The vulnerability stemmed from three key flaws: 1) _unprotect_cache_value's error handling allowed bypassing security checks by returning data even when verification failed. 2) verify_signed_data used non-constant-time HMAC comparison, vulnerable to timing attacks. 3) _get_cache_key's weak key derivation allowed potential cache key collisions. The patch replaced these with constant-time checks, proper error handling, and stronger key derivation via HMAC-SHA384, confirming these were the vulnerable points.