A compromised signer share in Fireblocks’ mpc-lib can expose that same party’s ancestor shares, including its root share, when the attacker also knows the chain code. CERT/CC’s October 26 vulnerability note, revised October 27, identifies the flaw as CVE-2026-90975. It affects integrators that use the library’s BIP32 hardened-derivation path with a missing private parent key. Those running digital-asset custody systems need to trace where signer shares and chain codes travel before treating a leaf-share compromise as confined to one wallet branch.
Fireblocks uses multiparty computation (MPC) to split a wallet’s private key among parties. Hardened derivation is another layer: it should keep a party’s child share from revealing its ancestor share. CERT/CC says the library accepts a derivation without the private parent contribution and substitutes a null buffer. That known value makes the derivation delta computable when the chain code is known.
Missing parent becomes zero
In normal hardened derivation, the private parent key contributes to the value used to derive a child. Here, when that private parent is absent, the implementation uses a buffer of zeroes. CERT/CC describes a fail-open operation: the library continues using the zero buffer. With the associated chain code, someone who has obtained a descendant signer share can calculate the delta between that share and its parent. The relationship can be followed upward through that party’s hierarchy.
A party’s root share is still one share; the other MPC parties’ shares are needed to reconstruct a spendable private key. The risk for a custody team is that a compromise it thought limited to a leaf share can reach the same party’s other derivation levels. In deployments that publish chain codes in watch-only extended public keys, the second prerequisite is already available to an untrusted party.
Derivation downgrade stays quiet
CERT/CC says the downgrade occurs “silently with no error or log signal.” The note credits researcher hodlturk for reporting the issue. CERT/CC lists Fireblocks’ vendor status as unknown, says it could not reach the company to coordinate, and records no vendor statement. At the time of the note, a library fix was unavailable; a complete repair requires a change in mpc-lib.
Identify the integrators and signing services that use mpc-lib’s hardened derivation, and identify derivation operations that proceed without private parent material. Then trace how those services distribute chain codes, particularly through watch-only extended public keys or exports to untrusted systems. Keep the affected party’s ancestor shares in scope when investigating a leaf-share compromise.
Restrict chain codes and signer access
For teams operating affected signing clusters, stop distributing chain codes to untrusted parties, including watch-only extended public keys that carry the chain code, where the deployment can keep them secret. CERT/CC calls chain-code secrecy the most effective available mitigation without a code change, because the delta requires that code. Limit access to signer shares as well; that reduces the chance of the other prerequisite being met.
Inspect the watch-only extended public key (xpub) export and its recipient access list. After restricting distribution, test the export using an untrusted recipient account: the expected result is an access denial, with no chain code delivered. Confirm that the same account has no access to a descendant signer share. Treat any previously exposed chain code and compromised share as an exposure to that party’s ancestors, even after distribution changes. Until a library change and a reliable ancestor-isolation test are available, keep the affected hierarchy in incident scope; test the deployment’s actual derivation path when that change arrives.