Shielded Labs on Thursday disclosed a critical vulnerability in Zcash's Orchard privacy pool that could have allowed an attacker to create an unlimited number of counterfeit ZEC tokens completely undetected, and the Zcash Open Development Lab coordinated an emergency fix on June 1.
The flaw was discovered May 29 by Taylor Hornby, whom Shielded Labs engaged in April 2026 to look for protocol vulnerabilities. Hornby used Anthropic's Opus 4.8 model during a targeted review of the Orchard circuit and wrote a complete exploit that generated unlimited, undetectable counterfeit ZEC in a local testing environment. Shielded Labs said the same tool would have generated unlimited, undetectable counterfeit tokens in Hornby's mainnet wallet if it had been run on Zcash mainnet.
The disclosure arrived as the zcash token slumped roughly 30% in the past 24 hours, falling to about $400 amid broader market weakness. The market move underscored the immediate stakes: a vulnerability in Orchard, described by Shielded Labs as Zcash's most advanced privacy pool, strikes at the integrity of supply — the central trust point for any cryptocurrency.
Shielded Labs said the bug had been present since Orchard's activation in May 2022, meaning the vulnerability existed undetected for four years. Hornby immediately disclosed the issue to the Zcash Open Development Lab, which implemented an emergency patch on June 1 to close the exploitable path identified in the Orchard circuit.
Shielded Labs framed the disclosure with an unusual admission about detectability: "What makes this particularly challenging is that, due to the privacy properties of Orchard and the nature of the bug, there is no definitive way to determine using only cryptography whether such exploitation occurred before the vulnerability was discovered and fixed. We believe it is important to be transparent about that uncertainty," the lab wrote.
The company added a further, stark assessment: "We think he probably succeeded," acknowledging that the exploit Hornby produced in a test environment likely would have worked on mainnet had it been run there. At the same time Shielded Labs said the bug was fixed quickly and that it is likely the vulnerability was not exploited in practice.
That claim sits at the center of the story's friction. The fix removes the immediate technical avenue for minting counterfeit coins, but because Orchard is a privacy pool the network cannot rely on on-chain cryptography alone to prove that fake ZEC were never created during the four-year window. The result is a structural uncertainty about past supply integrity that a simple patch cannot erase.
To address that uncertainty, Shielded Labs has proposed a network upgrade that would deploy a new shielded pool and enforce turnstile accounting on all coins originating from the Orchard pool. The proposal is intended to let anyone verify the integrity of the ZEC supply independently; Shielded Labs did not specify a deployment timetable in its disclosure.
The Zcash Open Development Lab's emergency fix on June 1 closed the immediate vulnerability, but it did not and could not retroactively prove the absence of prior exploitation. That leaves holders and market participants with one urgent, unresolved question: did counterfeit ZEC enter circulation at any point between Orchard's activation in May 2022 and the June 1 patch?
The next concrete step the community can take is procedural: decide whether to adopt the upgrade Shielded Labs recommended, and how quickly to implement the new shielded pool with turnstile accounting. If enacted, the change would provide a forward-looking mechanism to validate supply; it would not, however, change the historical uncertainty around the four-year period when the flaw existed.
