The vulnerability CVE-2015-0899 explicitly references the MultiPageValidator implementation in Apache Struts 1.x. The core issue stems from improper validation of the 'page' parameter, which is used to control multi-page form validation. The validate method in MultiPageValidator is responsible for applying validation rules based on the current page context. By manipulating this parameter, attackers can bypass validation steps. The advisory confirms that even applications not explicitly using MultiPageValidator are vulnerable if they rely on Struts' default validation framework, reinforcing the function's central role in the vulnerability.