CVE-2024-4109 - How a Flaw in Undertow HTTP/2 Handler Can Leak Your Inflight Secrets

On May 2024, a new security issue—CVE-2024-4109—was disclosed, affecting Red Hat’s highly used web server component, Undertow. If you use WildFly, JBoss, Quarkus, or similar Java stacks, this is a bug you shouldn't ignore. Let's break down what it is, how it works, demo an exploit, and how to protect your servers.

What Is CVE-2024-4109?

CVE-2024-4109 is a flaw in Undertow's handling of HTTP/2 connections. The bug allows headers from a previous HTTP/2 stream (which you can think of as a previous request over the same connection) to bleed into the next stream. In simpler words: secrets meant for one user/request might end up sent to another.

Why is this bad? HTTP/2 allows multiplexing—opening several request/response exchanges over a single connection. Each stream should be isolated, but this bug may cross streams, leaking session tokens, authorization headers, or other sensitive data.

Technical Deep Dive

### How HTTP/2 Should Work

Whenever a new request comes in on a stream, Undertow should build its headers *fresh*—nothing from earlier requests should remain.

What Went Wrong

Due to an Undertow bug, header values may not always be cleared correctly between HTTP/2 streams, allowing an attacker/request to see data from a previous exchange.

References

- Red Hat Security Advisory
- Undertow Issue Tracker
- Quarkus Upstream Issue

How to Exploit CVE-2024-4109

Let's walk through a simplified proof-of-concept attack. Imagine two users are sending requests over the same HTTP/2 connection hijacked by a proxy or an attacker.

User A sends a request with the Authorization header: Bearer secret_token_123.

2. User B, over the same HTTP/2 connection/next stream, sends a separate request, expecting clean headers.

Due to CVE-2024-4109, User B's request may get User A's Authorization header.

Here's a minimal PoC using Python’s httpx and hypercorn to simulate parallel clients:

import httpx

# Simulate two clients sharing the same HTTP/2 connection, using connection pool.
client = httpx.Client(http2=True)

# User A, send sensitive header
response_a = client.get('https://your-undertow-server/api/data';, headers={
    'Authorization': 'Bearer secret_token_123'
})

# Simulate a new request/stream right after
response_b = client.get('https://your-undertow-server/api/account';)

print(response_b.request.headers)  # See if Authorization header leaks here

Note: Triggering the flaw may require more fine-tuned manipulation or concurrency. Attackers may use crafted HTTP/2 libraries or custom proxies to replay parallel streams.

Real-World Impact

- Cloud APIs: HTTP/2 is broadly used for performance. In cloud environments, a leakage between requests could expose one user's credentials to another.

Session Hijacking: If session cookies or tokens leak, attackers can impersonate users.

- Regulatory Risk: Leakage of user data, even between internal consumers, *can* be a compliance violation.

Update your Java stack: WildFly, Quarkus, etc.

- Check your vendor’s or distro’s patched versions (e.g., Red Hat Security Alert).

Mitigation

- If unable to patch, consider disabling HTTP/2 support, if feasible, until you can update.

Conclusion

CVE-2024-4109 shows how subtle bugs in complex protocols like HTTP/2 can have sweeping impacts. If you manage Java web infrastructure, check your libraries and upgrade ASAP. Even if you aren’t using HTTP/2 heavily, someone upstream or a load balancer could be, putting your backends at risk.

Stay up-to-date and audit your dependencies regularly!

Timeline

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