Cryptocurrency hacks have become a common occurrence, but instances where attackers take significant risks only to gain minimal rewards are rare. One such instance occurred on Sunday, when an attacker exploited a vulnerability in Hyperbridge's cross-chain gateway to mint 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network, and then sold them for approximately $237,000 worth of ether. This exploit highlights the growing list of vulnerabilities in bridge protocols, which have been a weak point in cross-chain architecture.
The attack did not affect Polkadot's core network or its native token, DOT, but rather targeted the bridge contract. The vulnerability was found in the way Hyperbridge's EthereumHost contract validated incoming cross-chain messages before passing them to the TokenGateway. Bridges, which facilitate the transfer of coins between different blockchains, often hold admin-level control over token contracts on destination chains, making them susceptible to attacks. In this case, the attacker submitted a forged message via dispatchIncoming, which was routed to TokenGateway.onAccept.
The request receipts check failed to verify the message against a valid cross-chain state commitment from Polkadot, allowing the gateway to process the message as legitimate. As a result, the accepted message executed changeAdmin on the bridged Polkadot token contract, granting the attacker admin rights. With these rights, 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 roughly 108.2 ETH. However, the limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their profit.
If the same vulnerability were to be exploited on a deeper pool or a higher-value bridged asset, the losses could have been significantly larger. The exploit was flagged by CertiK, which confirmed the attack vector and the attacker's profit of approximately $237,000. Hyperbridge has yet to publicly comment on the exploit or disclose whether other bridged token contracts using the same gateway are vulnerable to the same attack.