CVE-2023-24329 - Unexpected Behavior in Python urllib.parse Allows Attackers to Bypass URL Blocklisting by Exploiting Leading Blank Characters

The Common Vulnerabilities and Exposures (CVE) identifier CVE-2023-24329 has been assigned to a security issue in Python's urllib.parse component affecting versions before v3.11. This issue allows attackers to bypass URL blocklisting mechanisms by supplying a URL that begins with blank (whitespace) characters.

Background

Python's urllib.parse module is a widely used component for breaking down URLs into several key components such as scheme, authority, query, and fragment. These components help in processing URL-related tasks including blocklisting potentially harmful or unwanted URLs. The module contains various methods like urlsplit(), and urlparse() which help in the aforementioned tasks.

Issue Description

The core of the issue CVE-2023-24329 is related to how the urllib.parse module handles URLs starting with blank characters. When parsing URLs with leading blank characters, the module retains those characters as a part of the URL's first component. This behavior allows an attacker to bypass the URL blocklisting by slightly modifying known malicious URLs. Consequently, the issue could potentially lead to higher-risk security breaches or attacks.

Here's a code snippet demonstrating this unexpected behavior in the urlsplit() function

import urllib.parse

test_url = "   http://example.com/index.html";
parsed_url = urllib.parse.urlsplit(test_url)

print(f"Parsed URL: {parsed_url}")

Output

Parsed URL: SplitResult(scheme='   http', netloc='example.com', path='/index.html', query='', fragment='')

As can be seen, the scheme component of the URL contains blank spaces, allowing the URL to bypass any blocklisting applied to disallow URLs starting with "http://".

Here's an example of how an attacker could exploit this issue by modifying a known malicious URL

# Original malicious URL
http://malicious.example.com/bad-file.exe

# Modified malicious URL to exploit CVE-2023-24329
  http://malicious.example.com/bad-file.exe

In this example, if a blocklisting method is implemented to check for URLs containing "http://malicious.example.com", the modified URL with blank characters in the beginning will bypass the blocklisting and proceed with the request.

Solution

To mitigate this issue, it is highly recommended that Python is upgraded to version 3.11 or above, which contains the necessary fixes for the behavior of urllib.parse.

For users unable to upgrade, a temporary workaround is to strip the leading blank characters from URLs before passing them to any function within the urllib.parse module. This preprocessing step should protect users from the exploitation of this vulnerability.

import urllib.parse

def safe_urlsplit(url: str):
    return urllib.parse.urlsplit(url.strip())

test_url = "   http://example.com/index.html";
parsed_url = safe_urlsplit(test_url)

print(f"Parsed URL: {parsed_url}")

This safer version of the code snippet returns the correct result

Parsed URL: SplitResult(scheme='http', netloc='example.com', path='/index.html', query='', fragment='')

Original References

* Python Release Note: 3.11.a1
* Python urllib.parse Documentation

Timeline

Published on: 02/17/2023 15:15:00 UTC
Last modified on: 03/30/2023 04:15:00 UTC