CVE-2026-54496: Zebra: Missing copy constraint in halo2_gadgets variable-base scalar multiplication allows under-constrained base, breaking Orchard Action circuit soundness

A soundness vulnerability in the variable-base scalar multiplication gadget of halo2_gadgets allowed a malicious prover to produce a valid proof for an Orchard Action with an under-constrained base point. Because this gadget enforces the diversified-address-integrity condition of the Orchard Action statement, the flaw let a prover satisfy that condition for an arbitrary (pkd, gd, ivk) triple, effectively bypassing the check that binds an Action to the correct incoming viewing key — and therefore to the correct nullifier (nf) and spend validating key (ak) — of the note being spent.

The main practical consequence is that an adversary could have performed a double-spend within the Orchard pool, resulting in a balance violation: the same note could be spent multiple times, each time revealing a distinct, valid-looking nullifier. The total ZEC supply was protected by Zcash’s turnstile mechanism, which bounds value flowing out of any pool, so unbounded inflation of the overall supply was not possible; the exposure was inflation of value within the Orchard pool up to the turnstile-enforced limit.

Exploiting the vulnerability via a double-spend is undetectable on-chain. Exploitation only requires setting private circuit inputs to chosen values, and nullifiers produced by a double-spend are indistinguishable from honest nullifiers, so the zero-knowledge property hides any signature of the attack.

Alternatively, an adversary could (before the fix) have stolen funds by forging a spend authorization for an existing note. To do so they would have to know the note plaintext, which in practice they might obtain by knowing the corresponding incoming viewing key. For example, this could be used to bypass the protection provided by holding spending keys on a hardware wallet, if the linked software wallet were compromised. This form of exploitation cannot be detected via the turnstile (which might be a motivation to exploit the vulnerability in this way rather than via balance violation). It can be detected by the legitimate holder of stolen funds being unable to spend them, or by seeing the note’s nullifier on-chain — but this cannot necessarily be distinguished from compromise of the spending key by other means.

The vulnerability has existed since the Orchard pool was introduced in the NU5 network upgrade (activated May 31, 2022). There is no evidence it was exploited prior to remediation.

Note that use of the vulnerability, either for balance violation or for theft of funds, would need to have occurred before it was remediated. Theft would be in principle detectable from that point (since the real nullifier would appear on-chain). Balance violation would not be detectable at the point of exploitation.