croixhaug Simply Schedule Appointments Plugin up to 1.6.12.31 on WordPress REST Controller complete_group authorization

Summaryinfo

A vulnerability identified as critical has been detected in croixhaug Simply Schedule Appointments Plugin up to 1.6.12.31. Affected is an unknown function of the component REST Controller. This manipulation of the argument complete_group causes authorization. This vulnerability is tracked as CVE-2026-91109. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability was found in croixhaug Simply Schedule Appointments Plugin up to 1.6.12.31. It has been declared as critical. Affected by this vulnerability is some unknown functionality of the component REST Controller. The manipulation of the argument complete_group with an unknown input leads to a authorization vulnerability. The CWE definition for the vulnerability is CWE-639. The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at wordfence.com. This vulnerability is known as CVE-2026-91109 since 09/14/2026. The exploitation appears to be easy. The attack can be launched remotely. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 10/01/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Productinfo

Type

• WordPress Plugin

Vendor

• croixhaug

Name

• Simply Schedule Appointments Plugin

Version

• 1.6.12.0
• 1.6.12.1
• 1.6.12.2
• 1.6.12.3
• 1.6.12.4
• 1.6.12.5
• 1.6.12.6
• 1.6.12.7
• 1.6.12.8
• 1.6.12.9
• 1.6.12.10
• 1.6.12.11
• 1.6.12.12
• 1.6.12.13
• 1.6.12.14
• 1.6.12.15
• 1.6.12.16
• 1.6.12.17
• 1.6.12.18
• 1.6.12.19
• 1.6.12.20
• 1.6.12.21
• 1.6.12.22
• 1.6.12.23
• 1.6.12.24
• 1.6.12.25
• 1.6.12.26
• 1.6.12.27
• 1.6.12.28
• 1.6.12.29
• 1.6.12.30
• 1.6.12.31

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