The biggest crypto hack in history began with a rehearsal: before the October 29 breach, attackers ran two test sequences of code on Ethereum accounts they controlled while trading on Binance carried on as normal.

The rogue code then gave them access to exchange-owned blockchain accounts. As the attackers emptied those wallets, Binance's own funds kept refilling them, and the drain continued.

Most of the estimated $6.3B left Binance hot wallets on 10 blockchains in about 20 minutes.

The account shows how automated hot wallet top-ups can amplify a breach once attackers control withdrawals from exchange accounts.