The legacy AtomicQueue contract of ether.fi was exploited, resulting in approximately 15.45 ETH being transferred away. The root cause was the lack of access control on the solver parameter passed to the solve() function.

This function neither verifies that the solver equals msg.sender nor requires a signature, registration, or authorization. The attacker first used updateAtomicRequest() to write a malicious AtomicRequest, then forcibly designated the victim's address as the solver; the queue subsequently called finishSolve on the victim, executing want.transferFrom(solver, users[i], assetsToUser), withdrawing the ERC-20 allowance previously granted to the contract. What was stolen was the user's residual authorization, not the principal of the treasury.

Public technical details reveal that the exploited system was the old withdrawal queue built by Veda, not the current Liquid treasury. The attacker also internally deployed nearly worthless tokens, forged quotes, and completed the extraction in a single transaction using a wrapper contract and the solver contract, with on-chain records showing about 15.4536 ETH and gas fees of approximately 0.000685 ETH. After being exchanged through a decentralized exchange, the funds were split into multiple transactions of 10, 1, and 0.1 ETH entering a mixer. ExVul issued a public alert first, while SlowMist claimed to have privately notified the ether.fi team in advance and published the attacker's address and the vulnerability contract address.

In a normal process, users would list their treasury share tokens liquidETH for exchange into target assets, with the solver calling solve() to complete the transaction. This time, solve(offer, want, users[], runData, solver) treated the solver as a freely fillable parameter, allowing any address that still had authorization on the contract to be named as the "solver." The protocol's response includes full compensation for affected users.

From a market mechanism perspective, this is the pricing of authorized stock, not a breach of TVL. The sell orders come from addresses that still have unlimited or unrevoked authorizations; the buy orders are from vulnerability hunters exchanging cheap fake tokens for real allowances. The beneficiaries are the attackers who can scan residual allowances, as well as the team that will handle compensation and contract retirement afterward; the pressured party is the users who treat "authorizing the protocol" as a one-time action. Funds moved from user wallets via transferFrom into the attack contract, then into the mixer, with the protocol treasury itself not being directly drained.

On-chain supplements: The vulnerability contract and attack address are publicly traceable; the incident occurred in the legacy queue rather than the main staking path, indicating that the old solver interface remained callable after the main product iteration.

Source: Public information