A new vulnerability (CVE-2023-3223) has been discovered in Undertow, the lightweight Java-based web server and servlet container. Specifically, this flaw affects servlets annotated with @MultipartConfig, which might result in an OutOfMemoryError due to large multipart content. This vulnerability could enable unauthorized users to launch remote Denial of Service (DoS) attacks. To make matters worse, this exploit can bypass the fileSizeThreshold limitation by setting the file name to null in the request.

Exploit Details

When a servlet utilizes the @MultipartConfig annotation, it allows the handling of file uploads within a multipart request. However, if an attacker successfully submits a large multipart request, it could potentially cause an OutOfMemoryError, leading to a DoS attack.

For example, given the following servlet with the @MultipartConfig annotation

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.MultipartConfig;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@MultipartConfig(fileSizeThreshold = 1024 * 1024)
public class VulnerableServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle file upload and other request processing
    }
}

An attacker can craft a malicious request, like the following, where the file name is set to null to bypass the fileSizeThreshold limit:

POST /vulnerableservlet HTTP/1.1
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryAbCdEfGh
Content-Length: ... 

------WebKitFormBoundaryAbCdEfGh
Content-Disposition: form-data; name="file"; filename=""
Content-Type: application/octet-stream

[LARGE FILE CONTENT]
------WebKitFormBoundaryAbCdEfGh--

This would cause the server to run out of memory, leading to a DoS attack.

- CVE-2023-3223: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-3223
- Undertow GitHub Repository: https://github.com/undertow-io/undertow

Mitigation

To mitigate this vulnerability, developers should ensure they are running the latest version of Undertow, which contains critical security updates. Additionally, developers can employ stricter validation techniques for multipart requests, such as checking file names and sizes before processing the data.

Furthermore, as a best practice, developers should always utilize the latest security patches, follow secure coding guidelines, and establish a robust vulnerability management process.

Conclusion

In summary, CVE-2023-3223 is a severe vulnerability affecting Undertow servlets annotated with @MultipartConfig. This flaw allows unauthorized users to launch remote DoS attacks by exploiting an OutOfMemoryError caused by large multipart content. By staying aware, up-to-date, and employing secure coding practices, developers can prevent potential security breaches like this one.

Timeline

Published on: 09/27/2023 15:18:00 UTC
Last modified on: 09/28/2023 17:43:00 UTC