Cryptocurrency hacks have become all too familiar, but instances where attackers take substantial risks only to gain minimal rewards are rare. Such a scenario unfolded on Sunday when an attacker leveraged a vulnerability in Hyperbridge's cross-chain gateway, which connects various blockchains, to mint 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 exploit highlights the growing list of vulnerabilities in bridge protocols, following a $270 million drain from Drift Protocol on Solana last month and a social engineering attack that compromised infrastructure. The Sunday attack targeted the bridge contract, not Polkadot's core network, leaving the native DOT token unaffected. The weakness lay in how Hyperbridge's EthereumHost contract validated incoming cross-chain messages before passing them to the TokenGateway.
Bridges, which facilitate the movement 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 submission of 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, contained an all-zeros commitment value, indicating either absent or circumventable proof validation for this specific call path. The gateway treated the message as legitimate, executing changeAdmin on the bridged Polkadot token contract and transferring admin rights to the attacker's address. With admin control, the attacker minted 1 billion tokens in a single transaction and funneled them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, extracting about 108.2 ETH across multiple swaps at slightly different prices. However, the limited liquidity of the bridged DOT pool on Ethereum worked against the attacker, capping the 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 could have led to significantly larger losses. As of Monday morning, DOT traded just under $1.20.
CertiK identified the exploit, confirming that the attack vector was indeed the Hyperbridge gateway contract and that the attacker profited approximately $237,000 from minting and selling the bridged tokens. Hyperbridge has yet to publicly address the exploit or disclose whether other bridged token contracts using the same gateway are vulnerable to similar forged-message attacks.