https://helpx.adobe.com/security/products/magento/apsb23-50.html
The vulnerability with the highest CVSS score of 8.8 (CVE-2023-38218) states that it is an unauthenticated privilege escalation due to improper input validation.
If we look at the diff we see a number of security related changes. One of those changes is the following in Magento\Customer\Plugin\Webapi\Controller\Rest\ValidateCustomerData::validateInputData: