CVE-2025-31510 is a cross-site scripting (XSS) vulnerability classified under CWE-79 that affects LemonLDAP::NG, an open-source web single sign-on (SSO) and access management system. The vulnerability exists in the portal component of LemonLDAP::NG versions before 2.21.0, specifically in the handling of the 'tab' parameter used during Choice authentication. An attacker can craft a malicious URL containing a specially crafted 'tab' parameter that injects arbitrary JavaScript or HTML code into the login page. Because the input is not properly neutralized or sanitized before being reflected in the web page, this leads to stored or reflected XSS. The vulnerability has a CVSS 3.1 base score of 7.2, indicating high severity, with an attack vector of network (remote), no privileges required, no user interaction needed, and a scope change (affecting components beyond the vulnerable code). The impact includes partial confidentiality and integrity loss, such as session hijacking, credential theft, or manipulation of the authentication process. No known public exploits have been reported yet, but the vulnerability's characteristics make it a likely target for attackers aiming to compromise authentication portals. LemonLDAP::NG is widely used in European public sector and enterprise environments for centralized authentication, making this vulnerability particularly relevant for organizations relying on it for secure access management.

For European organizations, exploitation of this XSS vulnerability can lead to significant security risks including theft of user credentials, session hijacking, and unauthorized access to protected resources. Since LemonLDAP::NG is often deployed in critical environments such as government portals, universities, and large enterprises, successful attacks could compromise sensitive personal data and internal systems. The vulnerability undermines the integrity of the authentication process, potentially allowing attackers to bypass security controls or impersonate legitimate users. This can result in data breaches, regulatory non-compliance (e.g., GDPR violations), and reputational damage. The lack of required authentication or user interaction lowers the barrier for attackers, increasing the likelihood of exploitation. Additionally, the scope change in the CVSS vector suggests that the vulnerability affects components beyond the immediate input handling, potentially impacting broader system security. Organizations using vulnerable versions must consider the risk of targeted attacks, especially in sectors with high-value data or critical infrastructure.

The primary mitigation is to upgrade LemonLDAP::NG to version 2.21.0 or later, where this vulnerability has been addressed. If immediate upgrading is not feasible, organizations should implement strict input validation and output encoding on the 'tab' parameter to neutralize any injected scripts. Web application firewalls (WAFs) can be configured to detect and block suspicious payloads targeting the 'tab' parameter. Additionally, security teams should monitor authentication portal logs for unusual URL parameters or access patterns indicative of exploitation attempts. Employing Content Security Policy (CSP) headers can help mitigate the impact of injected scripts by restricting script execution sources. Regular security assessments and penetration testing focused on authentication components are recommended to detect similar vulnerabilities. Finally, educating users about phishing and suspicious links can reduce the risk of successful exploitation via social engineering.