# Appointment Booking Calendar <= 1.4.06 - Unauthenticated Payment Bypass via Insufficient PayPal IPN Verification

- **ID:** WPSEC-2026-1774
- **Plugin:** Appointment Booking Calendar (`appointment-booking-calendar`), https://wordpress.org/plugins/appointment-booking-calendar/
- **Affected versions:** all versions before 1.4.07
- **Fixed in:** 1.4.07 (Update to 1.4.07 or later.)
- **Severity:** Medium 6.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N)
- **Weakness:** CWE-345
- **Usage among sites WPSec scans:** plugin Low, affected versions None seen (as of 2026-10-27)
- **Attack surface analysis:** https://attacksurface.wpsec.com/plugin/appointment-booking-calendar
- **Fix released:** 2026-10-26
- **Published:** 2026-10-27
- **URL:** https://wpsec.com/vuln/WPSEC-2026-1774/

## Description

The Appointment Booking Calendar plugin for WordPress is vulnerable to payment bypass via the PayPal IPN handler (cpabc_ipncheck) in all versions up to, and including, 1.4.06, due to insufficient verification of payment notifications. The handler took the booking ID from the cpabc_ipncheck URL parameter, which is not part of the message PayPal verifies. It did not check the receiver e-mail, currency, amount or payment status (pending eCheck payments were accepted), and it did not stop a transaction ID from being used again. This makes it possible for unauthenticated attackers to mark arbitrary pending bookings as paid and confirmed, without paying the site owner, by sending a genuine PayPal-verified IPN message from an unrelated or minimal payment against any booking ID. Because the confirmation e-mail is also sent to the payer_email in that message, attackers can also receive other customers' booking details (name, e-mail, phone and comments).

## References

- https://wpsec.com/vuln/WPSEC-2026-1774/
- https://plugins.svn.wordpress.org/appointment-booking-calendar/tags/1.4.07/
- https://wordpress.org/plugins/appointment-booking-calendar/

Source: WPSec, https://wpsec.com/vuln/WPSEC-2026-1774/

License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Free to use, share and adapt, also commercially, if you credit WPSec and link to this advisory.