Attacker Exploits Cross-Chain Vulnerability, Steals $250,000 Worth of Ethereum

Cryptocurrency hacks have become increasingly common, but instances where attackers take significant risks only to gain minimal rewards are rare. One such incident occurred on Sunday, where an attacker exploited a vulnerability in Hyperbridge's cross-chain gateway, connecting various blockchains, to mint 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network and sell them for approximately $237,000 in 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, rather than Polkadot's core network, leaving the native DOT token 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 unfolded when the attacker submitted a forged message via dispatchIncoming, which was 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 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 and sold them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, extracting roughly 108.2 ETH. However, the limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their profit. The bridged DOT 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 traded just under $1.20.

CertiK confirmed 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 attack vector.