The vulnerability stems from insufficient input validation in the toggleSubpalette handler of executePostActions. The pre-patch code lacked checks for whether the user-controlled 'field' parameter was a valid selector field or allowed for the current user's permissions. This allowed attackers to inject arbitrary SQL through the field parameter, as demonstrated in the exploit example. The patch adds critical validation checks (including field allowlist verification and user permission checks) to prevent SQL injection, confirming the original vulnerability location.