TokenBridge, which connects Ethereum and Alephium, was attacked on May 30. Blockchain security firm Blockaid stated that the attackers gained control of three of the four guardian keys and used them to forge cross-chain approval messages, transferring approximately $815,000 in assets within about seven minutes.
The attack point is in the Guardian's signature.
The Alephium TokenBridge connects Ethereum and the Alephium chain. When a user transfers ALPH from Alephium to Ethereum, the native asset is locked on one side first, and then the corresponding wrapped asset, wALPH, is minted on the Ethereum side.
This process relies on guardian signatures to confirm the validity of cross-chain messages. According to the bridge's design, at least three of the four guardians must sign for a transfer message to be approved. Blockaid claims that the attacker obtained the private keys of three of the guardians, thus enabling them to forge seemingly valid VAAs, i.e., cross-chain approval messages.
Release of multiple assets after fabricating information
After gaining signing capabilities, the attackers not only forged wALPH's minting process but also caused the bridge to erroneously execute asset releases. The bridge identified these forged messages as valid withdrawals, thus unlocking various assets originally held in custody.
- USDT
- USDC
- WBTC
- WETH
Blockaid also stated that the attackers minted an additional 13.76 million wALPH without actually depositing any ALPH. This number exceeds the total supply of previously circulating sealed assets, meaning these assets are not backed by any real collateral.
Similar to previous cross-chain bridge attacks
This incident bears similarities to the earlier Wormhole cross-chain bridge attack. Both involved forging cross-chain messages and generating assets without sufficient collateral backing.
The report also mentioned that the cross-chain bridge between Verus and Ethereum was recently attacked, resulting in a loss of approximately $11.58 million. These consecutive cross-chain bridge incidents once again expose that multi-signature verification and key management remain the main risks for bridge protocols.
