Zcash's Ironwood hard fork, designated NU6.3, activates around July 28 at block height 3,428,143 to seal a soundness flaw in the Orchard shielded pool. Security researcher Taylor Hornby discovered the bug in late May while working with Shielded Labs, prompting developers to implement emergency patches. The fork coincides with the July 15 launch of Zakura 1.0.0, a new node built by Zcash cofounder Sean Bowe and Osmosis cofounder Dev Ojha that syncs the mainnet in approximately four hours and 20 minutes compared to Zebra's roughly 20 hours and 46 minutes.
Taylor Hornby Discovers Orchard Pool Flaw in Late May
Taylor Hornby, working with Shielded Labs, found the flaw in late May. The bug sat inside Orchard's cryptographic circuits. In theory, it lets someone mint counterfeit ZEC inside the shielded pool without leaving a trace. Zcash's privacy design blocks outside observers from checking whether anyone has exploited it. Developers moved fast with a soft fork that shut down the Orchard pool temporarily, followed by hard fork NU6.2 that patched the immediate danger. ZEC's price dropped sharply on the news before recovering part of its losses.
Ironwood Activates Turnstile Mechanism at Block 3,428,143
Once Ironwood activates, the legacy Orchard pool stops accepting new deposits or internal transfers. Existing coins in the old pool still work, but only if they move through a checkpoint called a turnstile on their way to the new Ironwood pool. The turnstile enforces a rule where outflows from the old pool cannot exceed the coins that legitimately went in. The total circulating ZEC supply becomes verifiable the moment Ironwood turns on, without waiting for every user to migrate. If counterfeit coins existed and excess supply tries to leave the old pool, the turnstile blocks it from leaving and the extra coins get destroyed. Project Tachyon is running formal verification on the new Ironwood circuits, producing machine-checked proofs.
Node Operators Must Upgrade Before July 28
Node operators must upgrade before block 3,428,143 hits, expected around 8 a.m. EST on July 28. Wallet providers tied to the Zcash Open Development Lab are adding prompts to guide shielded fund migration. Users holding transparent balances or funds outside the old Orchard pool do not need to take action. The legacy zcashd node software is reaching end of life around the same window.
Zakura 1.0.0 Launches with Four-Hour Sync Time
Zakura 1.0.0 launched July 15, forked from the Zcash Foundation's Zebra codebase. Sean Bowe, a Zcash cofounder and cryptographic engineer leading Project Tachyon, built it with Dev Ojha, the Osmosis cofounder now leading Valar Group. Both projects are funded through private ZEC donations. Zakura syncs the Zcash mainnet in about four hours and 20 minutes, compared to roughly 20 hours and 46 minutes for Zebra in the same test. Pruned snapshots let a new node bootstrap in under two minutes. A zcashd compatibility mode keeps older wallets and integrations working during the transition. Bowe and Ojha state the long-term goal is Visa and Mastercard-level throughput, near 50,000 transactions per second, for private payments. The Zcash Foundation says it welcomes the new client, framing multiple independent node implementations as a safeguard against single points of failure.
Turnstile Design Provides Verifiable Supply Audit
For traders, the Ironwood fork works like a public audit with a deadline. The turnstile design means the market gets a signal within days about whether the May bug was ever exploited. Zakura's faster sync times and lower node costs make it easier for exchanges, miners, and block explorers to run independent infrastructure. ZEC has stabilized above $500 as the July 28 activation approaches, with developers, the Foundation, Shielded Labs, Project Tachyon, and Valar Group aligned around the same upgrade window.
FAQ
What does the Zcash Ironwood hard fork do on July 28?
Ironwood (NU6.3) activates around July 28 at block height 3,428,143. It closes the legacy Orchard shielded pool and opens a new pool built on corrected code after Taylor Hornby discovered a soundness flaw in late May.
How fast does Zakura sync compared to Zebra?
Zakura 1.0.0, launched July 15, syncs the Zcash mainnet in approximately four hours and 20 minutes. Zebra takes roughly 20 hours and 46 minutes in the same test.
What action must node operators take before July 28?
Node operators must upgrade before block 3,428,143, expected around 8 a.m. EST on July 28. Wallet providers tied to the Zcash Open Development Lab are adding prompts to guide shielded fund migration.
