Zcash Foundation issues emergency Zebra upgrade to fix Orchard bug

The Zcash Foundation released Zebra 4.5.3 and Zebra 5.0.0 after engineers found and fixed a critical soundness bug in the Orchard Action circuit. Zebra 4.5.3 disabled Orchard actions and activated an emergency soft fork at mainnet block height 3,363,426, temporarily rejecting transactions and blocks that contained Orchard actions while a corrected circuit was prepared.

The soft fork went live at about 02:00 UTC on June 2 after an earlier coordination attempt ran into patch deployment issues. Zebra 5.0.0 re-enabled Orchard by activating the NU6.2 hard fork at mainnet height 3,364,600, routing Orchard proofs to a new per-circuit verifying key.

The foundation said a hard fork was necessary because the circuit fix required a new pinned verifying key, and it described this as the second security-driven protocol upgrade in Zcash history since 2016. Node operators were strongly urged to upgrade to Zebra 5.0.0 as soon as possible.

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