The vulnerability stems from the XML parser's handling of '<key>' elements. The unpatched code in parsePlistXML() (lines 153-163 in lib/parse.js) directly used node.childNodes[0].nodeValue for dictionary keys without validating against prototype pollution vectors like 'proto'. The commit 96e2303 added an invariant check specifically in this key-processing path, confirming this was the vulnerable area. The parse() function is the exposed API that triggers this logic. The test cases in test/parse.js demonstrate exploitation via key manipulation, and the CVE description explicitly references .parse() as the attack vector.