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

On August 19, 2026, the cross-chain bridge Allbridge was attacked. The attacker had prepared on July 26 by calling Circle’s MessageTransmitterV2.sendMessage on Polygon to forge a CCTP-style message claiming a 1M USDC transfer (with no actual burn) and obtained a valid attestation. On Aug 19, after a real CCTP deposit brought the Base Router balance to ~191k USDC, the attacker used the forged message via receiveCctpMessage (which lacked proper verification and credited it as a real deposit), flash-loaned ~809k USDC from Aave to match the claimed amount, and withdrew ~999k USDC, netting ~$189,800.