Poly Network's bridge contract flaw let one attacker move $610M

On August 10, 2021, cross-chain interoperability platform Poly Network — which lets users transfer or swap tokens between blockchains — was drained of more than $610 million in one of the largest digital-currency thefts ever recorded. Security firm SlowMist confirmed that over $610 million was transferred to three addresses, with large losses in the O3 Swap cross-chain pool.

The root cause was a smart-contract design flaw, not a stolen key. In Poly's contracts, the verifyHeaderAndExecuteTx function of the EthCrossChainManager can execute cross-chain transactions, and it could be used to call putCurEpochConPubKeyBytes — changing the keeper role of the EthCrossChainData contract to an attacker-chosen address. With the keeper replaced, the attacker could construct transactions at will and withdraw any amount.

The attacker, later dubbed 'Mr. White Hat,' claimed the attack was done 'for fun' to expose the vulnerability and began returning funds the next day. Poly Network offered a $500,000 bounty and promised the person would not be held accountable. By August 12, about $342 million had been returned, with only about $33 million in Tether still frozen by the token issuer.

$610M+ stolen across Ethereum, Binance Smart Chain and Polygon — at the time the biggest DeFi heist ever; nearly all returned within days except $33M in Tether frozen.

The contract let a cross-chain transaction change the keeper address that guards all funds.

verifyHeaderAndExecuteTx and the keeper-update path shared a function boundary with no privilege separation.

The code was deployed across three chains, yet the same flaw existed on all of them.

There was no emergency pause mechanism quick enough to stop the drain once it started.

In cross-chain bridges, privilege is money: keeper-changing and message-verifying functions need separate audits, minimal privileges and logic tests — not just key custody.

The attacker returned nearly all the stolen assets over the following days and Poly Network declared the person a 'white hat,' transferring the remainder to a multi-signature wallet controlled by both sides. The $610 million figure far exceeded the $474 million in criminal losses recorded across all of DeFi in the first seven months of 2021, and the incident pushed bridge projects to treat contract logic — not just private keys — as the critical attack surface. Poly Network continued operating, but cross-chain bridges remained a favorite target in later years.