CVE-2025-57759

Executive Summary

CVE-2025-57759 is a missing authorization vulnerability in the Contao Open Source CMS. Under certain conditions, authenticated back end users may be able to edit fields of pages and articles without having the necessary permissions. This flaw allows privilege escalation within the CMS backend, potentially leading to unauthorized content manipulation. The vulnerability affects Contao versions starting from 5.3.0 up to (but not including) 5.3.38, and versions 5.6.0 up to (but not including) 5.6.1. The CVSS score is 4.3 (Medium), and the EPSS probability is 0.0%, indicating low active exploitation in the wild. Patches are available in versions 5.3.38 and 5.6.1.

Supply Chain & Package Security

This vulnerability is in the Contao CMS, which is a PHP-based open source content management system. It is distributed via Composer (Packagist) and GitHub. The supply chain risk is moderate; users should ensure they are using patched versions from the official Contao repository. No malicious packages have been reported in relation to this CVE.

CVSS Breakdown & Threat Level

• CVSS Score: 4.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: Low (authenticated back end user)
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: Low (unauthorized read of field data)
• Integrity Impact: Low (unauthorized modification of fields)
• Availability Impact: None

Threat Level: Medium. The vulnerability requires authentication but allows low-privilege users to bypass permission checks. Exploitation could lead to unauthorized content changes, but not full system compromise.

Deep Dive & Mechanics

The vulnerability lies in Contao's permission checking logic for back end users. When a user attempts to edit a page or article, the system should verify that the user has the necessary permissions for each field. Due to a flaw in the authorization checks, under certain conditions (likely involving specific field types or nested data structures), the permission validation is bypassed. This allows a user with limited rights to modify fields they should not have access to.

The root cause is a missing or incomplete authorization check in the backend controller or model layer. The exact code path is not publicly disclosed, but it likely involves a failure to properly validate the user's access rights against the field's permission configuration before allowing an edit operation.

Vulnerable vs. Patched Code

The exact code diff is not publicly available. However, the patch likely involves adding or correcting permission checks in the backend editing logic. A typical fix would be to ensure that before any field update, the system calls a method like $user->hasAccessToField($fieldName, $record) and denies the operation if the check fails.

Vulnerable Code (Conceptual):

// In a controller or model
public function updateField($recordId, $fieldName, $value) {
    $record = $this->findRecord($recordId);
    // Missing permission check
    $record->$fieldName = $value;
    $record->save();
}

Patched Code (Conceptual):

public function updateField($recordId, $fieldName, $value) {
    $record = $this->findRecord($recordId);
    $user = BackendUser::getInstance();
    if (!$user->hasAccessToField($fieldName, $record)) {
        throw new AccessDeniedException();
    }
    $record->$fieldName = $value;
    $record->save();
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to a Contao backend with a low-privilege user account (e.g., editor role).
• Target Contao version 5.3.0 to 5.3.37 or 5.6.0.

Exploitation Steps:

1. Log in to the Contao backend as a low-privilege user.
2. Navigate to a page or article that the user should not have full edit rights on.
3. Intercept the edit request (e.g., using Burp Suite) and modify the field parameter to target a restricted field (e.g., published, access, groups).
4. Send the modified request. If the vulnerability is present, the server will accept the change without proper authorization.
5. Verify the change by viewing the page or checking the database.

Example Request (Conceptual):

POST /contao/backend/page/edit/123 HTTP/1.1
Host: target.com
Cookie: contao_backend_session=...
Content-Type: application/x-www-form-urlencoded

field=published&value=1

Blue Team

Detection:

• Monitor backend access logs for unusual edit operations by low-privilege users.
• Look for HTTP requests to /contao/backend/*/edit/* with unexpected field parameters.
• Alert on changes to sensitive fields (e.g., published, access, groups) by users who normally do not have permission to modify them.

Hunting:

• Query database for records where tstamp (timestamp) has been updated by users with low privilege roles.
• Review audit logs for permission denied errors followed by successful edits on the same record.

Indicators of Compromise (IOCs)

• Network: HTTP requests to /contao/backend/*/edit/* with unusual field parameters.
• Host: Modified database entries in tl_page or tl_article where the tstamp field is updated by a user with insufficient permissions.
• Logs: Backend access logs showing successful edits on restricted fields by low-privilege users.
• File: No specific file-based IOCs.

Detection Engineering

YARA Rule

rule CVE_2025_57759_Contao_Backend_Edit {
    meta:
        description = "Detects potential exploitation of CVE-2025-57759 in Contao CMS"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-57759"
    strings:
        $s1 = "/contao/backend/" ascii wide nocase
        $s2 = "/edit/" ascii wide nocase
        $s3 = "field=" ascii wide nocase
        $s4 = "value=" ascii wide nocase
    condition:
        all of ($s1, $s2, $s3, $s4)
}

Sigma Rule

title: Contao Backend Unauthorized Field Edit
id: 8a4b3c2d-1e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects attempts to exploit CVE-2025-57759 by editing restricted fields in Contao backend
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains:
            - '/contao/backend/'
            - '/edit/'
            - 'field='
            - 'value='
    condition: selection
falsepositives:
    - Legitimate admin edits
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-57759 Contao Backend Unauthorized Field Edit";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/contao/backend/";
    http_uri;
    content:"/edit/";
    http_uri;
    content:"field=";
    http_client_body;
    content:"value=";
    http_client_body;
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Contao to version 5.3.38 or 5.6.1 (or later).
2. If immediate upgrade is not possible, restrict backend access to trusted IPs and enforce strict user role reviews.

Long-term Measures:

• Implement regular security audits of Contao backend permissions.
• Enable detailed logging for all backend edit operations.
• Use a Web Application Firewall (WAF) to block suspicious edit requests.

Workarounds:

• No official workarounds are available. Patching is the only solution.

Valters IT
Securing Digital Frontiers