Zcash Dodges $6.5M Bullet: Sprout Pool Vulnerability Fixed Before Anyone Could Drain It

A critical vulnerability in Zcash nodes that could have allowed malicious miners to drain over 25,000 $ZEC (about $6.5 million) from the network's deprecated Sprout shielded pool has been patched. The bug caused zcashd nodes to skip proof verification for transactions involving the legacy Sprout pool, but no funds were compromised. In what might be the most expensive game of "whoops, forgot to check the math," Zcash just pulled off a close-call victory that would have made even the most degenerate DeFi gambler sweat.

Security researcher Alex "Scalar" Sol discovered the flaw on March 23 using AI assistance and disclosed it to Shielded Labs, according to a disclosure report released Tuesday. The vulnerability affected releases from July 2020 through the present. That's roughly four years of everyone sending Sprout transactions thinking their privacy was intact while their coins were essentially held together with digital duct tape and optimism. Sol out here using AI to find bugs like he's got ChatGPT on retainer for zero-days.

Zcash developers released v6.12.0 on Tuesday to contain the fix. Major mining pools moved quickly—Luxor deployed the patch on March 25, while F2Pool, ViaBTC, and AntPool all followed by March 26. The mining pool response time was so fast you almost wonder if they were already awake at 3 AM staring at their monitors anyway. This is what peak operational readiness looks like in an industry where sleep is apparently optional.

The Zebra full node implementation was unaffected and would have triggered a chain fork if exploitation had been attempted, providing an additional layer of protection. Zebra just sat there in the corner like that one friend who doesn't even play the game but somehow ends up saving everyone's ass anyway. Sometimes the unassuming one really does come through.

For his disclosure, Sol will receive a 200 $ZEC bounty (valued above $51,000), with contributions from Shielded Labs, ZODL, the Zcash Foundation, and Bootstrap. That's a pretty solid bug bounty for basically printing money for whitehat hackers. Sol found a $6.5 million vulnerability and got paid in approximately 0.2% of the potential damage. The math checks out, the market for finding these things remains fascinatingly inefficient.

The Sprout pool was closed to new deposits in November 2020 but still holds approximately 25,424 $ZEC that users haven't migrated. While the vulnerability could have drained these funds, Zcash's "turnstile" mechanism would have prevented broader supply inflation by requiring that any coins leaving the Sprout pool must have verifiably entered it. So basically, someone could have stolen the candy, but they definitely couldn't have printed more. Small mercies in the wild west of crypto.

This isn't the network's first rodeo—Zcash patched an