The Zcash Foundation released Zebra 4.5.3 and Zebra 5.0.0 as a coordinated response to a critical soundness bug in the Orchard Action circuit. In a June 3, 2026 release note, the foundation said Zebra 4.5.3 temporarily disabled Orchard actions through an emergency soft fork, while Zebra 5.0.0 activated NU6.2 and re-enabled Orchard with a corrected circuit.
The issue was discovered on May 29 by independent security researcher Taylor Hornby during a protocol audit for Shielded Labs, then confirmed by ZODL engineers within hours. The foundation said the vulnerability was contained before any known exploitation, with no evidence of unauthorized value creation and total ZEC supply remaining intact under Zcash’s turnstile mechanism.
Orchard Was Temporarily Disabled Before the Fix
The emergency soft fork in Zebra 4.5.3 activated at mainnet block height 3,363,426, around 02:00 UTC on June 2. After that activation point, Zebra nodes rejected transactions and blocks containing Orchard actions, giving engineers time to finalize the corrected zero-knowledge proof circuit without exposing too much detail about the flaw in advance.
The vulnerability affected soundness in the Orchard zero-knowledge proof circuit, implemented in the halo2_gadgets crate. In practical terms, Zcash Foundation said the bug could have allowed invalid state transitions inside Orchard, potentially enabling double-spending within that pool, although the turnstile mechanism prevented total ZEC supply inflation.
NU6.2 Re-Enables Orchard Under New Consensus Rules
Zebra 5.0.0 activated the NU6.2 hard-fork network upgrade at mainnet block height 3,364,600 and testnet block height 4,052,000. NU6.2 re-enables Orchard actions using the fixed Orchard Action circuit, adds the consensus branch ID 0x5437f330, and routes Orchard proofs to a new per-circuit verifying key.
The foundation said a hard fork was required because fixing a zero-knowledge proof circuit bug requires updating the pinned verifying key, which cannot be handled through a standard node patch alone. Zebra 5.0.0 also adds a consensus rule rejecting Orchard bundles with non-canonical proof sizes from the NU6.2 activation height, permanently closing the vulnerability mitigated by the emergency soft fork.
For node operators, the operational message is clear: older Zebra releases should be upgraded to Zebra 5.0.0 to remain aligned with the post-NU6.2 consensus rules. The foundation said nodes that followed an incorrect fork after activation may need to resync from scratch or restore from a backup taken before the activation height.
The confirmed post-upgrade status is that Orchard has been restored under the corrected circuit, while Sapling and transparent transactions continued operating normally throughout the incident. Broader claims about user behavior, market impact or long-term shielded-pool confidence would require separate data beyond the foundation’s release notes.