CVE-2026-39601 in Booking Calendar Plugin
Summary
by MITRE • 10/02/2026
Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') vulnerability in WPdevelop Booking Calendar booking allows Leveraging Race Conditions.This issue affects Booking Calendar: from n/a through 11.8.4.
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Analysis
by VulDB Data Team • 10/02/2026
The identified vulnerability represents a classic race condition within the WPdevelop Booking Calendar plugin, specifically categorized under CWE-362 as Concurrent Execution using Shared Resource with Improper Synchronization. This security flaw arises when multiple threads or processes access shared data concurrently without adequate locking mechanisms, leading to unpredictable and potentially dangerous outcomes. In the context of this WordPress plugin, the vulnerability allows an attacker to exploit timing gaps in how booking requests are processed by the server. By sending rapid, concurrent requests that modify the same resource state simultaneously, such as a specific date slot or room availability status, an adversary can bypass intended validation checks designed to prevent double bookings or unauthorized access. This type of flaw is particularly insidious because it relies on precise timing and network latency variations rather than direct code injection or logic errors in single-request flows, making it difficult to detect through standard static analysis tools that do not simulate concurrent execution environments.
From an operational perspective, the impact of this race condition can vary significantly depending on how the plugin handles state transitions for bookings. If the underlying database transactions are not properly isolated, an attacker might successfully book a resource that has already been reserved by another user, leading to data integrity issues and customer disputes. In more severe scenarios involving administrative functions or payment processing hooks triggered during booking confirmation, this vulnerability could potentially be leveraged to escalate privileges or bypass financial controls. For instance, if the plugin checks for availability before creating a record but fails to lock that resource until after the transaction is committed, an attacker can exploit the window between the check and the commit to create duplicate entries. This undermines the core functionality of the application, which relies on accurate real-time inventory management, thereby eroding user trust and potentially causing financial loss if associated with paid bookings or service commitments.
The technical root cause lies in the lack of atomic operations when handling concurrent requests against shared resources like database rows representing available slots. Without proper synchronization primitives such as row-level locking, optimistic concurrency control, or transaction isolation levels set to SERIALIZABLE, the application logic proceeds based on stale data views. This aligns with common patterns found in web applications where performance optimizations inadvertently sacrifice consistency guarantees. The vulnerability affects versions of Booking Calendar from n/a through 11.8.4, indicating a long-standing issue that may have persisted across multiple releases if not addressed by implementing robust concurrency controls. Attackers can utilize automated tools to flood the endpoint with parallel requests, increasing the probability of hitting the race window and successfully manipulating the booking state in their favor.
To mitigate this vulnerability, developers must implement strict synchronization mechanisms for all operations involving shared resources. This includes using database-level locking strategies such as SELECT FOR UPDATE when checking availability before insertion, ensuring that no other transaction can modify or read the specific resource until the current operation is complete. Alternatively, implementing optimistic concurrency control with version numbers or timestamps can help detect and reject conflicting updates after they occur. From a defensive standpoint, administrators should ensure their WordPress environment runs on web servers capable of handling connection queuing effectively to reduce the likelihood of simultaneous request overlap, although this is not a primary fix for application-level race conditions. Furthermore, applying input validation and rate limiting at the API level can help mitigate the volume of concurrent requests an attacker might generate, adding another layer of defense in depth.
In terms of industry standards mapping, this vulnerability corresponds to CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition). It also relates to MITRE ATT&CK technique T1059, specifically Command and Scripting Interpreter if the race condition is leveraged to execute arbitrary code through file upload or inclusion vulnerabilities that are triggered during booking processing. Additionally, it falls under CWE-841: Improper Enforcement of Behavioral Workflow, as the application fails to enforce a strict sequence of operations required for secure state transitions. Security professionals should prioritize patching this vulnerability by upgrading to a version where these concurrency issues have been resolved or applying custom code patches that introduce proper locking mechanisms around critical booking logic. Regular security audits focusing on transactional integrity and concurrent access patterns are essential to prevent similar flaws in future development cycles, ensuring the resilience of web applications against sophisticated timing-based attacks.