CVE-2025-6639

Executive Summary

CVE-2025-6639 is an Insecure Direct Object Reference (IDOR) vulnerability discovered in the Tutor LMS Pro WordPress plugin, affecting all versions up to and including 3.8.3. This flaw resides in the tutor_assignment_submit() function, which fails to validate a user-controlled key when viewing or editing assignment submissions. An authenticated attacker with Subscriber-level access or higher can exploit this to view and modify assignment submissions belonging to other students. The vulnerability has a CVSS score of 5.4 (Medium), with a low attack complexity and no user interaction required beyond authentication. As of this report, no official patch has been released, and no public proof-of-concept (PoC) or exploit code is available.

Supply Chain & Package Security

This vulnerability affects the WordPress ecosystem specifically through the Tutor LMS Pro plugin. Since it is a commercial plugin distributed via third-party marketplaces (e.g., CodeCanyon), there is no direct NPM, PyPI, Go, or Ruby package involved. However, the plugin may be bundled with themes or other plugins, increasing the attack surface. Administrators should verify the integrity of their plugin files and monitor for unauthorized modifications.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs Subscriber-level access.
• User Interaction (UI): None (N) – No victim action needed.
• Scope (S): Unchanged (U) – Exploit does not affect other components.
• Confidentiality Impact (C): Low (L) – Limited data exposure (assignment submissions).
• Integrity Impact (I): Low (L) – Limited data modification (assignment submissions).
• Availability Impact (A): None (N) – No denial of service.

Threat Level: Medium. The vulnerability requires authentication but allows unauthorized access to sensitive student data and the ability to tamper with submissions. In an educational context, this could lead to grade manipulation or privacy breaches.

Deep Dive & Mechanics

The vulnerability is a classic Insecure Direct Object Reference (CWE-639) in the tutor_assignment_submit() function. This function is responsible for handling assignment submissions via AJAX requests. The flaw occurs because the function uses a user-controlled parameter (likely assignment_id or submission_id) to fetch and update submission records without verifying that the current user owns the submission or has permission to access it.

Root Cause: Missing authorization check on the submission ID parameter. The function retrieves the submission object directly from the database using the provided ID and then allows the authenticated user to view or edit it, assuming the user is the owner.

Attack Flow:

1. An attacker authenticates as a Subscriber (or higher role).
2. The attacker crafts a POST request to the WordPress AJAX endpoint wp_ajax_tutor_assignment_submit with a manipulated submission_id parameter.
3. The server processes the request without verifying ownership, returning the submission data or allowing modifications.
4. The attacker can iterate through submission IDs to access all student submissions.

Affected Component: tutor_assignment_submit() function in the Tutor LMS Pro plugin (file likely located in tutor/tutor-lms-pro.php or similar).

Vulnerable vs. Patched Code

Since no patch is available, the following is a hypothetical comparison based on the vulnerability description.

Vulnerable Code (Conceptual):

function tutor_assignment_submit() {
    $submission_id = $_POST['submission_id'];
    $submission = get_post($submission_id);
    // No ownership check
    if ($submission) {
        // Process view/edit
        wp_send_json_success($submission);
    }
}

Patched Code (Conceptual):

function tutor_assignment_submit() {
    $submission_id = $_POST['submission_id'];
    $submission = get_post($submission_id);
    // Verify ownership
    if ($submission && $submission->post_author == get_current_user_id()) {
        // Process view/edit
        wp_send_json_success($submission);
    } else {
        wp_send_json_error('Unauthorized');
    }
}

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• WordPress instance with Tutor LMS Pro <= 3.8.3.
• Valid user account with Subscriber role or higher.
• Knowledge of a valid assignment submission ID.

Exploitation Steps:

1. Log in to WordPress as a Subscriber.
2. Intercept a legitimate assignment submission request using Burp Suite or browser dev tools.
3. Identify the submission_id parameter in the POST data.
4. Modify the submission_id to another value (e.g., increment by 1).
5. Send the request to view or edit another student's submission.

Example Request (View):

POST /wp-admin/admin-ajax.php HTTP/1.1
Host: target-site.com
Content-Type: application/x-www-form-urlencoded
Cookie: wordpress_logged_in_...

action=tutor_assignment_submit&submission_id=123&task=view

Example Request (Edit):

POST /wp-admin/admin-ajax.php HTTP/1.1
Host: target-site.com
Content-Type: application/x-www-form-urlencoded
Cookie: wordpress_logged_in_...

action=tutor_assignment_submit&submission_id=123&task=edit&content=malicious_data

Blue Team (Detection & Hardening)

Detection:

• Monitor AJAX requests to admin-ajax.php with action=tutor_assignment_submit.
• Look for rapid sequential submission_id values from the same user.
• Alert on any Subscriber-level user accessing submission IDs that do not belong to them.

Hardening (Temporary Workaround):

• Disable the tutor_assignment_submit AJAX action for non-admin users via a custom plugin or .htaccess rule.
• Implement a Web Application Firewall (WAF) rule to block requests with suspicious submission_id patterns.

Example WAF Rule (ModSecurity):

SecRule ARGS:submission_id "!^\d+$" "id:1000,phase:2,deny,status:403,msg:'IDOR attempt blocked'"

Indicators of Compromise (IOCs)

• Network IOCs:

  • Repeated POST requests to /wp-admin/admin-ajax.php with action=tutor_assignment_submit.
  • Sequential or out-of-range submission_id values in POST data.
  • Unusual task parameters (e.g., view or edit) from low-privileged users.

• Host IOCs:

  • WordPress debug logs showing tutor_assignment_submit calls with mismatched user IDs.
  • Modified assignment submission content in the database (e.g., wp_posts table).

• File IOCs:

  • Suspicious entries in wp-content/plugins/tutor/tutor-lms-pro.php or related files.

Detection Engineering

YARA Rule

rule CVE_2025_6639_Tutor_LMS_IDOR {
    meta:
        description = "Detects exploitation attempts of CVE-2025-6639 in Tutor LMS Pro"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-6639"
    strings:
        $ajax_action = "tutor_assignment_submit"
        $submission_id = "submission_id"
        $task_view = "task=view"
        $task_edit = "task=edit"
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-6639 Tutor LMS Pro IDOR Exploitation
id: 8a4b3c2d-1e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects potential exploitation of IDOR vulnerability in Tutor LMS Pro
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'action=tutor_assignment_submit'
        cs-uri-query|contains: 'submission_id='
    condition: selection
falsepositives:
    - Legitimate assignment submissions by the owner
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-6639 Tutor LMS Pro IDOR Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=tutor_assignment_submit"; http_client_body; content:"submission_id="; http_client_body; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin if not critical for operations until a patch is released.
2. Restrict Subscriber permissions – Remove the ability to submit assignments if possible.
3. Implement a custom patch – Add ownership checks in the tutor_assignment_submit() function (see patched code example above).

Long-Term Fix:

• Monitor the vendor's website for an official update (version > 3.8.3).
• Apply the patch as soon as it becomes available.
• Conduct a security audit of all WordPress plugins for similar IDOR vulnerabilities.

Workaround Code (Add to theme's functions.php or custom plugin):

add_action('wp_ajax_tutor_assignment_submit', 'custom_tutor_assignment_submit', 1);
function custom_tutor_assignment_submit() {
    $submission_id = intval($_POST['submission_id']);
    $submission = get_post($submission_id);
    if ($submission && $submission->post_author != get_current_user_id()) {
        wp_send_json_error('Unauthorized');
        exit;
    }
}

Valters IT
Securing Digital Frontiers