# Tokens with callbacks may allow malicious attackers to steal the protocol Crit/High · Three Sigma · DeFi infrastructure · 10th November, 2023 Finding 3S-Clip Finance-M08 of the Clip Finance Strategies security review. - Protocol: https://www.clip.finance/ - Report: /reports/clip-finance-i - Source: https://cdn.sanity.io/files/qoqld077/production/c23d04c8223879d2443221caf3ccb55ac118441a.pdf --- ### Description In Batch.sol, function withdraw() [burns](https://github.com/ClipFinance/strategy-router/blob/master/contracts/Batch.sol#L217-L218) the receiptId after transferring the tokens to the user. This means that if the token has a callback, the user may use it to sell the nft somewhere else, getting some extra value for it, and then the execution continues in withdraw(), burning the nft of the other new owner. In BatchOut.sol, withdrawFulfill(), the withdrawStatus is set to [true](https://github.com/ClipFinance/strategy-router/blob/master/contracts/BatchOut.sol#L379-L381) only after the tokens are transferred to the user. An attacker could use a callback to reenter the function and withdraw the shares until the contract is drained. ### Recommendation Follow the checks-effects-interactions pattern. In Batch.sol, burn the receipt and only then transfer the tokens to the receiptOwner. In BatchOut.sol, set the withdrawStatus to true before transferring tokens. ### Status Addressed in [#972526f](https://github.com/ClipFinance/StrategyRouter-private/commit/972526f894796b2388f79393cf5d169f7aed3ef8) --- Related findings: - [Wrong tokensToCheck logic in BaseRouter enables attackers to steal funds](https://0xsimao.com/findings/fuji-finance-logic-enables-attackers-steal): Fuji Finance - [ERC20AssetPool and ERC721AssetPool should have the nonReentrant modifier as ERC721 and some tokens have callbacks](https://0xsimao.com/findings/singularity-erc20-erc721-reentrant-callbacks): Singularity - [Attackers will steal the reserve from the `Vault` by receiving `ra` in `FlashSwapRouter::__swapDsforRa()`](https://0xsimao.com/findings/cork-protocol-steal-receiving-flash-dsfor): Cork Protocol - [Signatures missing some parameters being vulnerable to attackers using them coupled with malicious parameters](https://0xsimao.com/findings/crestal-network-signatures-parameters-them-coupled): Crestal Network