The vulnerability description explicitly identifies the built-in URLValidator as the source of improper input validation. The Struts S2-047 advisory confirms that the default regex in URLValidator is flawed, and the solution involves upgrading to a version with a patched regex. The validate method in the URLValidator class is directly responsible for applying the vulnerable regex pattern, making it the clear entry point for this ReDoS vulnerability. The high confidence stems from the direct correlation between the advisory's workaround (regex replacement) and the function's role in validation.