The vulnerability identified as CVE-2025-12788 affects the Hydra Booking — Appointment Scheduling & Booking Calendar plugin for WordPress, specifically versions up to and including 1.1.27. The root cause is the plugin's acceptance of client-controlled payment confirmation data within the tfhb_meeting_paypal_payment_confirmation_callback function without performing proper server-side verification against PayPal's API. This design flaw constitutes a CWE-602 issue, where client-side enforcement is incorrectly trusted for server-side security decisions. Consequently, an unauthenticated attacker can craft and send fake payment confirmation data, tricking the system into marking bookings as paid without any real payment transaction. This bypasses the intended payment requirement, potentially allowing attackers to obtain services or appointments without paying. The vulnerability does not affect confidentiality or availability directly but compromises the integrity of payment verification. Exploitation requires no privileges or user interaction, making it relatively easy to abuse. Although no public exploits are known at this time, the vulnerability's nature and ease of exploitation make it a significant risk for affected sites. The CVSS 3.1 base score is 5.3 (medium), reflecting network attack vector, low complexity, no privileges required, no user interaction, and an impact limited to integrity loss. The vulnerability is particularly relevant for organizations relying on this plugin for revenue-generating appointment scheduling and booking services.

For European organizations, this vulnerability can lead to unauthorized free bookings, resulting in direct financial losses and potential service disruptions due to overbooking or resource misallocation. Businesses such as clinics, salons, consultancy services, and other appointment-based services using the Hydra Booking plugin may experience revenue leakage and reputational damage if customers exploit this flaw. The integrity of payment processing is compromised, undermining trust in the booking system. While the vulnerability does not expose sensitive data or cause denial of service, the financial and operational impacts can be significant, especially for SMEs that rely heavily on online bookings. Additionally, fraudulent bookings may complicate scheduling and resource planning, causing indirect operational inefficiencies. The absence of known exploits reduces immediate risk but does not eliminate the threat, as attackers could develop exploits given the straightforward nature of the flaw.

Organizations should immediately verify if they use the Hydra Booking plugin and identify the version in use. Since no official patch links are currently available, administrators should consider the following mitigations: (1) Disable or deactivate the plugin until a vendor patch is released. (2) Implement server-side validation of payment confirmations by integrating direct API calls to PayPal to verify transaction authenticity before confirming bookings. (3) Employ web application firewalls (WAFs) to detect and block suspicious requests that attempt to forge payment confirmations. (4) Monitor booking and payment logs for anomalies such as unusually high numbers of paid bookings without corresponding PayPal transactions. (5) Restrict access to the payment confirmation callback endpoint to trusted IP ranges if feasible. (6) Engage with the plugin vendor or community to obtain updates or patches and apply them promptly once available. (7) Educate staff to recognize and report suspicious booking activity. These steps go beyond generic advice by focusing on immediate operational controls and compensating technical measures until an official fix is deployed.