Zcash resolves double-spend crisis! Emergency hard fork upgrade to fix zero-knowledge proof vulnerability, official confirms no funds lost

According to an announcement released by the Zcash Foundation on June 3, the well-known privacy coin Zcash's Orchard zero-knowledge proof circuit was recently found to have a serious soundness vulnerability. The development team urgently initiated the second security protocol upgrade since 2016, successfully fixing the issue through deploying Zebra 4.5.3 soft fork and Zebra 5.0.0 (NU6.2) hard fork. Due to swift reporting and response, the vulnerability was resolved before exploitation, preventing any loss of funds or privacy.
(Background: Zcash stopped block production for over 4 hours! Emergency fork caused height 3,364,601 to get stuck, worsening governance crisis)
(Additional context: Arthur Hayes: Now buying privacy-focused Zcash beats Bitcoin)

The well-known privacy cryptocurrency Zcash (ZEC) recently experienced a tense security crisis. On June 3, 2026, the Zcash Foundation officially announced the release of Zebra 4.5.3 (emergency soft fork) and Zebra 5.0.0 (NU6.2 network upgrade) to fix a critical soundness vulnerability in the Orchard zero-knowledge proof circuit. The incident was triggered on May 29 when independent security researcher Taylor Hornby, conducting a protocol audit on behalf of Shielded Labs, discovered this potentially network-threatening flaw and promptly reported it to core engineers.

Details of the vulnerability and Turnstile protection mechanism

According to official explanations, the soundness vulnerability mainly exists in the implementation of halo2_gadgets. If maliciously exploited, an attacker could cause the Orchard pool to accept invalid state transitions, such as double-spending within Orchard. Fortunately, thanks to the Turnstile mechanism in Zcash’s architecture, the total token supply is strictly protected, preventing attackers from fabricating or inflating the overall ZEC issuance arbitrarily.

Orchard, as the core privacy framework introduced in Zcash’s 2022 NU5 upgrade, is based on the trustless Halo 2 proof system and holds a large amount of circulating ZEC. Therefore, the discovery and patching of this vulnerability are critical to the network’s overall security.

Confidential coordination and soft/hard fork upgrades

To prevent the vulnerability details from leaking and being exploited by hackers during the fix, Zcash core engineers, infrastructure operators, miners, and exchanges conducted discreet private coordination. The team first successfully activated the emergency soft fork (Zebra 4.5.3) at mainnet block height 3,363,426 on June 2, temporarily rejecting all transactions containing Orchard to buy time for the fix.

Subsequently, the NU6.2 hard fork (Zebra 5.0.0) was officially activated at midnight Eastern Time on June 3 on mainnet block height 3,364,600, not only re-enabling the patched Orchard but also adding a consensus rule to reject non-canonical proof sizes.

The official emphasized that during this crisis response, user privacy was not compromised, and Sapling and transparent transactions continued to operate normally. This is the second protocol upgrade since Zcash’s mainnet launch in 2016 due to a major security issue. The foundation strongly recommends all node operators to upgrade to Zebra 5.0.0 as soon as possible to ensure they remain on the correct blockchain fork.