Zcash's Ironwood upgrade sealed the Orchard pool holding ~3.66M ZEC after a 4-year counterfeiting bug was found, replacing it with a new pool that opened from zero.
Blockchain Security News
Zcash (ZEC) activated a network upgrade called Ironwood at block 3,428,143 on July 28. The upgrade, formally known as NU6.3, permanently closed the Orchard shielded pool. A replacement pool opened at the same time with zero coins inside. Every ZEC holder with funds in Orchard must move their coins manually into the new pool.
Orchard held approximately 3.66 million ZEC at the time of the upgrade. At prices around $463 per coin on July 28, that supply was worth roughly $1.7 billion. No new coins can enter Orchard now that the upgrade has activated.
A 4-Year Bug Triggered the Response
Shielded Labs researcher Taylor Hornby identified a flaw in Orchard's proof circuit on May 29. The flaw would have allowed an attacker to mint counterfeit ZEC without leaving any visible record on the blockchain. The bug had been present since Orchard launched in May 2022, meaning it was active for four years before discovery. Developers issued a patch within days of finding it.
The patch fixed the code going forward but could not resolve four years of uncertainty. Orchard is a shielded pool, which means it uses zero-knowledge proofs to hide transaction amounts and participant identities. Those proofs confirm a transaction is valid without revealing any underlying details. Because of that design, the blockchain holds no record of what actually moved inside the pool during those four years.
A CoinDesk Research report published in July found that available evidence points against exploitation. Orchard's balance grew steadily across every year the bug was active, including during last year's price rally. Any counterfeiting followed by a sale would have shown up as net outflows from the pool. No such outflows were recorded across the entire four-year period.
Related Article: Zcash Ironwood Upgrade Locked In for July 28 After Orchard Pool Bug
The Turnstile Locks In What Cannot Be Proven
Ironwood addresses the remaining uncertainty through a rule called a turnstile. The turnstile sits at the boundary between Orchard and the new pool. It caps total withdrawals from Orchard at the amount that was verifiably deposited into it. Any counterfeit coins that may exist inside cannot leave because the pool will not release more than what demonstrably entered.
As of activation, 1,500 ZEC had already moved into the new pool according to a tracker. The remaining 3.66 million ZEC will stay in Orchard until each holder chooses to migrate. The new pool launched with two protections Orchard never had. Transaction records are designed to stay recoverable if quantum computers eventually break current cryptographic standards, a property defined under ZIP 2005. The proof circuit is also undergoing formal verification, a mathematical process that confirms the code behaves correctly across every possible input. That process targets the same category of flaw that affected Orchard. ZEC traded near $463 on July 28, down about 8% on the day and 15% over the prior week, though it remained roughly tenfold higher than one year earlier.
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