Cryptocurrency hacks have become all too common, but instances where attackers take significant risks and end up with relatively modest gains are rare. One such incident 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 around $237,000 worth of ether. This exploit highlights the growing list of vulnerabilities in bridge protocols, following a $270 million drain on Solana's Drift Protocol last month.
The attack targeted the bridge contract, rather than Polkadot's core network, and the native DOT token remained unaffected. The vulnerability lay in the validation process for incoming cross-chain messages on Hyperbridge's EthereumHost contract. Bridges, which facilitate the transfer of coins between blockchains, are often the weakest link 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 minting capabilities.
The attack unfolded when the perpetrator 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 that the proof validation was either absent or circumventable. As a result, the gateway processed the message as legitimate. The accepted message executed a changeAdmin function on the bridged Polkadot token contract, transferring admin rights to the attacker's 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 at varying prices. However, the limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their profit. The pool's shallow 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 below $1.20. CertiK identified the exploit, confirming that the attack vector was the Hyperbridge gateway contract and that the attacker profited around $237,000 from minting and selling the bridged tokens. 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 forged-message attack vector.