Zcash's Sprout Pool Came This Close to a $6.5M Drain—But the Turnstile Saved the Day

A security researcher discovered a critical vulnerability in Zcash nodes that bypassed proof verification for the deprecated Sprout shielded pool. The flaw could have allowed malicious miners to drain more than 25,000 ZEC—worth about $6.5 million at writing. Thankfully, nobody actually pulled this off, so we can all laugh about it now instead of witnessing the privacy coin's equivalent of a bank heist movie.

Alex "Scalar" Sol disclosed the vulnerability on March 23, revealing that zcashd nodes were skipping proof verification for transactions involving the legacy Sprout pool. The bug spanned releases from July 2020 through the present, but was not exploited and all users' funds remain safe. That's four years of "oopsie" hiding in the code, just waiting for someone to notice.

Major mining pools deployed the patch within three days. Luxor mining pool confirmed deployment on March 25, while F2Pool, ViaBTC, and AntPool all deployed the fix by March 26. Zcash developers released v6.12.0 on Tuesday to contain the fix. When $6.5 million is on the line, apparently even miners can move fast—shocking, we know.

The Zebra full node implementation was not affected by the vulnerability and would have triggered a chain fork if exploitation had been attempted, providing an additional layer of network protection. Picture it: some degenerate tries to drain the Sprout pool, and Zebra just sits there like "nope, not on my watch" while splitting the chain in protest. Sometimes the boring node wins.

Sol, who discovered the vulnerability using AI assistance, reported it to Shielded Labs on March 23. The organization coordinated with the Zcash Open Development Lab (ZODL), whose engineer Jack "str4d" Grigg authored the patch. For his disclosure, Sol will receive a 200 ZEC total bounty—valued above $51,000—with Shielded Labs, ZODL, the Zcash Foundation,