Cryptocurrency hacks have become all too familiar, but it's rare for attackers to take significant risks only to end up with relatively modest gains. However, this unusual scenario unfolded on a recent Sunday. An attacker discovered a vulnerability in Hyperbridge's cross-chain gateway, which connects various blockchains, and proceeded to mint 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network. They then sold these tokens 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, not Polkadot's core network, and the native DOT token remained unaffected.
The vulnerability was found in the way Hyperbridge's EthereumHost contract verifies incoming cross-chain messages before passing them to the TokenGateway. Bridges, which facilitate the transfer of coins between blockchains, remain a weak point 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 with the attacker submitting 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. The gateway processed the message as legitimate, and 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 in a single transaction and sold them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, extracting roughly 108.2 ETH across multiple swaps. The limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their 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 would have led to significantly larger losses. As of Monday morning, DOT was trading just under $1.20. CertiK flagged the exploit, confirming that 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 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.