SlowMist Unveils Details of Allbridge Bridge Attack: Forged CCTP Messages + Flash Loans, Insufficient Mint Verification

Odaily News, SlowMist Security Team disclosed that the cross-chain bridge project Allbridge suffered an attack on August 19, 2026, with losses of approximately $190,000. Notably, this attack was not executed instantaneously—the attacker began laying the groundwork nearly a month in advance, bypassing the verification mechanism through forged cross-chain messages.

According to SlowMist'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 CCTP-style, claiming a transfer of 1 million USDC, despite no actual USDC burn operation occurring. Subsequently, Circle generated a valid attestation for this complete message following standard procedures.

Approximately 24 days later, on August 19, the attacker waited for the Base Router to receive a genuine CCTP deposit, bringing its balance to approximately 191,000 USDC, then launched the attack just 6 seconds later. Using the previously forged message and attestation, the attacker called Allbridge's receiveCctpMessage function. Due to the project's lack of critical validation, the system mistook the fraudulent cross-chain message for a genuine deposit and recorded a 1 million USDC credit.

Subsequently, the attacker borrowed approximately 809,000 USDC temporarily via an Aave flash loan to match the Router's balance with the forged amount, then utilized the internal credit record to call the transfer function, ultimately moving out approximately 999,000 USDC (after deducting a 0.1% fee). After repaying the flash loan and fees, the attacker netted approximately $189,800 in profit. The root cause of this vulnerability lies in Allbridge's failure to verify the identity of the cross-chain message sender and receiver, as well as its failure to confirm whether USDC was genuinely minted or whether the balance actually increased—instead directly trusting the amount and message hash data constructed by the attacker.

SlowMist emphasized that on-chain message verification does not equate to actual asset arrival. Cross-chain protocols must not only verify message authenticity but also ensure the message source is trustworthy, confirm the receiver is Circle's official TokenMessengerV2, and only record assets after confirming actual minting and balance changes. This incident once again highlights the security risks in cross-chain bridges' message verification and asset settlement processes.