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On February 17, 2023, a URL parsing vulnerability in certain versions of the Python programming language was published with the ID CVE-2023-24329. The issue lies in the urllib.parse module which contains functions for breaking URLs into components and combining them back into full URLs.

According to the description provided on NVD, the vulnerability has a CVSS v3 base score of 7.5 which indicates it is highly severe. If exploited, this flaw could enable attackers to bypass security protections and filters that rely on URL blocklisting. Essentially, by supplying specially crafted URLs, malicious actors may be able to bypass implemented domain or protocol blacklists.

This creates serious security implications, as failure to filter dangerous URLs could lead to scenarios like arbitrary file reads, SSRF attacks, unauthorized access to internal networks, and remote code execution. Organizations using affected Python versions are strongly advised to update as soon as possible to mitigate any potential attacks leveraging CVE-2023-24329.

In this blog post, we will dive into what the urllib.parse module does understand the technical details of the vulnerability, learn which Python versions are impacted, and, most importantly – how to fix CVE-2023-24329, a URL parsing issue in Python.

A Short Note About urllib.parse Function

The urllib.parse module in Python provides functions for manipulating URLs and their components. It can break down a URL string into its constituent parts like scheme, network location, path, parameters, query, and fragment. Some key functions include:

  • urlparse() – Parses a URL into six components and returns a tuple.
  • urlsplit() – Similar to urlparse() but does not split params.
  • urlunparse() – Takes a parsed tuple and combines it back into a full URL.
  • urldefrag() – Removes the fragment identifier from a URL.
  • urljoin() – Joins a base URL with another relative URL.
  • urlencode() – Encodes query parameters into a URL encoded string.
  • parse_qs() – Parses a query string into a Python dictionary.

The urllib.parse module is extensively used while working with URLs in Python. But as with any parsing logic, it also poses some risks if the input validation is not robust enough.

Summary of CVE-2023-24329

CVE-2023-24329 refers to a security vulnerability in the urllib.parse library of Python. According to the details on CERT/CC, the issue is tied to improper input validation in the URL parsing functions. Specifically, the flaw allows an attacker to provide URLs that can bypass blacklist filtering by starting the URL with whitespace characters.

The improper parsing stems from the fact that urllib.parse does not raise errors for unusual whitespace and instead tries to extract a hostname or scheme regardless. This causes the blocklisting checks on dangerous URLs to fail.

With a CVSS v3 score of 7.5, the issue is classified as high severity. Successful exploitation could mean circumvention of domain, protocol, or IP address filtering put in place as a security measure. Depending on where such filters are implemented, the impact can include SSRF, remote code execution, unauthorized data access, and other threats.

Associated CVE ID CVE-2023-24329
Description A serious security vulnerability in Python’s urllib.parse module. This vulnerability, with a CVSS score of 7.5 (High severity), could allow attackers to bypass URL blocklisting methods by supplying specially crafted URLs.
Associated ZDI ID
CVSS Score 7.5 High
Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Impact Score
Exploitability Score
Attack Vector (AV) Network
Attack Complexity (AC) Low
Privilege Required (PR) None
User Interaction (UI) None
Scope Unchanged
Confidentiality (C) None
Integrity (I) High
availability (a) None

Overall, CVE-2023-24329 represents a serious vulnerability that needs to be addressed promptly, especially for organizations running vulnerable Python versions that rely on URL filtering to secure their applications and infrastructure.

Understanding Technical Details About the CVE-2023-24329 Vulnerability

As per the technical description provided on CERT/CC, the vulnerability arises due to a flaw in URL parsing behavior:

An issue in the urllib.parse component of Python before v3.11 allows attackers to bypass blocklisting methods by supplying a URL that starts with blank characters.

urlparse has a parsing problem when the entire URL starts with blank characters. This problem affects both the parsing of hostname and scheme and eventually causes any blocklisting methods to fail.

The main takeaway is that urllib.parse does not handle URLs starting with whitespace appropriately. Instead of raising an exception, it tries to extract a scheme and hostname regardless.

For example, normally, passing a URL like “” would result in the hostname being “”. But if we add whitespace like ”, urlparse will still try to extract a hostname.

This causes issues when such URLs are checked against blocklists, as most security products do. If a domain like “” is blocked, an attacker could bypass it via ”” which tricks the parser.

So, in essence, CVE-2023-24329 allows malformed URLs to bypass security protections implemented via domain, IP, and protocol blacklists. Proper validation of the input URL is missing, which leads to this vulnerability.

Python Versions Affected

Based on the references provided on NVD, the vulnerable Python versions are:

  • Python 3.7 prior to 3.7.17
  • Python 3.8 prior to 3.8.17
  • Python 3.9 prior to 3.9.17
  • Python 3.10 prior to 3.10.12
  • Python 3.11 prior to 3.11.4

So essentially, all Python 3 versions from 3.7 to 3.11 are impacted unless patched. Organizations running any unpatched Python installs are exposed to potential attacks abusing CVE-2023-24329.

How to Fix CVE-2023-24329 – URL Parsing Issue in Python?

Given the severity of CVE-2023-24329, it is highly recommended to update Python to the latest version as soon as possible. The fix has been released in the following versions:

  • Python 3.12 (and above)
  • Python 3.11.4
  • Python 3.10.12
  • Python 3.9.17
  • Python 3.8.17
  • Python 3.7.17

So upgrading Python to any of these patched releases will resolve the security vulnerability and prevent exploitation.

For example:

# Upgrade Python in Ubuntu/Debian
sudo apt update
sudo apt install python3.11

# Upgrade Python in RHEL/CentOS
sudo yum update python3

If upgrading the Python runtime is not feasible, the vulnerability note mentions an option to use string.lstrip() as a workaround:

from urllib.parse import urlparse
import string

url = string.lstrip(url)
parsed = urlparse(url)

This strips any leading whitespace from the URL before parsing to mitigate the issue. However, upgrading Python is still the recommended solution.

For mitigating potential attacks, organizations should also review their usage of URL allowlists and blocklists in security products like Web Application Firewalls (WAFs), API Gateways, etc. The rules may need to be updated to account for edge cases. Monitoring for anomalies in traffic patterns can also help detect any exploitation attempts.

Bottom Line

CVE-2023-24329 represents a high-severity security vulnerability in Python’s URL parsing module that could lead to a bypass of URL blocklisting filters. Though no active exploits have been reported yet, Python users should aim to patch this issue quickly. Upgrading to the latest 3.11, 3.10, and 3.9 versions that contain the fixes is highly recommended. For anyone unable to upgrade, extra caution needs to be exercised and alternative mitigations like input validation implemented. As with all software, keeping Python up-to-date is key to avoiding potential security problems.


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