CVE-2024-7885 - Exploiting Undertow ProxyProtocolReadListener StringBuilder Reuse

In 2024, a significant vulnerability surfaced in the Undertow web server, tracked as CVE-2024-7885. The issue lies in how the ProxyProtocolReadListener class manages a StringBuilder object when parsing incoming HTTP requests. If not patched, this can potentially lead to information leakage across different HTTP requests — a nightmare scenario for multi-user or high-security deployments.

This article gives you a clear, step-by-step look at CVE-2024-7885, including technical details, sample code, and practical exploitation scenarios. The aim is to make the vulnerability simple to understand, even if you’re not a security expert or Java wizard.

What Is Undertow?

Undertow is a lightweight Java web server known for its flexibility and performance. It’s used in many enterprise environments, directly or within frameworks like WildFly and JBoss EAP.

Type: Information Disclosure

- Severity: Medium (leakage mostly, but sometimes fatal error/connection drop)

ROOT CAUSE

The bug occurs because the method that parses requests, *parseProxyProtocolV1*, reuses the same StringBuilder instance for multiple requests on the same HTTP connection. In Java, a StringBuilder is mutable: if it isn’t properly emptied after each use, data from an old request can stick around and accidentally get included in a new one.

Vulnerable Pseudocode

// Simplified: ProxyProtocolReadListener.java

public class ProxyProtocolReadListener {
    private StringBuilder buffer = new StringBuilder();

    private void parseProxyProtocolV1(ByteBuffer input) {
        // Appends new data to buffer
        while (input.hasRemaining()) {
            char c = (char) input.get();
            buffer.append(c);
            // ... process each char until end of proxy line
        }
        // Suppose parsing finishes, but buffer isn't reset:
        String header = buffer.toString();
        // header now might include leftover data from a previous request!
    }
}

If a new HTTP request comes in on the same connection, the buffer may *already contain extra data* from the previous one! This can:

Exploit Scenario — Step by Step

Environment: An Undertow server with proxy protocol enabled, with multiple clients making requests via the same HTTP connection (e.g., HTTP/1.1 keep-alive, or using a reverse proxy/balancer upstream).

Some old data from Client A gets mixed up.

#### 3. Client B (or application/business logic) receives data that may contain traces of Client A’s request.

First Request (Client A)

PROXY TCP4 192.168.1.2 192.168.1.1 12345 80\r\nGET /test1 HTTP/1.1\r\n

Second Request (Client B)

PROXY TCP4 10.10.10.10 10.10.10.1 54321 808\r\nGET /test2 HTTP/1.1\r\n

But due to the bug, the buffer contains

PROXY TCP4 192.168.1.2 192.168.1.1 12345 80
PROXY TCP4 10.10.10.10 10.10.10.1 54321 808

Now, data meant only for Client A leaks to Client B.

Proof-of-Concept Code

The following snippet simulates what happens if the buffer isn’t cleared. (Don’t use in production!)

public class SimplePPV1Demo {
    private static StringBuilder buffer = new StringBuilder();

    // Simulate two proxy requests arriving on same connection
    public static void main(String[] args) {
        // Client A
        String proxyLineA = "PROXY TCP4 192.168.1.2 192.168.1.1 12345 80\r\n";
        parseProxyProtocolV1(proxyLineA);

        // Client B (same buffer, not cleared)
        String proxyLineB = "PROXY TCP4 10.10.10.10 10.10.10.1 54321 808\r\n";
        parseProxyProtocolV1(proxyLineB);
    }

    private static void parseProxyProtocolV1(String line) {
        buffer.append(line);
        String header = buffer.toString();
        System.out.println("Header parsed: " + header);
        // Oops, buffer must be cleared here!
        // buffer.setLength(); // <== missing in vulnerable code
    }
}

Output

Header parsed: PROXY TCP4 192.168.1.2 192.168.1.1 12345 80
Header parsed: PROXY TCP4 192.168.1.2 192.168.1.1 12345 80
PROXY TCP4 10.10.10.10 10.10.10.1 54321 808

You use Undertow with the proxy protocol feature.

- Your server handles multiple HTTP requests per connection (common with HTTP keep-alive, or behind load balancers).

What’s The Risk?

This bug usually causes outright parsing errors and closes the connection — but sometimes, old data can wrongly cross over between requests. This is especially risky on:

Applications with sensitive headers or session data in the proxy protocol lines.

Bottom Line: If a malicious client guesses the timing, they could steal another user’s proxy line or metadata.

Patch Status & Fix

Upstream fix: Developers patched this by resetting the buffer (buffer.setLength()) after every parse, ensuring no ghost data is carried over.

Reference

- Undertow GitHub Issue #1509
- Red Hat Security Advisory

Upgrade to Undertow version: Any version with the buffer-reset fix — check for the latest releases on Undertow Releases.

Patch immediately: Update Undertow to the latest release if you use the proxy protocol feature.

- If you can’t patch: Disable the proxy protocol feature, or use isolation controls to ensure only trusted connections.

Closing Thoughts

CVE-2024-7885 is a prime example of how a small programming mistake — like not clearing a buffer — can turn into a real-world risk for web security. If you run Java web apps that rely on Undertow and the proxy protocol, don’t wait — apply the patch today.

Timeline

Published on: 08/21/2024 14:15:09 UTC
Last modified on: 08/29/2024 18:18:13 UTC

CVE-2024-7885 - Exploiting Undertow ProxyProtocolReadListener StringBuilder Reuse — A Hands-On Overview