Zcash developers have locked in consensus rule changes for the Ironwood upgrade, set to activate in late July at block height 3,417,100. The upgrade plugs a critical vulnerability in the Orchard shielded pool — a bug that could have let an attacker mint unlimited counterfeit ZEC. The flaw was uncovered during an AI-assisted security review by external researchers in early 2026.
What the bug allowed
The Orchard shielded pool, introduced in May 2022 with the NU5 upgrade, uses the Halo 2 proof system, which doesn't require a trusted setup. But the vulnerability meant someone who knew about it could effectively break the network's supply cap from inside that pool. No limit on how much fake ZEC could be created — the only barrier was discovery.
The researchers who found it didn't publish details, but the fix is now public. Ironwood replaces the compromised Orchard pool with a redesigned one, enforces supply controls through the existing turnstile mechanism, and disables new incoming payments to the old pool.
How Ironwood stops the exploit
The upgrade introduces a redesigned Orchard circuit with a special flag. Once activated at block 3,417,100, that flag will be permanently enabled for the legacy Orchard pool. It blocks new payments to other users inside that pool while letting users still generate change notes — so no one gets stuck mid-transaction. All new Orchard-addressed payments will automatically route to the replacement pool.
The supply controls hinge on the turnstile mechanism, already part of Zcash's shielded system. Every ZEC leaving the old pool must pass through that turnstile, and the total exiting can't exceed the value that verifiably entered it. That prevents any counterfeit coins minted before the upgrade from leaking out.
Ironwood was proposed jointly by ZODL, Tachyon, Valar Group, the Zcash Foundation, and Shielded Labs. The activation target also marks the end-of-support block for zcashd, the original reference client.
Price bounces back
The vulnerability news didn't go unnoticed by traders. ZEC dropped to an intraday low of roughly $252 on June 5. But it quickly recovered, bouncing more than 80% from that floor. The upgrade's details and the fact the bug never got exploited appear to have calmed markets.
What wallet users need to do
Wallet providers that support Orchard are expected to roll out one-click migration tooling. Users won't need to change their existing Orchard addresses — the migration preserves them. But they will need to update their wallet software to stay on the right side of the fork. Anyone still using the old pool after activation could find their incoming payments silently redirected.
