Attacker Exploits Polkadot Token Bridge, Gains $250,000 from $1 Billion Mint

Cryptocurrency hacks have become commonplace, 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, which connects multiple blockchains. The attacker successfully minted 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network and sold them for around $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, 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 administrative control over token contracts on destination chains. A single validation failure can grant an attacker unlimited minting capabilities. 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 a changeAdmin function on the bridged Polkadot token contract, granting the attacker administrative rights.

With these rights, the attacker minted 1 billion tokens in a single transaction and sold them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, earning approximately 108.2 ETH. However, the limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their profits.

The same vulnerability could have resulted in significantly larger losses if the pool had greater depth or if the bridged asset had a higher value. As of Monday morning, DOT was trading at 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 not publicly commented on the exploit or disclosed whether other bridged token contracts using the same gateway are vulnerable to the same attack vector.