CVE-2025-31510: LemonLDAP::NG XSS Vulnerability

CVE-2025-31510 Overview

A cross-site scripting (XSS) vulnerability has been identified in the portal component of LemonLDAP::NG before version 2.21.0. This vulnerability allows remote attackers to inject arbitrary web script or HTML into the login page via the tab parameter when using Choice authentication. The flaw stems from improper input validation of user-supplied data in the authentication selection interface.

Critical Impact

Remote attackers can exploit this XSS vulnerability to inject malicious scripts into the login page, potentially enabling session hijacking, credential theft, phishing attacks, or malware distribution targeting users of the affected authentication portal.

Affected Products

• LemonLDAP::NG versions prior to 2.21.0
• LemonLDAP::NG portal with Choice authentication enabled
• Debian systems running vulnerable LemonLDAP::NG packages

Discovery Timeline

• 2026-01-16 - CVE CVE-2025-31510 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2025-31510

Vulnerability Analysis

This reflected XSS vulnerability exists in the LemonLDAP::NG portal's Choice authentication mechanism. The tab parameter, which is used to select between different authentication methods presented to users, fails to properly sanitize user input before rendering it in the HTML response. This allows an attacker to craft a malicious URL containing JavaScript code that executes in the context of the victim's browser session when they click the link.

The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), indicating that the application does not adequately neutralize special characters in user-controllable input before including it in the output. Because the vulnerability exists in the login page—a critical security boundary—successful exploitation could lead to credential harvesting through fake login forms, session token theft, or redirection to malicious sites.

Root Cause

The root cause of this vulnerability is insufficient input validation and output encoding in the portal's handling of the tab parameter within the Choice authentication workflow. When a user selects an authentication method, the tab parameter value is reflected in the page without proper HTML entity encoding or JavaScript escaping, allowing script injection.

Attack Vector

The attack is network-based and requires no authentication or special privileges. An attacker crafts a malicious URL containing JavaScript payload in the tab parameter and distributes it via phishing emails, social engineering, or other means. When a victim clicks the link and visits the compromised login page, the injected script executes in their browser context.

The malicious script can then perform actions such as capturing keystrokes on the login form, stealing session cookies, redirecting users to phishing pages, or modifying the page content to display fake authentication prompts. Since LemonLDAP::NG is commonly used as a Single Sign-On (SSO) solution, successful exploitation could compromise access to multiple downstream applications.

Detection Methods for CVE-2025-31510

Indicators of Compromise

• Unusual or malformed URLs in web server access logs containing the tab parameter with encoded script tags or JavaScript content
• User reports of suspicious login page behavior, unexpected redirects, or modified page appearance
• Browser security warnings or Content Security Policy violations originating from the LemonLDAP::NG portal
• Detection of outbound connections to unknown domains from client browsers during authentication attempts

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block XSS patterns in the tab parameter
• Monitor web server logs for requests containing suspicious payloads such as <script>, javascript:, onerror=, or encoded variants
• Deploy browser-based XSS detection mechanisms and Content Security Policy headers to identify and report injection attempts
• Conduct regular security scans of the LemonLDAP::NG portal for XSS vulnerabilities

Monitoring Recommendations

• Enable verbose logging on the LemonLDAP::NG portal to capture all authentication-related requests including parameter values
• Set up alerting for anomalous patterns in authentication logs, such as requests with unusually long parameter values
• Monitor for client-side JavaScript errors that may indicate attempted XSS exploitation
• Review referrer logs for links to the login page from unexpected external sources

How to Mitigate CVE-2025-31510

Immediate Actions Required

• Upgrade LemonLDAP::NG to version 2.21.0 or later immediately
• If immediate upgrade is not possible, temporarily disable Choice authentication until patching is complete
• Implement Content Security Policy (CSP) headers to mitigate the impact of potential XSS attacks
• Review web server access logs for evidence of exploitation attempts
• Notify users to be cautious of suspicious links to the authentication portal

Patch Information

The vulnerability has been addressed in LemonLDAP::NG version 2.21.0. Organizations should upgrade to this version or later to remediate the vulnerability. Additional details are available in the GitLab Issue #3341. Debian users should refer to the Debian LTS Announcement for package update information.

Workarounds

• Deploy a Web Application Firewall with rules to sanitize or block requests containing XSS payloads in the tab parameter
• Implement strict Content Security Policy headers including script-src 'self' to prevent inline script execution
• Use HTTP-only and Secure flags on session cookies to limit the impact of potential session theft
• Consider temporarily restricting access to the portal to trusted networks until patching is complete

# Example Apache configuration for Content Security Policy
<IfModule mod_headers.c>
    Header set Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; frame-ancestors 'self';"
    Header set X-Content-Type-Options "nosniff"
    Header set X-XSS-Protection "1; mode=block"
</IfModule>