CVE-2020-10821

What it actually does

Stored or reflected cross-site scripting flaw allowing arbitrary JavaScript injection via the theme parameter in account/main.php, executing in the context of authenticated administrative users.

Why it matters to the business

Attackers may hijack monitoring dashboards, exfiltrate credentials, or perform unauthorized configuration changes, disrupting visibility into critical infrastructure and enabling further lateral movement.

How an attack unfolds

An attacker supplies a crafted theme value containing script tags or event handlers to account/main.php; when an administrator loads the affected page the payload executes, enabling cookie theft or action hijacking. Public exploit code has been available since shortly after disclosure.

How to fix it

Upgrade Nagios XI to the latest available release that resolves the theme-parameter XSS. Until upgrades are complete, restrict direct access to /account/main.php and apply input sanitization or WAF filtering on the theme parameter.

How to confirm the fix worked

Confirm the installed Nagios XI version is newer than 5.6.11; re-test the theme parameter with a benign payload such as theme=<script>alert(1)</script> and verify the response is properly encoded or rejected.

Typical attack path

1. Identify a reachable Nagios XI instance and locate the account/main.php endpoint
2. Craft a URL or form submission embedding a malicious JavaScript payload in the theme parameter
3. Deliver the link via phishing or stored configuration to an authenticated administrator
4. Payload executes in the administrator browser context, exfiltrating session data or performing privileged actions

How to detect it

Inspect web access logs for requests to account/main.php containing theme values with HTML tags, JavaScript keywords, or unusual encoding; correlate with administrator session activity for unexpected redirects or cookie exfiltration attempts.

• account/main.php?theme=
• theme=<script
• theme=javascript:
• theme=%3Cscript

What your security team should do

• Prioritize upgrade of all Nagios XI instances to a patched release
• Add web-application firewall rules blocking script tags and event handlers in the theme parameter
• Review administrative accounts for anomalous logins or configuration changes since publication
• Monitor vendor advisories for any subsequent Nagios XI security updates