Appointment Booking Calendar < 1.6.11.9 - Unauthenticated SQL Injection via 'append_where_sql' Parameter

Description

The Appointment Booking Calendar — Simply Schedule Appointments Booking Plugin plugin for WordPress is vulnerable to time-based blind SQL Injection via the 'append_where_sql' parameter in all versions up to, and including, 1.6.11.8 due to insufficient escaping on the user supplied parameter and lack of sufficient preparation on the existing SQL query. This makes it possible for unauthenticated attackers to append additional SQL queries into already existing queries that can be used to extract sensitive information from the database. The /appointments/bulk REST endpoint is reachable by unauthenticated attackers because its permission check accepts a public nonce that is embedded in the booking widget's frontend JavaScript (ssa.api.public_nonce) and visible to all site visitors; exploitation requires issuing the request as a PUT with an application/x-www-form-urlencoded body so that PHP's superglobals are not populated and the blocklist check silently passes.