The vulnerability stems from the set function's incomplete validation of keypaths. Though it explicitly blocks 'proto' (the fix for CVE-2020-7618), it doesn't account for alternative prototype pollution vectors like 'constructor.prototype'. The recursive property creation logic (lines 23-34 in the provided set.js) combined with the lack of validation for prototype chain manipulation allows attackers to inject properties into Object.prototype. This matches the CWE-1321 pattern of improper prototype attribute control and aligns with the advisory's description of an incomplete fix for a previous prototype pollution vulnerability.