SlowMist said cross-chain bridge project Allbridge was attacked on Aug. 19, 2026, with losses of about $190,000. The security team said the exploit was not carried out instantly. Instead, the attacker began setting up nearly a month earlier and used a forged cross-chain message to get past the protocol’s checks.
The setup began on July 26 on Polygon
According to SlowMist’s analysis, the attacker directly called Circle’s MessageTransmitterV2.sendMessage function on Polygon on July 26 and built a message made to resemble a CCTP-style cross-chain transfer. The message claimed a transfer of 1 million USDC, but no USDC burn actually occurred.
Circle then generated a valid attestation for that complete message through its standard process.
The exploit was triggered six seconds after a real deposit arrived
About 24 days later, on Aug. 19, the attacker waited until the Base Router received a real CCTP deposit and its balance increased to about 191,000 USDC. Just six seconds later, the attack was launched.
The attacker used the previously forged message and attestation to call Allbridge’s receiveCctpMessage function. SlowMist said the project lacked critical checks, so the system treated the fake cross-chain message as a real deposit and recorded a 1 million USDC credit.
Aave flash loan used to align the balance
The attacker then temporarily borrowed about 809,000 USDC through an Aave flash loan, bringing the Router balance in line with the forged amount. Using the internal credit record, the attacker called the transfer function and moved out about 999,000 USDC after a 0.1% fee.
After repaying the flash loan and related fees, the attacker’s net profit came to about $189,800.
SlowMist says the failure was in message and asset settlement checks
SlowMist said the root cause was that Allbridge did not verify the identities of the cross-chain message sender and recipient, and did not confirm whether USDC had actually been minted or whether the balance had truly increased. Instead, the protocol directly trusted the amount and message hash data constructed by the attacker.
The firm said on-chain message verification is not the same as confirming that real assets have arrived. For cross-chain protocols, checking message authenticity alone is not enough. They also need to verify that the message source is trusted, that the recipient is Circle’s official TokenMessengerV2, and that asset booking happens only after actual minting and balance changes are confirmed.
The incident, SlowMist said, again exposed security risks in cross-chain bridges at the message verification and asset settlement stages.