VU#701544: Retraction of mpc-lib hardened derivation fails to protect ancestor isolation

Fireblocks' mpc-lib library contains a cryptographic fail-open vulnerability in its BIP32 hardened derivation implementation in the case of a missing private parent key. This breaks the ancestor isolation property of hardened derivation, allowing an attacker with a compromised lower-level signer share to recover the higher-level share it was derived from. …