Cryptocurrency hacks have become commonplace, but instances where attackers take substantial risks only to gain minimal rewards are rare. Such an unusual scenario unfolded recently. An attacker leveraged a vulnerability in Hyperbridge's cross-chain bridge, which connects various blockchains, to mint 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network.
The attacker then sold these tokens for approximately $237,000 in ether. This exploit highlights the growing list of vulnerabilities in bridge protocols, following a $270 million Drift Protocol exploit on Solana last month. The recent attack targeted the bridge contract, not the core Polkadot network, and the native DOT token remained unaffected. The weakness 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, 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 the ability to mint an unlimited supply of tokens.
The attack began when the attacker submitted a forged message via dispatchIncoming, which was then routed to TokenGateway.onAccept. The request receipts check failed to verify the message against a valid cross-chain state commitment from Polkadot, and the gateway processed the message as legitimate. The accepted message executed changeAdmin on the bridged Polkadot token contract, transferring admin rights to the attacker's address.
With admin control, the attacker minted 1 billion tokens and sold them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, extracting roughly 108.2 ETH. However, limited liquidity in the bridged DOT pool on Ethereum restricted the attacker's 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 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 approximately $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 attack vector.