# Potential ETH Loss Due to transfer Usage in Requestor Contract on `zkSync` Medium · Sherlock · Raffle · 16th May 2024 Finding M-7 of the PoolTogether Prize Layer competition. - Protocol: https://audits.sherlock.xyz/contests/225 - Report: /reports/pooltogether-the-prize-layer-for-defi - Codebase: https://github.com/0xsimao/2024-05-pooltogether/tree/1aa1b8c028b659585e4c7a6b9b652fb075f86db3 - Source: https://github.com/sherlock-audit/2024-05-pooltogether-judging/issues/80 --- ## Summary - The [Requestor](https://github.com/sherlock-audit/2024-05-pooltogether/blob/1aa1b8c028b659585e4c7a6b9b652fb075f86db3/pt-v5-rng-witnet/src/Requestor.sol#L36) contract uses `transfer` to send `ETH` which has the risk that it will not work if the gas cost increases/decrease(low Likelihood), but it is highly likely to fail on `zkSync` due to gas limits. This may make users' `ETH` irretrievable. ## Vulnerability Detail - Users (or bots) interact with [RngWitnet](https://github.com/sherlock-audit/2024-05-pooltogether/blob/1aa1b8c028b659585e4c7a6b9b652fb075f86db3/pt-v5-rng-witnet/src/RngWitnet.sol#L132) to request a random number and start a draw in the `DrawManager` contract. To generate a random number, users must provide some `ETH` that will be sent to [WitnetRandomness](https://github.com/witnet/witnet-solidity-bridge/blob/6cf9211928c60e4d278cc70e2a7d5657f99dd060/contracts/apps/WitnetRandomnessV2.sol#L372) to generate the random number. ```js function startDraw(uint256 rngPaymentAmount, DrawManager _drawManager, address _rewardRecipient) external payable returns (uint24) { (uint32 requestId,,) = requestRandomNumber(rngPaymentAmount); return _drawManager.startDraw(_rewardRecipient, requestId); } ``` - The `ETH` sent with the transaction may or may not be used (if there is already a request in the same block, it won't be used). Any remaining or unused `ETH` will be sent to [Requestor](https://github.com/sherlock-audit/2024-05-pooltogether/blob/1aa1b8c028b659585e4c7a6b9b652fb075f86db3/pt-v5-rng-witnet/src/Requestor.sol#L36), so the user can withdraw it later. - The issue is that the `withdraw` function in the `Requestor` contract uses `transfer` to send `ETH` to the receiver. This may lead to users being unable to withdraw their funds . ```js function withdraw(address payable _to) external onlyCreator returns (uint256) { uint256 balance = address(this).balance; >> _to.transfer(balance); return balance; } ``` - The protocol will be deployed on different chains including `zkSync`, on `zkSync` the use of `transfer` can lead to issues, as seen with [921 ETH Stuck in zkSync Era](https://medium.com/coinmonks/gemstoneido-contract-stuck-with-921-eth-an-analysis-of-why-transfer-does-not-work-on-zksync-era-d5a01807227d).since it has a fixed amount of gas `23000` which won't be anough in some cases even to send eth to an `EOA`, It is explicitly mentioned in their docs to not use the `transfer` method to send `ETH` [here](https://docs.zksync.io/build/quick-start/best-practices.html#use-call-over-send-or-transfer). > notice that in case `msg.sender` is a contract that have some logic on it's receive or fallback function the `ETH` is definitely not retrievable. since this contract can only withdraw eth to it's [own addres](https://github.com/sherlock-audit/2024-05-pooltogether/blob/1aa1b8c028b659585e4c7a6b9b652fb075f86db3/pt-v5-rng-witnet/src/RngWitnet.sol#L99-L102) which will always revert. ## Impact - Draw Bots' `ETH` may be irretrievable or undelivered, especially on zkSync, due to the use of `.transfer`. ## Code Snippet - https://github.com/sherlock-audit/2024-05-pooltogether/blob/1aa1b8c028b659585e4c7a6b9b652fb075f86db3/pt-v5-rng-witnet/src/Requestor.sol#L27-L30 ## Tool used Manual Review , Foundry Testing ## Recommendation - recommendation to use `.call()` for `ETH` transfer. --- Related findings: - [Unconditional lastUpdated advance in RangePool.sync leads to loss of streamed BMX when pool liquidity == 0](https://0xsimao.com/findings/bmx-unconditional-range-streamed-bmx): BMX - [Usage of transferFrom() could revert if used from itself](https://0xsimao.com/findings/glacier-usage-transfer-revert-itself): Glacier - [When changing addresses, use 2 step transfer and/or contract size and/or address 0x0 checks](https://0xsimao.com/findings/fuji-finance-changing-addresses-transfer-0x0): Fuji Finance - [Reward Token Loss for LPs During NFT Position Transfer](https://0xsimao.com/findings/bmx-reward-nft-position-transfer): BMX