The vulnerability stems from insufficient sandbox isolation in the code evaluation mechanism. The PoC demonstrates prototype pollution via Object.defineProperty targeting the proto chain within safeEval's execution context. As the primary sandboxed evaluation function, safeEval's failure to properly restrict access to the global prototype chain (despite being marketed as a 'safe' evaluator) directly enables this attack. The vulnerability is explicitly tied to sandbox escape attempts through prototype manipulation, which would be mediated through the core evaluation function.