Ankr Bridge Attack Used Five Forged Receipts With Valid Signatures

An attacker drained roughly $304,500 from Ankr's cross-chain bridge by submitting five forged withdrawal receipts that all carried valid signatures from trusted consensus signers, according to an account of the incident. The bridge released the assets without independently verifying the event on the source network.

The mechanics were simple and that's what makes the case notable. All five receipts referenced the same legitimate transaction on Optimism. Each one altered three fields: the recipient address, the token and the withdrawal amount. The signatures on those modified receipts were valid, and the bridge's withdrawal function accepted them.

What exactly failed?

The withdrawal function did not check the source-chain event before releasing assets. It treated a valid signature as sufficient proof that a withdrawal was legitimate. In practice, that meant the contract accepted withdrawal details that had never been confirmed against the actual event on the originating network.

The signature proved a trusted signer had signed something. It did not prove that the details inside the receipt matched a real, confirmed withdrawal on Optimism. That distinction — signature validity versus event authenticity — is the gap the attacker exploited.

This is a verification design failure, not a cryptography failure. The signatures were not broken. The data the signatures covered was not bound tightly enough to a verified source-chain event, so altered withdrawal details could pass through the system as long as the signature check itself succeeded.

How did the attacker use one real transaction?

The attacker took one legitimate transaction on Optimism and reused it as the anchor for five separate forged receipts, changing the recipient, token and amount in each. Because the receipts were built around a real transaction and carried valid signatures from trusted consensus signers, they cleared the bridge's checks one after another.

Five forged receipts. Five releases. One real underlying event on Optimism serving as cover for all of them.

Is the signature leak solved?

No. The origin of the unauthorized signatures remains under investigation, and no compromise of the signers has been confirmed, according to the incident account. That leaves two live possibilities for investigators: either the signing keys or signing process were abused in a way not yet identified, or the signature scheme itself allowed valid signatures to be produced over attacker-chosen data.

Until that question is answered, the trusted signer set behind the bridge cannot be assumed clean or compromised — and that uncertainty is itself an operational problem for any system that depends on those signers.

What's the broader lesson for bridge design?

The incident demonstrates a point that bridge architects keep relearning: valid signatures alone do not establish that withdrawal details match a real source-chain event. A signature is a statement that a signer signed data. It is not, by itself, a statement that the data reflects a confirmed occurrence on the origin network.

For a bridge, the release of assets should be conditioned on verifying the source-chain event directly — the transaction exists, it was confirmed, and the withdrawal parameters presented on the destination chain match it field for field. Ankr's withdrawal function skipped that independent check and paid out roughly $304,500 as a result.

The pattern matters beyond Ankr. Cross-chain bridges remain among the most attacked corners of the crypto infrastructure stack, and the failure mode here — accepting attested data without re-validating it against the chain of record — is a design decision, not an exotic exploit. Any bridge that trusts signed messages without binding them to independently verified source-chain state carries the same structural exposure.

The investigation into the unauthorized signatures is the open item to watch. Its outcome will determine whether this was a signer-security failure, a message-design flaw, or both — and it will shape how other protocols that rely on trusted consensus signers review their own withdrawal paths.