Ankr’s Ethereum cross-chain bridge was emptied on Monday by five withdrawals that reused a single small Optimism deposit, and the contract was only paused about an hour and 40 minutes after the first one went through. Security firms put the loss at between $286,000 and $304,500. The withdrawals passed the bridge’s signature check, which shifts the focus to how the attacker obtained those signatures.
What happened on the bridge
Blockaid flagged the attack first. An attacker “submitted five withdraw calls on Ankr’s Ethereum cross-chain bridge, using a tiny Optimism deposit to unlock the bridge’s full locked inventory to a new wallet,” the security firm said, putting the haul at about $292,000 and warning that the attack was still under way.
Onchain records show all five withdrawals landed within two and a half minutes. In order, they took 82.91 ankrETH, about 7.98 million ANKR, 515,206 ankrPOL, 5,372 units of a legacy Ankr Polygon staking token and 0.82 aETHb. Every token went to the same fresh address. By our reading of the token contracts, the bridge’s balance of all five assets is now zero.
SlowMist described the method in more detail. The attacker “submitted five forged cross-chain receipts referencing the same legitimate Optimism transaction, altering the recipient, token, and withdrawal amounts,” the firm said. It added that all five receipts carried valid signatures from the bridge’s trusted consensus signer, so the contract released funds “without independently verifying the receipts against the actual source-chain events.” SlowMist estimated the loss at about $304,500 and said signer compromise “has not been confirmed.” PeckShield’s count was about $286,000. Most of the gap is in the price used for ankrETH: PeckShield valued the 83 ankrETH at about $192,000.
A key that moved hours earlier
Onchain forensics outfit TraceFast went further. It said all five withdrawals recover to the same signer, 0x4069d8a3de3a72eca86ca5e0a4b94619085e7362, the key that also signed a legitimate November 3 withdrawal for the same Optimism deposit. “The attacker didn’t bypass verification. They held the notary key,” TraceFast said.
Two parts of that account check out onchain. That address is written into the bridge contract’s stored configuration. And the wallet behind it, which has a long record of routine “Update Ratio” calls on Ankr staking tokens, sent out its 1.797745 ETH balance at 02:08 UTC on Monday, about six hours before the bridge was drained. TraceFast says other Ankr operational wallets were swept in the same window. Ankr has not said whether any of its keys were compromised.
Swapped to ETH before the pause
The attacker moved fast. Between 09:12 and 09:36 UTC, the receiving wallet sold all of the ANKR and 77.91 of the ankrETH through MetaMask’s swap router in about 30 small trades. As of about 10:10 UTC it held roughly 104.7 ETH, worth about $220,000, along with 5 ankrETH and the untouched ankrPOL. PeckShield said most of the stolen funds had been swapped for 103 ETH. About 1.5 ETH more arrived from the wallet that sent the withdrawals.
The bridge was paused at 09:51 UTC by a transaction from the contract’s owner, a Safe multisig that needs three of its five signers to approve. By then the bridge’s balances of the drained tokens had already gone to zero, so the pause blocks reuse of the contract but does not recover anything. Ankr’s X account had posted nothing about the incident at the time of writing.
Ankr’s history with key security
This is not the first time Ankr has had to explain a key problem. In December 2022, an attacker used a compromised private key to mint vast amounts of its aBNBc token on BNB Chain and cashed out about $5 million. In its after-action report, Ankr blamed a former team member and wrote: “The exploit was possible partly because there was a single point of failure in our developer key.” It promised multi-sig sign-off for contract updates.
Monday’s bridge pause did go through a multisig. But if TraceFast is right, withdrawals only needed one signer’s approval, and that one key was enough. As with the ACryptoS vault exploit over the weekend, the dollar loss is small next to this year’s big hacks. What stands out is the weak point: one trusted key signing off on cross-chain messages, the same kind of single point of failure Ankr described in 2022.
What to watch
The main open question is how the attacker got valid signatures, whether by stealing the signer key or by abusing the service that uses it. Ankr also has not said whether its bridges on other chains run on the same signer, or whether users whose tokens were locked in the Ethereum contract will be made whole. Exchanges and trackers following the attacker’s wallets should be careful: TraceFast warned that lookalike “address poisoning” wallets are already sending spam transfers to confuse anyone copying addresses from transaction history.
FAQ
How much was taken from the Ankr bridge?
Estimates range from about $286,000 (PeckShield) to $304,500 (SlowMist), with Blockaid at roughly $292,000. The assets were ankrETH, ANKR, ankrPOL and two smaller legacy Ankr tokens.
Is the Ankr bridge still open?
The Ethereum bridge contract is paused. Its owner multisig paused it at 09:51 UTC on November 9, after the targeted token balances had already been drained.
Was this a smart contract bug?
Security firms say the withdrawals carried valid signatures from the bridge’s trusted signer. SlowMist said a signer compromise has not been confirmed, and Ankr has not published an explanation yet.
Disclaimer: This article is for informational purposes only and does not constitute financial advice.
