Cryptocurrency attacks are not uncommon, but it's rare for attackers to take significant risks and walk away with relatively minor gains. This unusual scenario unfolded on Sunday when an attacker exploited a vulnerability in the Hyperbridge cross-chain gateway, which connects different blockchain networks. The attacker successfully minted 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network and then sold them for around $237,000 worth of ether. This exploit highlights the growing list of vulnerabilities in bridge protocols, following a $270 million Drift Protocol incident on Solana last month.
The attack targeted the bridge contract rather than Polkadot's core network, leaving the native DOT token unaffected. The vulnerability was found in the validation process of incoming cross-chain messages by Hyperbridge's EthereumHost contract before passing them to the TokenGateway. Bridges, which facilitate the transfer of coins between blockchains, remain a weak point in cross-chain architecture due to their admin-level control over token contracts on destination chains. A single validation failure can grant an attacker unlimited supply.
The attack began when the attacker submitted a forged message via dispatchIncoming, which was then routed to TokenGateway.onAccept. The request receipts check, intended to verify the message against a valid cross-chain state commitment from Polkadot, stored an all-zeros commitment value, indicating either absent or circumventable proof validation.
The gateway processed the message as legitimate, allowing the attacker to execute changeAdmin on the bridged Polkadot token contract and transfer admin rights to their address. With admin control, the attacker minted 1 billion tokens in a single transaction and sold them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, extracting approximately 108.2 ETH across multiple swaps.
However, the limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their profit. The pool's limited depth meant that the 1 billion tokens overwhelmed the available liquidity, resulting in the attacker receiving only a fraction of a cent per token. On a deeper pool or with a higher-value bridged asset, the same vulnerability could have led to significantly larger losses.
As of Monday morning, DOT was trading just under $1.20. CertiK identified the exploit, confirming that the attack vector was the Hyperbridge gateway contract and the attacker profited around $237,000 from minting and selling the bridged tokens. Hyperbridge has not publicly commented on the exploit or disclosed whether other bridged token contracts using the same gateway are vulnerable to the same attack vector.