AI-Assisted Audit Uncovers Critical Zcash Orchard Vulnerability That Could Have Minted Unlimited Counterfeit ZEC

A security researcher using Anthropic’s Opus 4.8 model uncovered a critical vulnerability in Zcash’s Orchard shielded pool that could have been exploited to create unlimited undetectable counterfeit ZEC, according to adisclosure published Thursday by Shielded Labs, the nonprofit that funds Zcash development.

Independent security engineer Taylor Hornby, hired by Shielded Labs in April for an ongoing protocol review, discovered the bug on May 29 using a custom AI auditing framework paired with Opus 4.8, which Anthropic released the day before on May 28. Hornby wrote a complete exploit program that successfully generated unlimited counterfeit ZEC in a local test environment. The vulnerability had been live since Orchard’s activation in May 2022, evading years of scrutiny by some of the world’s top cryptographers. The Zcash Open Development Lab and the Zcash Foundation coordinated an emergency two-phase network upgrade, with the fix completed on June 2.

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The flaw stemmed from an under-constrained element of the Orchard circuit, which allowed arbitrary false inputs to pass through an elliptic curve multiplication check and still validate. Because Orchard hides balances and amounts using zero-knowledge proofs, any counterfeit ZEC produced through the exploit would have been indistinguishable from legitimate tokens. Shielded Labs said prior exploitation appears unlikely but cannot be ruled out cryptographically. “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,” the post said.

ZEC fell 31% in 24 hours to $409.64 by Thursday evening, with most of the decline coming in the five hours after disclosure. Arthur Hayes posted that he had dumped his entire ZEC position, writing the privacy thesis “demands perfection” and that uncertainty over supply integrity was disqualifying.

The disclosure is the cleanest counterpoint yet to the AI-attacker thesis that has dominated DeFi security discourse this spring. Former OpenZeppelin CTO Manuel Aráoz argued last week that AI gives attackers asymmetric advantage because defenders must fix every bug while attackers need only one.

Helius CEO Mert Mumtaz wrote on X that the team’s proactive use of advanced AI red-teaming and rapid patch coordination should be read as bullish for the protocol, not bearish. Shielded Labs is now preparing a follow-up network upgrade proposal that would deploy a new shielded pool with turnstile accounting on coins exiting Orchard, allowing anyone to verify the integrity of the Zcash supply.

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