Slow Mist Reveals Details of the Allbridge Cross-Chain Bridge Attack: Forged CCTP Messages, Flash Loans, Insufficient Minting Result Verification
The Slow Mist security team disclosed that the cross-chain bridge project Allbridge was attacked on August 19, 2026, resulting in a loss of approximately $190,000. Notably, this attack was not executed instantly; the attacker had begun laying the groundwork nearly a month prior and bypassed the verification mechanism by forging cross-chain messages. According to Slow Mist's analysis, on July 26, the attacker directly called Circle's MessageTransmitterV2.sendMessage function on the Polygon chain, constructing a cross-chain message disguised as a CCTP style message, claiming that a transfer of 1 million USDC existed, but in reality, no USDC destruction operation took place. Subsequently, Circle generated a valid verification proof (attestation) for this complete message according to normal procedures.About 24 days later, on August 19, the attacker waited for the Base Router to receive a real CCTP deposit, increasing the balance to approximately 191,000 USDC, and initiated the attack just 6 seconds later. The attacker utilized the previously forged message and verification proof to call Allbridge's receiveCctpMessage function. Due to the project's lack of critical verification, the system mistakenly recognized the false cross-chain message as a real deposit and recorded a limit of 1 million USDC. The attacker then temporarily borrowed approximately 809,000 USDC through an Aave flash loan, matching the Router balance with the forged amount, and used the internal credit record to call the transfer function, ultimately transferring out approximately 999,000 USDC (after a 0.1% fee). After repaying the flash loan and fees, the attacker netted a profit of about $189,800. The root cause of this vulnerability lies in Allbridge's failure to verify the identities of the sender and receiver of the cross-chain message, as well as not confirming whether USDC was genuinely minted and whether the balance actually increased, instead directly trusting the amounts and message hash data constructed by the attacker. Slow Mist emphasizes that on-chain message verification does not equate to the actual arrival of real assets. Cross-chain protocols not only need to verify the authenticity of messages but must also ensure that the message source is trustworthy, that the receiver is Circle's official TokenMessengerV2, and that asset accounting can only proceed after confirming the actual minting of assets and changes in balance. This incident once again highlights the security risks of cross-chain bridges in the message verification and asset settlement processes.
