Zcash’s Ironwood upgrade activated on Tuesday at block height 3,428,143 as part of the NU6.3 network upgrade, introducing a new shielded pool called Ironwood. The upgrade follows concerns about a zero‑knowledge circuit flaw in the previous Orchard pool, which was limited to withdrawal functions after the activation. The Orchard pool, launched in 2022, had represented nearly 88% of the blockchain’s shielded token supply at its peak, while older Sapling and Sprout pools accounted for the remainder, according to The Block’s data. A migration tracker indicated that 40,207 ZEC had moved into Ironwood, while roughly 3.6 million tokens remained in Orchard. The Orchard vulnerability, disclosed by Shielded Labs researcher Taylor Hornby in late May, involved a flaw in the pool’s zero‑knowledge circuit that could theoretically permit counterfeit ZEC creation without detection. Analysts stated there was no evidence the flaw had been exploited, but developers opted to patch the pool and deploy a more mathematically sound alternative. Ironwood reuses the corrected Orchard circuit and its Halo 2 proof system, has undergone formal verification, and includes quantum‑recoverable notes under ZIP 2005 and a “turnstile” mechanism that caps outflows from Orchard and allows independent auditing of the circulating supply. Josh Swihart, executive director of the Zcash Open Development Lab, said 14 ZODL engineers contributed 82% of the merged protocol changes, and the upgrade was a collaborative effort involving ZODL, Project Tachyon, Valar Group, the Zcash Foundation, and Shielded Labs. Sean Bowe, a Zcash co‑founder, noted that the cross‑team collaboration eliminated sources of undetectable counterfeiting bugs from the new protocol. The upgrade installs a turnstile that limits the amount of funds that can exit Orchard and enables auditing of ZEC’s circulating supply, supporting the protocol’s 21 million coin cap. The token price fell about 9% on the day of the upgrade before recovering to roughly $475.