Cryptocurrency hacks have become increasingly common, but it's rare for attackers to take significant risks and walk away with relatively modest gains. However, that's exactly what happened on Sunday when an attacker exploited a vulnerability in the Hyperbridge cross-chain gateway. The attacker managed to mint 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network and sold them for approximately $237,000 worth of ether. This incident highlights the growing list of vulnerabilities in bridge protocols, which are designed to facilitate the transfer of assets between different blockchain networks.
Last month, a $270 million exploit occurred on the Solana network, and a social engineering attack compromised infrastructure. The recent exploit targeted the bridge contract, not the core Polkadot network, and the native DOT token was unaffected. The vulnerability stemmed from the way Hyperbridge's EthereumHost contract validated incoming cross-chain messages before passing them to the TokenGateway.
Bridges are often the weakest link in cross-chain architecture because they hold admin-level control over token contracts on destination chains. A single validation failure can grant an attacker unlimited control over the token supply. The attack unfolded when 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. 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.
The limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their profit. If the same vulnerability had been exploited on a deeper pool or a higher-value bridged asset, the losses would have been significantly larger.
The bridged DOT pool on Ethereum held limited depth, meaning the 1 billion tokens overwhelmed the available liquidity, and the attacker received only a fraction of a cent per token. As of Monday morning, DOT was trading just under $1.20. CertiK flagged the exploit, confirming the attack vector was the Hyperbridge gateway contract and that the attacker profited approximately $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 forged-message attack vector.