CVE-2026-30876: Chamilo LMS Information Disclosure Flaw

CVE-2026-30876 Overview

CVE-2026-30876 is a user enumeration vulnerability in Chamilo LMS, an open-source learning management system. Prior to version 1.11.36, Chamilo allows attackers to determine whether a username exists within the system by observing differences in application responses when submitting valid versus invalid usernames. This information disclosure weakness (CWE-204) enables attackers to build lists of valid user accounts for subsequent targeted attacks.

Critical Impact

Attackers can enumerate valid usernames in Chamilo LMS installations, facilitating targeted credential stuffing, brute-force attacks, and social engineering campaigns against legitimate users.

Affected Products

• Chamilo LMS versions prior to 1.11.36
• All Chamilo LMS installations using default authentication configurations
• Self-hosted and cloud-deployed Chamilo instances

Discovery Timeline

• 2026-03-16 - CVE-2026-30876 published to NVD
• 2026-03-17 - Last updated in NVD database

Technical Details for CVE-2026-30876

Vulnerability Analysis

This vulnerability stems from observable response discrepancies in the Chamilo LMS authentication mechanism. When a user attempts to authenticate or interact with user-related functionality, the application returns different responses depending on whether the submitted username corresponds to an existing account. This behavioral difference allows unauthenticated attackers to systematically probe the system and enumerate valid usernames.

The vulnerability is classified under CWE-204 (Observable Response Discrepancy), which occurs when an application reveals the existence of sensitive information through differences in its behavior. In this case, the sensitive information is the validity of user accounts within the educational platform.

Root Cause

The root cause is improper handling of authentication error messages and response timing in the Chamilo LMS user validation logic. Instead of providing generic, uniform responses regardless of username validity, the application reveals whether an account exists through observable differences in HTTP responses, error messages, or processing times.

Attack Vector

The attack vector is network-based, requiring no authentication or privileges. An attacker can exploit this vulnerability remotely by sending HTTP requests to the Chamilo LMS login endpoint or user lookup functionality. The attack requires some precision in timing or response analysis but does not require user interaction.

The exploitation workflow typically involves:

1. An attacker sends authentication requests with different usernames to the target Chamilo instance
2. The application returns subtly different responses for valid versus invalid usernames
3. By analyzing response content, status codes, or timing differences, the attacker identifies which usernames exist
4. The compiled list of valid usernames can then be used for password spraying, credential stuffing, or targeted phishing attacks against users

For technical details on the vulnerability mechanism, refer to the GitHub Security Advisory GHSA-cqr8-vvqp-4mp4.

Detection Methods for CVE-2026-30876

Indicators of Compromise

• High volume of authentication attempts with distinct usernames from single IP addresses
• Sequential or pattern-based username queries in access logs
• Automated tool signatures in User-Agent strings targeting login endpoints
• Unusual timing patterns in authentication request sequences

Detection Strategies

• Monitor authentication logs for repeated failed login attempts across multiple usernames from the same source
• Implement rate limiting detection on login and user lookup endpoints
• Analyze web application firewall (WAF) logs for enumeration tool patterns
• Deploy behavioral analysis to identify automated probing activity

Monitoring Recommendations

• Enable detailed logging on Chamilo LMS authentication endpoints
• Configure alerting for threshold-based authentication failures
• Review access logs regularly for patterns consistent with enumeration attacks
• Monitor for known attack tool signatures and automated request patterns

How to Mitigate CVE-2026-30876

Immediate Actions Required

• Upgrade Chamilo LMS to version 1.11.36 or later immediately
• Implement rate limiting on authentication endpoints as a defense-in-depth measure
• Deploy Web Application Firewall (WAF) rules to detect and block enumeration attempts
• Review user accounts for signs of compromise following potential enumeration

Patch Information

Chamilo has addressed this vulnerability in version 1.11.36. The patch normalizes authentication responses to prevent attackers from distinguishing between valid and invalid usernames. Organizations should update to the patched version as soon as possible.

For detailed release information, see the Chamilo LMS Release v1.11.36.

Workarounds

• Implement application-level rate limiting to slow enumeration attempts
• Deploy a reverse proxy or WAF with brute-force protection capabilities
• Consider implementing CAPTCHA on authentication forms to impede automated attacks
• Enable multi-factor authentication (MFA) to reduce the impact of successful enumeration

# Example: Rate limiting configuration for nginx reverse proxy
# Add to server or location block for Chamilo login endpoint
limit_req_zone $binary_remote_addr zone=login_limit:10m rate=5r/m;

location /main/auth/login.php {
    limit_req zone=login_limit burst=3 nodelay;
    # Additional proxy configuration...
}