Zcash sealed its main shielded pool and the migration moved fast

@Zcash activated its Ironwood (NU6.3) network upgrade at block 3,428,143 on Tuesday, sealing the Orchard shielded pool that held about 3.66 million $ZEC and opening a new private pool that starts with zero coins. Every coin remaining inside Orchard must now be walked out by its owner through a protocol-level gate.

How the turnstile works

The only route out of Orchard is a turnstile, an accounting rule at the pool's boundary that caps total withdrawals at the amount verifiably deposited. Because the turnstile rejects any attempt to move out more ZEC than entered, users gain an immediate, trustless guarantee that no more than the correct amount of ZEC can be circulating, without waiting for anyone to migrate. Node output posted by @Zcash hours after activation showed the new Ironwood pool holding 104,035 ZEC against 3.56 million still sitting in Orchard, putting early migration at close to 3% on day one.

For ZEC holders, wallet software supporting Orchard will display a prompt to migrate balances to the new Ironwood pool. Node operators who had not yet upgraded to Zebra 6.0.0 or the alternative Zakura client needed to do so before block 3,428,143 or risk falling onto a minority chain.

The bug that forced the rebuild

The vulnerability was discovered on May 29 by Taylor Hornby, an independent security researcher conducting a protocol audit for Shielded Labs. Hornby used an AI-assisted auditing framework with Anthropic's Opus 4.8 model, released the day before, to surface the flaw. Prior audits of the same circuit with older models had not found the bug.

"The vulnerability could have been exploited to undetectably create an unlimited amount of counterfeit ZEC within Orchard," according to Zcash founder Zooko Wilcox. The vulnerability had been present since Orchard's introduction in May 2022, raising concerns about the integrity of Zcash's private pools. An emergency soft fork on June 2 temporarily disabled Orchard transactions, and the NU6.2 network upgrade on June 3 introduced a corrected circuit, restoring Orchard functionality.

Although the vulnerability was quickly fixed through an emergency network upgrade in early June 2026, the privacy features of Orchard meant there was no cryptographic way to prove whether anyone had exploited it before the patch was released. The Zcash Foundation said there was no evidence the bug was exploited, no unauthorized value creation, and no impact on user privacy. Ironwood addresses this concern by introducing a completely new shielded pool instead of continuing to rely on the existing Orchard pool.

Ironwood incorporates formal verification and independent security audits to strengthen the shielded pool's cryptographic foundation. The upgrade was coordinated across multiple teams in the Zcash ecosystem, including Shielded Labs, Project Tachyon, Valar Group, the Zcash Foundation, and wallet provider Zodl.

Sources:
CoinDesk: Zcash Ironwood goes live
Zodl: Ironwood, a new shielded pool for Zcash
Crypto Briefing: Zcash fixes critical Orchard bug after emergency network upgrade