Fireblocks MPC Flaw Lets a Compromised Share Expose Its Root

A compromised signer share in Fireblocks’ mpc-lib can expose that party’s ancestor shares, up to its root, when the attacker also knows the associated chain code. CERT/CC’s note identifies CVE-2026-90975 in BIP32 hardened derivation: with no private parent key, the library substitutes a known zero buffer and continues. The delta between child and parent shares then becomes computable. In watch-only extended public keys that expose chain codes, one of the two prerequisites is already available.

MPC still splits the complete wallet key among parties. One recovered party’s root share alone is not a spendable private key. CERT/CC says the downgrade is silent, with no error or log signal, and records no available library fix or Fireblocks vendor statement.

Find signing services using the affected mpc-lib derivation path. Trace operations that lack private parent material and inspect where chain codes go, especially watch-only exports and access granted to untrusted parties. Stop distributing chain codes to those parties where they can be kept secret; restrict signer-share access. Inspect the watch-only xpub export and recipient access list. Test it with an untrusted account: expect access denied and no chain code delivered. If a chain code and a descendant share were exposed together, include that party’s ancestors in the investigation. A complete fix requires a library change, followed by checking the actual derivation path against it.