While cryptocurrency hacks are not uncommon, it is rare for attackers to take significant risks and end up with relatively small gains. This unusual scenario unfolded on Sunday when an attacker exploited a weakness in Hyperbridge's cross-chain gateway, which connects different blockchains, and minted 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 incident highlights the growing list of vulnerabilities in bridge protocols, including a recent $270 million drain on Solana's Drift Protocol and a social engineering attack that compromised infrastructure.
The exploit targeted the bridge contract, not Polkadot's core network, and the native DOT token was not affected. The vulnerability was found in how Hyperbridge's EthereumHost contract validates incoming cross-chain messages before passing them to the TokenGateway.
Bridges, which facilitate the movement 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 supply. 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, giving the attacker admin rights. With this 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 around 108.2 ETH.
However, limited liquidity worked against the attacker, capping their profit. The bridged DOT pool on Ethereum had limited depth, and the 1 billion tokens overwhelmed the available liquidity, resulting in the attacker receiving only a fraction of a cent per token. If the vulnerability had been exploited on a deeper pool or a higher-value bridged asset, the losses would have been significantly larger.
As of Monday morning, DOT was trading just under $1.20. CertiK confirmed the exploit, stating 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 not publicly commented on the exploit or disclosed whether other bridged token contracts using the same gateway are vulnerable to the same attack vector.