Fireblocks mpc-lib hardened derivation fails to isolate ancestor shares
Original title: VU#701544: mpc-lib Hardened Derivation Fails to Protect Ancestor Isolation
CVE-2026-90975 lets a known chain code turn a leaf share into that party's ancestor shares; no vendor fix is available.
CERT/CC says Fireblocks' mpc-lib fails open in BIP32 hardened derivation when the private parent key is missing, substituting a null buffer so the derivation delta becomes computable. An attacker who already holds one party's descendant signer share and knows the chain code can walk up to that party's root share. The flaw does not by itself produce a spendable key, because MPC still blocks reconstructing a full private key from one party's shares. Fireblocks could not be reached, and no library fix is available.
Unconfirmed: Fireblocks had not confirmed the issue or shipped a fix when the note was published.
- CVE-2026-90975 was published October 26, 2026.
- A missing parent secret is replaced with a null buffer, a known constant 0.
- The delta is HMAC-SHA512 of the chain code and related inputs.
- Keeping the chain code confidential blocks ancestor-share recovery without a code change.
- The downgrade is silent, with no error or log signal.
Impact: Recover one party's ancestor shares from a leaf share
| Product | Affected |
|---|---|
| Fireblocks mpc-lib | — |
- 1.Treat the chain code as secret and keep it out of watch-only extended public keys.
- 2.Do not rely on hardened derivation to isolate signer shares.
- 3.Treat compromise of any share as potentially exposing that party's other shares.
- CVEs
- CVE-2026-90975
- Vendors
- Fireblocks
- Products
- mpc-lib
- Organizations
- CERT/CC
Vulnerabilities mentioned
All| CVE | Vulnerability | CVSS | EPSS | Signals | Published |
|---|---|---|---|---|---|
| CVE-2026-90975 | — | — | — |
From the original article
3 min read at kb.cert.orgOverview 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.
kb.cert.org
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