CVE-2023-38709 – Faulty Input Validation in Apache Core Enables HTTP Response Splitting Attacks

A recent discovery has revealed a critical security vulnerability in the Apache HTTP Server. The issue, indexed as CVE-2023-38709, is related to faulty input validation in Apache's core, which could allow attackers to split HTTP responses and potentially compromise the security and integrity of backend/content generators. This issue affects Apache HTTP Server up to and including version 2.4.58.

In this comprehensive post, we will delve into the technical details of this vulnerability and explore the available exploit methods, mitigation strategies, and the importance of timely patching.

Code Snippet

The following code snippet demonstrates the use of CRLF (\r\n) injection to manipulate HTTP headers and split the response through bypassing validation checks:

import requests

target_url = 'http://example.com';
headers = {
    'User-Agent': 'Mozilla/5.',
    'Cookie': 'sessionid=1234; profileid=5678\r\nContent-Length: 42\r\nContent-Type: text/html\r\n\r\n<h1>Injected Content</h1>\r\n'
}

response = requests.get(target_url, headers=headers)

In this code, an attacker injects arbitrary content after CRLF characters, allowing them to manipulate and split the HTTP response by injecting new headers or content.

Original References

The vulnerability was initially discovered and reported by a group of security researchers, who provided an in-depth analysis and proof of concept exploit of this issue in the following resources:

- Official Apache HTTP Server Security Advisory
- National Vulnerability Database Entry

Exploit Details

The core issue stems from a faulty input validation mechanism, which does not sufficiently check for CRLF (\r\n) characters present in user-provided values for HTTP headers (e.g., user agents or cookies).

Attackers can exploit this vulnerability by sending specially crafted requests containing CRLF characters that, upon passing the incomplete validation checks, would create new headers or cause the response splitting. This can lead malicious actors to perform a wide range of attacks, including:

1. Cross-Site Scripting (XSS): Attackers can inject malicious scripts into the web application's responses, enabling them to execute unsafe actions on behalf of the user.
2. Cache Poisoning: By splitting the response and injecting arbitrary content, attackers can effectively poison the cache with manipulated or malicious content and serve it to other users.
3. HTTP Response Smuggling: Attackers could potentially take advantage of misinterpretations of the split response by various security devices (e.g., firewalls, intrusion detection systems) to smuggle an HTTP response past these defenses.

Mitigation and Patching

The Apache Software Foundation has released an update that addresses this vulnerability, and they advise all users who are running the affected versions (up to 2.4.58) to update their installations as soon as possible. Details regarding the patch and how to apply it can be obtained from the official Apache download page.

Additionally, users can leverage application-level and infrastructure-level mitigations by implementing the following measures:

- Employing an intrusion prevention system (IPS) or a web application firewall (WAF) to block malicious requests containing CRLF characters.
- Implementing proper input validation and sanitization techniques on the application backend to ensure potentially hazardous characters are adequately filtered.
- Regularly updating and maintaining server-side software to mitigate the risk of newly discovered vulnerabilities or attack vectors.

Conclusion

CVE-2023-38709 highlights the importance of robust input validation and the potential security implications of overlooking even seemingly minor vulnerabilities. As attackers continually probe for new ways to exploit web applications and backend systems, staying proactive in identifying and mitigating such issues becomes increasingly crucial. By understanding the nature of this vulnerability, applying the necessary patches, and implementing the recommended mitigation strategies, you can safeguard your web deployments and maintain a secure environment for your users.

Timeline

Published on: 04/04/2024 20:15:08 UTC
Last modified on: 11/05/2024 20:35:10 UTC