Zcash Ironwood Upgrade Finalizes to Patch Orchard Pool Flaw, Targets July

Zcash developers have finalized consensus rule changes for the Ironwood upgrade, targeting late July. It aims to get an activation at block height 3,417,100 to address a critical vulnerability in the Orchard shielded pool that exposed the network to unlimited counterfeit ZEC minting.

The upgrade introduces a replacement shielded pool, enforces supply controls via an existing turnstile mechanism, and disables new incoming payments to the compromised Orchard pool, all backed by formal verification of the underlying zero-knowledge proof circuits and independent third-party security audits.

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The Orchard Pool Bug: Ironwood to Fix Zcash

The Orchard pool was introduced in May 2022 as part of the NU5 upgrade, which brought the Halo 2 proof system to Zcash and positioned Orchard as the protocol’s next-generation privacy layer. It is a shielded environment leveraging zero-knowledge proofs to obscure transaction amounts and participant addresses without a trusted setup.

The flaw discovered in early 2026 resided in the Orchard protocol’s circuit integrity: an attacker exploiting it could have minted counterfeit ZEC without any on-chain trace detectable through normal node verification.

The bug meant the total ZEC supply enforced by consensus was not actually bounded within the Orchard pool. Because Zcash’s zero-knowledge architecture is precisely what makes Orchard private, the same properties that protect legitimate user transactions also make unauthorized issuance invisible to external observers, and, critically, to the Zcash development team itself.

An AI-assisted security review by external researchers surfaced the flaw, leading to a quiet patch and coordinated disclosure before Ironwood was formally proposed.

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Turnstile, New Pool, and Supply Verification

Ironwood was proposed jointly by ZODL, Tachyon, Valar Group, the Zcash Foundation, and Shielded Labs, a multi-stakeholder governance structure that distinguishes this response from a single-team patch.

The upgrade’s core mechanism is a redesigned Orchard circuit that includes a flag capable of disabling payments to other users within a pool while preserving the ability to generate change notes, which Bowe has described as a privacy safeguard.

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Once activated, that flag will be permanently enabled for the legacy Orchard pool, constraining the valueBalance field and routing all new Orchard-addressed payments automatically to the replacement pool.

The supply controls enforced by Ironwood depend on the protocol’s pre-existing turnstile mechanism; every ZEC exiting the old Orchard pool must pass through the turnstile before entering the new pool, and the turnstile enforces that the total value leaving the old pool cannot exceed the value that verifiably entered it.

Bowe stated: “This combination enforces a bound on the circulating supply of ZEC through the use of the existing turnstile mechanism; the amount of ZEC that anyone can transact with is no more than the amount that is supposed to exist.”

Once migration is complete, on-chain data will allow any full node to independently verify that no counterfeit ZEC crossed into the new pool, restoring trustless supply verification at the protocol level for the first time since the vulnerability was introduced.

The activation target coincides with zcashd end-of-support at block height 3,417,100. Testnet trials, ecosystem coordination, and final security audits remain outstanding before mainnet activation. Wallet providers supporting Orchard are expected to offer one-click migration tooling, and the new pool is designed to preserve existing Orchard addresses, avoiding disruptive key rotation for active users.

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