Allbridge

Allbridge’s CCTPTokenMessenger treated a valid Circle-attested message as proof that USDC had actually been transferred. An attacker created an arbitrary CCTP message on Polygon declaring a 1,000,000 USDC transfer without burning any USDC and directed its recipient to an attacker-controlled contract rather than Circle’s TokenMessengerV2.

Circle legitimately attested the generic message, but redeeming it on Base minted no USDC. Allbridge nevertheless trusted the attacker-controlled amount encoded in the message and recorded a 1,000,000 USDC deposit without checking for a corresponding token balance increase.

The attacker flash-loaned 808,844 USDC to temporarily bring the router’s balance to the declared amount, redeemed the phantom credit for 999,000 USDC, repaid the loan, and drained the router’s existing liquidity. A copycat subsequently used the same technique to take the remaining funds.