The vulnerability stems from improper input validation when handling locale parameters in HTTP requests to access files. The Stapler framework's resource resolution in the 'open' method used unvalidated locale components to build file paths, allowing path traversal. The patch adds regex validation for language, country, and variant fields, confirming the vulnerability was in this function. The test cases in Security914Test.java validate() that crafted Accept-Language headers can no longer traverse directories, further confirming this as the attack vector.