CVE-2026-91109 in Simply Schedule Appointments Plugin
Summary
by MITRE • 10/01/2026
The Simply Schedule Appointments plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 1.6.12.31 via the 'complete_group' parameter due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with subscriber-level access and above, to disclose every co-booker's private per-appointment id_token (exposed as public_token) alongside their PII (name and email address), then use each leaked token to read, overwrite arbitrary appointment meta on, or cancel the co-booker's appointment via the same REST controller. Exploitation requires the attacker to possess a valid id_token for any single appointment within the targeted group booking.
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Analysis
by VulDB Data Team • 10/01/2026
The vulnerability identified in Simply Schedule Appointments versions up to 1.6.12.31 represents a critical failure in access control mechanisms, specifically classified as an Insecure Direct Object Reference or IDOR under CWE-639. This flaw resides within the plugin's handling of group booking functionalities, where the system fails to adequately validate user permissions against the specific resources being accessed via the REST API endpoint associated with the complete_group parameter. The core technical deficiency lies in the absence of strict authorization checks that verify whether the authenticated user initiating the request is actually authorized to interact with the data object referenced by the provided identifier. Instead, the application relies on predictable or directly exposed identifiers without cross-referencing them against a secure access control list tied to the current session's privileges.
Exploitation of this vulnerability requires an attacker to possess at least subscriber-level access and above within the WordPress environment, which is often achievable through registration or compromised credentials. The attack vector begins with the acquisition of a valid id_token for any single appointment within a targeted group booking structure. Once this token is obtained, typically by viewing source code on a publicly accessible page or intercepting API traffic if logged in as another user, the attacker can manipulate the complete_group parameter to reference other appointments belonging to different users within the same group context. Because the server does not verify that the requesting user has ownership or explicit permission for these specific co-booked slots, it processes the request blindly based on the provided identifier.
The operational impact of this flaw is severe and multifaceted, encompassing both confidentiality and integrity violations against sensitive personal data and business logic. Upon successful exploitation, an attacker can disclose private per-appointment identifiers that are exposed as public tokens, alongside personally identifiable information such as names and email addresses associated with each co-booker. This constitutes a significant privacy breach under regulations like GDPR or CCPA due to the unauthorized exposure of PII. Furthermore, beyond mere data disclosure, the lack of validation allows for active manipulation of appointment states. An attacker can read arbitrary meta fields attached to appointments, overwrite critical configuration details that may affect scheduling logic, or cancel other users' appointments entirely. This capability disrupts business operations and erodes trust in the booking platform's reliability.
From a threat modeling perspective aligned with MITRE ATT&CK, this vulnerability facilitates unauthorized access to data (T1530) and potential disruption of services through appointment cancellation (T1499). The exploitation chain leverages broken object level authorization, allowing lateral movement within the application's logical boundaries without requiring privilege escalation beyond initial low-level authentication. This highlights a common pitfall in RESTful API design where resource identifiers are treated as trusted inputs rather than untrusted data requiring rigorous validation against session context and ownership records.
Mitigation strategies must prioritize immediate patching to version 1.6.12.32 or later, which addresses the missing validation logic by implementing strict object-level access controls. Developers should ensure that every API endpoint verifies that the authenticated user has explicit permission for each specific resource ID referenced in the request parameters before processing any read, write, or delete operations. Additionally, input sanitization and output encoding practices should be reviewed to prevent secondary issues such as cross-site scripting if token values are reflected in responses without proper context handling. For organizations unable to patch immediately, implementing a Web Application Firewall rule that restricts access to the specific REST endpoint for users lacking administrative privileges can provide temporary relief, though this is not a substitute for fixing the underlying authorization logic within the application code itself.