CVE-2026-123128 in GemBooking Plugin
Summary
by MITRE • 11/09/2026
The GemBooking WordPress plugin before 1.7.1 does not serialise concurrent booking requests per team member, allowing unauthenticated users to create overlapping confirmed bookings for the same team member across different bookable items by sending simultaneous requests.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 11/09/2026
The vulnerability identified in GemBooking versions prior to 1.7.1 represents a critical race condition within the plugin's core scheduling logic, specifically affecting how concurrent booking requests are processed for individual team members. This flaw stems from a lack of proper synchronization mechanisms when handling multiple simultaneous HTTP POST requests aimed at reserving time slots. In a typical web application environment, especially one dealing with finite resources like appointment slots or service availability, it is imperative that the system validates resource availability and locks those resources during the transaction process to prevent double-booking. However, in this specific implementation, the plugin fails to serialize these concurrent requests on a per-team-member basis. This architectural oversight allows an unauthenticated attacker to exploit the timing gap between the initial validation of slot availability and the final persistence of the booking record into the database.
From a technical perspective, this is a classic Time-of-Check to Time-of-Use (TOCTOU) vulnerability combined with insufficient concurrency control. When two or more requests are sent in rapid succession for the same team member but potentially different bookable items that share underlying resource constraints or simply compete for the same time slot, the application processes them as independent transactions without mutual exclusion. The first request may successfully validate that a slot is free and begin the insertion process, while the second request simultaneously validates the same slot as still being available because the database transaction of the first request has not yet committed. Consequently, both requests are accepted by the system, resulting in overlapping confirmed bookings for the same team member at the same time. This behavior violates the fundamental principle of data integrity and consistency required for scheduling applications.
The operational impact of this vulnerability is significant for businesses relying on accurate appointment management. It leads to overbooking scenarios where a service provider or staff member is scheduled for multiple appointments simultaneously, causing logistical chaos, customer dissatisfaction, and potential financial loss due to no-shows or rescheduling fees that cannot be enforced effectively. Furthermore, because the exploitation requires only unauthenticated access, any visitor to the website can trigger this condition without needing valid credentials. This ease of exploitation increases the likelihood of successful attacks, potentially leading to denial-of-service conditions for specific team members by flooding their schedules with conflicting appointments, thereby rendering them unavailable for legitimate customers.
This vulnerability aligns closely with CWE-362, which describes concurrent execution using shared resources with inconsistent intermediate states, and specifically relates to race conditions in web applications. In the context of the MITRE ATT&CK framework, this behavior can be categorized under T1498 Network Denial of Service or more broadly within exploitation techniques that leverage timing differences for unauthorized access or state manipulation. The lack of serialization is a fundamental design flaw rather than a simple coding error, indicating that the underlying architecture did not account for high-concurrency scenarios common in modern web traffic patterns.
To mitigate this vulnerability, immediate action should be taken to upgrade the GemBooking plugin to version 1.7.1 or later, where these concurrency issues have been addressed through improved serialization logic. For organizations unable to update immediately due to compatibility constraints with other plugins or custom code, temporary mitigations include implementing rate limiting at the web server level to restrict the frequency of booking-related API calls from a single IP address. Additionally, introducing application-level locking mechanisms such as database row locks or distributed locks for team member schedules during the booking transaction can help enforce mutual exclusion. It is also advisable to implement idempotency keys in booking requests to ensure that duplicate submissions are detected and rejected before they impact the schedule integrity. Regular security audits focusing on race conditions and concurrency controls should be integrated into the development lifecycle to prevent similar issues in future updates or custom extensions.