Attacker Exploits Polkadot Token Vulnerability on Ethereum, Gains Limited Profit

Cryptocurrency hacks have become commonplace, but instances where attackers take significant risks only to gain minimal rewards are rare. Such a scenario unfolded recently. An attacker leveraged a vulnerability in Hyperbridge's cross-chain bridge, 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, including a $270 million exploit 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 stemmed from the validation process of incoming cross-chain messages in Hyperbridge's EthereumHost contract. 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.

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, stored an all-zeros commitment value, suggesting that the proof validation was either absent or circumventable.

The gateway processed the message as legitimate, allowing the attacker to execute changeAdmin on the bridged Polkadot token contract and transfer admin rights to their address. With admin control, the attacker minted 1 billion tokens and sold them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, extracting around 108.2 ETH.

However, 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 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 confirmed the exploit, stating that the attack vector was the Hyperbridge gateway contract and that the attacker profited around $237,000. 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.