnerability, including the original references, code snippets, and possible exploit scenarios.

Official References

- Django Security Advisory
- CVE-2022-23833 on NIST's NVD

Code Snippet

The issue resides in MultiPartParser, which is part of Django's multipart form parsing mechanism. Here's an example of the code affected by this vulnerability:

# django/http/multipartparser.py

def _find_boundary(self, data):
    """
    Find the boundary string in the input data.
    """
    idx = data.find(self._boundary)
    if idx < :
        return None
    return idx

This _find_boundary() function is responsible for finding the boundaries of multipart form inputs. The problem occurs when the given input data is crafted in a way that creates a loop in the data search for the boundary.

Exploit Details

To exploit this vulnerability, an attacker can create a specially crafted input, which, when submitted as part of a multipart form, causes the server to enter an infinite loop. This will lead to resource exhaustion and, in turn, render the server unresponsive, causing a Denial of Service (DoS) condition.

Due to the use of simple string matching, an attacker can craft a multipart boundary that consists of only a part of the actual boundary string. This will cause the _find_boundary() function to always find a partial match in the input data and never progress beyond it, resulting in an infinite loop.

For example, if the expected boundary is "----WebKitFormBoundary", an attacker could submit a form with the custom boundary "----WebKitForm" multiple times. The _find_boundary() function would get stuck in a loop, causing a DoS.

Solutions and Workarounds

The Django team has released patches to address this issue in all affected versions. The patched function now checks for partial boundary matches before entering the loop and also handles any edge cases that might cause the function to run infinitely.

To protect your application from this vulnerability, update Django to the latest secured version as follows:

You can update Django using pip, the Python package manager

pip install --upgrade "Django~=2.2.27"
pip install --upgrade "Django~=3.2.12"
pip install --upgrade "Django~=4..2"

Conclusion

CVE-2022-23833 is a serious DoS vulnerability that affects several versions of Django's MultiPartParser. Developers using the affected versions should take immediate action and upgrade to patched versions to ensure the security of their applications. This post provided a comprehensive look at the vulnerability, including code snippets, original references, and potential exploit scenarios, helping developers better understand the issue and how it can be mitigated.

Timeline

Published on: 02/03/2022 02:15:00 UTC
Last modified on: 02/22/2022 10:18:00 UTC