Cryptocurrency hacks have become increasingly common, but instances where attackers take significant risks only to gain minimal rewards are rare. Such an incident occurred on Sunday, where an attacker exploited a vulnerability in the Hyperbridge cross-chain gateway, which connects various blockchains, to mint 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network, and subsequently sold them 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 targeted bridge contract, rather than Polkadot's core network, was the weak point in this incident, with the native DOT token remaining 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 administrative control over token contracts on destination chains, making them susceptible to attacks that can grant unlimited token minting capabilities. The attack unfolded when the hacker submitted a forged message via the dispatchIncoming function, 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, instead storing an all-zeros commitment value, indicating a potential absence or circumvention of proof validation for this specific call path. As a result, the gateway processed the message as legitimate, allowing the attacker to execute the changeAdmin function on the bridged Polkadot token contract and transfer administrative rights to their address. With administrative control, the attacker minted 1 billion tokens in a single transaction and sold them through Odos Router V3 on a Uniswap V4 DOT-ETH pool, extracting around 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 profits. The pool's weak depth meant that 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 below $1.20. The exploit was flagged by CertiK, 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.