CVE-2026-110034 in Salon Booking System Plugin
Summary
by MITRE • 10/19/2026
The Salon Booking System WordPress plugin before 10.31.11 does not enforce a minimum of zero when subtracting a fixed-amount discount from a booking's total, allowing an unauthenticated visitor who knows a valid discount code to create a confirmed booking stored with a negative total.
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Analysis
by VulDB Data Team • 10/19/2026
The vulnerability identified in the Salon Booking System WordPress plugin prior to version 10.31.11 represents a critical logic flaw within the application’s pricing and checkout calculation engine. This issue stems from an insufficient validation of numerical boundaries during the processing of discount codes. Specifically, when a user applies a fixed-amount discount code to a booking total, the system performs arithmetic subtraction without verifying that the resulting value remains non-negative. In secure software design, input sanitization must extend beyond simple format checks to include semantic and logical constraints, ensuring that calculated values adhere to expected business rules such as minimum order totals or zero-balance requirements. The absence of this boundary check allows the internal state of a booking record to transition into an invalid negative integer value for its total cost.
From a technical perspective, this flaw is classified under CWE-20: Improper Input Validation and often intersects with CWE-841: Improper Enforcement of Behavioral Constraints. The application fails to enforce the invariant that financial totals must be greater than or equal to zero before persisting data to the database. An unauthenticated attacker who possesses a valid discount code can exploit this logic error by applying a discount amount that exceeds the original price of the service being booked. Because the system does not cap the discount at the total value, it proceeds to store the booking with a negative monetary figure. This behavior indicates a lack of server-side validation for financial integrity, relying instead on client-side assumptions or incomplete backend checks that do not account for edge cases involving high-value discounts relative to low-cost services.
The operational impact of this vulnerability extends beyond mere accounting discrepancies. By creating bookings with negative totals, an attacker can effectively manipulate the system’s inventory and scheduling logic without incurring any financial cost. This can lead to resource exhaustion if an adversary creates a large volume of such fraudulent appointments, potentially blocking legitimate customers from accessing services or overwhelming administrative interfaces used for managing schedules. Furthermore, this flaw may facilitate broader security risks depending on how downstream systems handle negative values. For instance, if the plugin integrates with payment gateways that expect positive transaction amounts, these integrations might fail unpredictably or log erroneous data into financial reporting tools. In more complex e-commerce environments, such logic errors can sometimes be chained with other vulnerabilities to achieve unauthorized access or privilege escalation, although in this specific context, the primary risk is operational disruption and data integrity compromise.
This type of vulnerability aligns with techniques observed in the MITRE ATT&CK framework under T1496: Resource Hijacking, where an attacker uses a system's resources for their own purposes without authorization, albeit here through logical manipulation rather than computational hijacking. It also reflects patterns associated with business logic abuse, which is often difficult to detect via standard vulnerability scanners because it requires understanding the specific workflow of the application. The ability to confirm bookings at no cost undermines the core value proposition of the service and can cause significant reputational damage if customers encounter errors or missing services due to fraudulent appointments occupying available time slots.
To mitigate this risk, immediate action is required by upgrading the Salon Booking System plugin to version 10.31.11 or later, where developers have presumably implemented proper validation logic to ensure that discount applications do not result in negative totals. In addition to updating software, administrators should review their server-side code for similar patterns of arithmetic operations involving user-supplied inputs. Implementing robust input validation is essential; this includes enforcing minimum and maximum bounds on all financial calculations before they are committed to the database. Developers should also consider implementing idempotency checks and transactional integrity constraints within the database layer to prevent invalid states from being persisted regardless of application-level logic errors. Regular security audits focusing on business logic flows, particularly those involving payments and discounts, are recommended to identify and remediate such vulnerabilities before they can be exploited in production environments.