Key Highlights

  • Researcher Taylor Hornby discovered the flaw on May 29 using an AI-assisted audit, finding it had been present in Zcash's Orchard circuit since its activation in May 2022, giving it a four-year window of undetected exposure.

  • The vulnerability stemmed from an under-constrained elliptic curve multiplication element that could allow false inputs to pass verification, theoretically enabling unlimited ZEC minting in the shielded Orchard pool.

  • Shielded Labs patched the flaw by June 1 and said actual exploitation is unlikely, but investor Arthur Hayes publicly announced he exited his entire ZEC position following the disclosure.

Zcash fell 31% after developer organization Shielded Labs publicly disclosed a critical vulnerability in the protocol's Orchard shielded transaction pool. The flaw, discovered on May 29 by security researcher Taylor Hornby during a targeted audit using Anthropic's Opus 4.8 model, had been present in the codebase since Orchard's activation in May 2022.

The vulnerability arose from an under-constrained element in the Orchard zero-knowledge circuit governing elliptic curve multiplication. In practice, an attacker who understood the flaw could have submitted false inputs that still passed verification, potentially allowing the creation of counterfeit ZEC within the shielded pool without detection. As reported following the disclosure, the bug had a four-year window of potential exposure before being caught.

Shielded Labs moved quickly to contain the fallout. The organization patched the vulnerability by June 1 and stated that actual exploitation of the bug is unlikely, noting no evidence of malicious activity in the Orchard pool. The team is now exploring a proposed network upgrade that would allow anyone to cryptographically verify the integrity of the total ZEC supply and prove no counterfeit coins exist in the shielded pool.

Market reaction was sharp. ZEC dropped to $409.64 in the 24 hours following the announcement. Prominent investor Arthur Hayes added further pressure when he announced he had sold his entire ZEC position after the disclosure, citing concerns about whether the privacy properties of the shielded pool could continue to guarantee supply integrity.