Zcash Upgrade Could Lock $16 Million in Crypto Forever

Zcash is preparing to shut down its oldest privacy technology, potentially leaving millions of dollars in cryptocurrency inaccessible to their owners—even if they still have the passwords and private keys needed to access their wallets.

Approximately 12,224 ZEC remain in Sprout, the cryptocurrency's original privacy pool, according to ZecStats data updated Thursday.

At recent market prices, those holdings represent roughly $15 million to $16 million that could become effectively frozen when the network activates its NU7 upgrade.

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“NU7 (block 3,543,405) makes v4 transactions invalid, so what remains in Sprout is unspendable: frozen, not burned,” Zecstats says in its information card for Sprout.

The upgrade is scheduled for block 3,543,405, expected Thursday evening in the United States. The exact deadline depends on how quickly the blockchain processes new blocks, rather than a predetermined clock time.

Why is Zcash freezing millions in crypto?

Unlike Bitcoin, where transactions are generally visible to anyone examining the blockchain, Zcash allows users to conceal transaction amounts and the addresses involved. It accomplishes this through technology called zero-knowledge proofs, which can verify that a transaction is valid without revealing its private details.

Sprout introduced this technology when Zcash launched in 2016, but the team subsequently introduced improved privacy systems, including Sapling, Orchard, and Ironwood. Sprout has been closed to new deposits for years, but existing holders could still withdraw their funds.

NU7 changes that by eliminating version-4 transactions, the only format capable of moving coins out of Sprout. According to the network's technical specification, transactions using that format will become invalid after activation.

The decision addresses a problem familiar to anyone maintaining old software: supporting obsolete technology creates additional complexity and potential security vulnerabilities.

Sprout's original privacy system required substantial computing resources, and its outdated transaction-verification mechanisms create risks that developers want to eliminate.

Developers haven't technically destroyed the coins. They will continue counting toward Zcash's circulating issuance calculations, and a future network upgrade could theoretically restore access, but that is not guaranteed, and the team has not shown any interest in doing so.

In fact, the specification explicitly warns that spending those funds may never be re-enabled.

What happens to Zcash holders?

Legacy holders are the people most exposed to the change. Anyone whose ZEC remains in Sprout must move it through a compatible withdrawal mechanism before the upgrade activates, or risk losing access indefinitely.

Sprout addresses begin with "zc," which can help users recognize old wallet records. Regular ZEC holders using modern supported wallets do not need to migrate their coins out of Sprout, although keeping wallet software updated remains important.

The blockchain also cannot reveal who owns the remaining Sprout balance, meaning nobody knows how much belongs to active users and how much is already inaccessible because its owners lost their keys.

Meanwhile, NU7 brings a benefit for everyone else: Zcash's target block interval falls from 75 seconds to 25 seconds, meaning transactions should receive their first confirmation approximately three times faster, according to the Zcash Foundation.

For Sprout holders, however, the relevant number is block 3,543,405 which will happen later today. As of the time of publication, the latest available block is 3,543,027. Once the network reaches block 3,543,405, any remaining coins in the old pool will be frozen under the new rules.