Cryptocurrency hacks have become all too familiar, but instances where attackers take significant risks only to gain minimal rewards are rare. One such unusual case occurred on Sunday, where an attacker exploited a weakness in Hyperbridge's cross-chain gateway, connecting different blockchains, to mint 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network, and then sold them for approximately $237,000 in ether. This incident highlights the growing list of vulnerabilities in bridge protocols, following a $270 million exploit 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 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, making them susceptible to attacks that can grant unlimited supply. The attack unfolded with the attacker submitting 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, allowing the gateway to process the message as legitimate.
The accepted message then 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 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 would 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 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 attack vector.