CVE-2023-27537 - Libcurl Double Free Vulnerability in HSTS Data Sharing and Exploit Details

A new vulnerability, identified as CVE-2023-27537, has been discovered in libcurl versions older than 8.. which can lead to a double-free or use-after-free scenario when sharing HSTS data. This poses a significant security risk, as an attacker may take advantage of these vulnerabilities to compromise the host application of libcurl, possibly allowing remote code execution, denial-of-service, or obtaining sensitive information. This post aims to provide an overview of the vulnerability, a code snippet illustrating the issue, and links to original references and exploit details.

Background

HSTS (HTTP Strict Transport Security) is a web security policy mechanism that helps protect websites against man-in-the-middle attacks, particularly those involving protocol downgrade attacks and cookie hijacking. Libcurl supports HSTS within its library, and this vulnerability arises when two separate "handles" are created, each sharing the same HSTS data. In previous versions of libcurl, sharing HSTS data across multiple threads was introduced without proper documentation and without taking into account the need for mutexes or thread locks to avoid concurrency issues.

Details of the Vulnerability

The libcurl library (versions < 8..) contains a dangerous double-free vulnerability due to improper sharing of HSTS data between separate "handles" and the lack of necessary mutexes or thread locks. When two threads share the same HSTS data without proper synchronization, a double-free or use-after-free scenario may occur, potentially crashing the host application or leading to further exploitation by an attacker.

Code Snippet

The following code snippet illustrates an example of improper sharing of HSTS data between two separate "handles":

CURL *handle1, *handle2;
CURLSH *share;

// Initialize libcurl share handle and set HSTS sharing
share = curl_share_init();
curl_share_setopt(share, CURLSHOPT_SHARE, CURL_LOCK_DATA_HSTS);

// Initialize libcurl handles and associate with the share handle
handle1 = curl_easy_init();
handle2 = curl_easy_init();
curl_easy_setopt(handle1, CURLOPT_SHARE, share);
curl_easy_setopt(handle2, CURLOPT_SHARE, share);

// Perform HTTP requests with both handles concurrently (e.g., in separate threads)

The code above initializes and shares the HSTS data between two libcurl handles, creating a potential double-free or use-after-free vulnerability when invoked concurrently by separate threads.

Links to Original References

1. Libcurl Security Advisory (CVE-2023-27537)
2. Libcurl GitHub Commit Fixing the Issue

Exploit Details

An exploit for this vulnerability would likely involve triggering the double-free or use-after-free scenario by deliberately crafting and sending specially prepared requests to the vulnerable application that uses libcurl. Additionally, the attacker may have an opportunity to inject arbitrary code for execution on the target system, depending on the context of the vulnerable application. Further research and analysis are required to determine the exact exploit conditions and potential impacts.

Conclusion

This libcurl vulnerability (CVE-2023-27537) poses a significant security risk, and the best mitigation strategy would be to update to libcurl 8.. or later to ensure proper protection. Adhering to current best practices and thoroughly reviewing library implementations is essential for maintaining software security. Developers using libcurl must be aware of this vulnerability and take appropriate action to secure their applications, such as ensuring proper thread synchronization and mutexes when sharing HSTS data between separate "handles."

Timeline

Published on: 03/30/2023 20:15:00 UTC
Last modified on: 04/20/2023 09:15:00 UTC