Attacker Exploits Polkadot Token Bridge, Steals $250,000 Despite Minting $1 Billion

Cryptocurrency hacks have become increasingly common, but it's rare for attackers to take significant risks and end up with relatively modest gains. However, this unusual scenario unfolded on Sunday when an attacker exploited a vulnerability in Hyperbridge's cross-chain gateway, which connects different blockchain networks.

The attacker successfully minted 1 billion Polkadot tokens, valued at $1.19 billion, on the Ethereum network and sold them for approximately $237,000 worth of 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 was found in the way Hyperbridge's EthereumHost contract validated incoming cross-chain messages before passing them to the TokenGateway. Bridges, which facilitate the transfer of coins between blockchain networks, 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 when the attacker submitted a forged message via dispatchIncoming, which was routed to TokenGateway.onAccept.

The request receipts check, responsible for verifying the message against a valid cross-chain state commitment from Polkadot, stored an all-zeros commitment value, indicating that proof validation was either absent or circumventable for this specific call path. As a result, the gateway processed 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 in a single transaction and routed them through Odos Router V3 into a Uniswap V4 DOT-ETH pool, extracting roughly 108.2 ETH across multiple swaps at slightly different prices.

The limited liquidity in the bridged DOT pool on Ethereum worked against the attacker, capping their profit. 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 under $1.20. CertiK identified the exploit, 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 not publicly commented on the exploit or disclosed whether other bridged token contracts using the same gateway are vulnerable to the same forged-message attack vector.