CVE-2024-13993 is a reflected cross-site scripting (XSS) vulnerability identified in Nagios XI, a widely used IT infrastructure monitoring solution. The flaw exists in versions prior to 2024R1.1.2 and is triggered via the login page when accessed using older, legacy web browsers. The root cause is insufficient validation or escaping of user-supplied input that is reflected back in the login page's HTML output. This improper neutralization of input (CWE-79) allows an attacker to craft a malicious URL containing JavaScript payloads. When a victim clicks this URL using a vulnerable browser, the script executes within the context of the Nagios XI web application, potentially allowing theft of session cookies, redirection to malicious sites, or execution of arbitrary actions on behalf of the user. Modern browsers implement stricter XSS protections and content security policies that may block or mitigate some attack vectors, but legacy browsers lack these defenses, making them susceptible. The vulnerability requires no authentication or privileges but does require user interaction (clicking the malicious link). The CVSS 4.0 base score of 5.1 reflects a network attack vector, low complexity, no privileges required, user interaction needed, and limited scope impact. No public exploits or active exploitation campaigns have been reported to date. The vulnerability is particularly concerning for organizations relying on Nagios XI for critical monitoring, as successful exploitation could undermine trust in monitoring data or facilitate further attacks within the network.

For European organizations, this vulnerability poses a moderate risk primarily to the confidentiality and integrity of user sessions and potentially the availability of monitoring services if exploited to inject malicious scripts. Attackers could leverage the XSS flaw to hijack sessions of administrators or users accessing Nagios XI via legacy browsers, leading to unauthorized access or manipulation of monitoring data. This could disrupt incident detection and response, delay remediation efforts, or cause false alarms. The impact is heightened in sectors with critical infrastructure monitored by Nagios XI, such as energy, telecommunications, and finance. Additionally, organizations with lax browser update policies or remote users relying on older browsers are more vulnerable. While the vulnerability does not directly compromise the underlying Nagios XI server or network, the ability to execute arbitrary scripts in users' browsers can facilitate phishing, credential theft, or lateral movement within the network. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers often weaponize such vulnerabilities post-disclosure. Therefore, European entities must consider the operational impact of potential exploitation and the risk to their monitoring integrity.

1. Upgrade Nagios XI installations to version 2024R1.1.2 or later, where the vulnerability is patched. 2. Enforce strict browser usage policies that mandate modern, up-to-date browsers with built-in XSS protections for all users accessing Nagios XI, especially administrators. 3. Implement web application firewalls (WAFs) with rules to detect and block reflected XSS payloads targeting the Nagios XI login page. 4. Conduct user awareness training to recognize and avoid clicking suspicious links, particularly those purporting to lead to Nagios XI login pages. 5. Review and harden Nagios XI configurations to minimize exposure, such as restricting access to the login page via VPN or IP whitelisting where feasible. 6. Monitor web server logs for unusual query parameters or repeated suspicious requests to the login page indicative of attempted exploitation. 7. Employ Content Security Policy (CSP) headers on the Nagios XI web server to restrict execution of inline scripts and reduce XSS impact. 8. Regularly audit and update legacy systems and browsers in the environment to reduce the attack surface for browser-based vulnerabilities.