Reentrancy
Reentrancy is the bug that stole $60M in The DAO hack: a contract sends funds *before* updating its own balance, so a malicious receiver contract calls bac
Reentrancy is the bug that stole $60M in The DAO hack: a contract sends funds *before* updating its own balance, so a malicious receiver contract calls back in and withdraws again — and again — draining the vault in one transaction. The fix (update state first) is one line; forgetting it cost a blockchain its innocence.
Why it matters
It’s the canonical example of why “audited” matters and why ordering operations correctly is security-critical in finance code. Modern libraries and reentrancy guards make it rarer, but every new developer rediscovers it — when an audit flags reentrancy risk, treat it as a five-alarm finding, not a note.