The vulnerability arises from vm2's failure to properly sandbox 'Promises', the component that handles asynchronous operations. While vm2 sanitizes callbacks attached to its own internal Promise implementation, async functions return a global Promise whose .then() and .catch() callbacks are not properly sanitized. This allows Promise.prototype.then and Promise.prototype.catch callback sanitization to be bypassed, enabling attackers to escape the sandbox and run arbitrary code on the host system.