A recently discovered vulnerability (CVE-2024-4109) affects Undertow, a widely-used web server in the Java ecosystem. This vulnerability can potentially lead to information leakage between requests on an HTTP/2 connection. In this post, we'll explore the details of this vulnerability, examine a code snippet to demonstrate the flaw, and provide references to the original reports and resources to mitigate the issue.

Background

Undertow is a popular high-performance web server used by many projects, including the popular JBoss/Wildfly application servers. It supports multiple protocols, including HTTP/1.x and HTTP/2. One of the features in HTTP/2 is the ability to use a single TCP connection for serving multiple requests (known as streams) simultaneously. This helps improve performance significantly.

Exploit Details

The vulnerability lies in the way Undertow handles HTTP/2 request headers. Specifically, an HTTP request header value from a previous stream can be incorrectly reused for a request associated with a subsequent stream on the same HTTP/2 connection. This can lead to potential information leakage between requests.

Here's an example Code Snippet to demonstrate the issue

undertow = Undertow.builder()
        .addHttp2Listener(8443, "localhost", sslContext)
        .setHandler(new HttpHandler() {
            @Override
            public void handleRequest(final HttpServerExchange exchange) throws Exception {
                String user = exchange.getRequestHeaders().getFirst("User");
                String secret = exchange.getRequestHeaders().getFirst("Secret");
                exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain");
                exchange.getResponseSender().send("Hello, " + user + ". Your secret is: " + secret);
            }
        }).build();
undertow.start();

While this is a simplified example, it demonstrates a server implementation that processes two request headers (User and Secret) and returns their values in the response body. If these header values are incorrectly reused from a previous request, it can lead to information leakage - users may receive data intended for other users.

Original References

1. The official Undertow announcement.
2. The CVE details.
3. The GitHub issue and patch.

Mitigation

The Undertow team has already acknowledged the issue and released a patch to fix it. Users of Undertow should upgrade to a version that includes the fix:

2..35.Final

Additionally, users should review their server implementations to ensure that they are correctly handling request headers and not relying on assumptions about their value reuse across requests.

Conclusion

The CVE-2024-4109 vulnerability in Undertow is a reminder that even widely-used, high-performance libraries can have security issues. Developers should stay vigilant, keep their dependencies up-to-date, and follow secure coding practices to minimize the impact of such vulnerabilities.

Timeline

Published on: 12/12/2024 09:15:06 UTC