The vulnerability stems from improper handling of user input in template processing functions. JFinal's template engine evaluates expressions during rendering, and the advisory explicitly identifies the 'template function' as the attack vector. The Engine.getTemplate method is the entry point for template processing, and Template.render executes the logic. Together they form the chain for SSTI exploitation when user input isn't properly sanitized, consistent with CWE-94 code injection patterns in template engines.