Lines of code

https://github.com/code-423n4/2022-04-backd/tree/main/backd/contracts/strategies/BkdEthCvx.sol#L77 https://github.com/code-423n4/2022-04-backd/tree/main/backd/contracts/strategies/BkdEthCvx.sol#L93 https://github.com/code-423n4/2022-04-backd/tree/main/backd/contracts/strategies/BkdEthCvx.sol#L117 https://github.com/code-423n4/2022-04-backd/tree/main/backd/contracts/pool/EthPool.sol#L30 https://github.com/code-423n4/2022-04-backd/tree/main/backd/contracts/vault/EthVault.sol#L29 https://github.com/code-423n4/2022-04-backd/tree/main/backd/contracts/vault/EthVault.sol#L37

Vulnerability details

Impact

Using Solidity's transfer function has some notable shortcomings when the withdrawer is a smart contract, which can render ETH deposits impossible to withdraw. Specifically, the withdrawal will inevitably fail when:

The withdrawer smart contract does not implement a payable fallback function.
The withdrawer smart contract implements a payable fallback function which uses more than 2300 gas units.
The withdrawer smart contract implements a payable fallback function which needs less than 2300 gas units but is called through a proxy that raises the call’s gas usage above 2300.

The sendValue function available in OpenZeppelin Contract’s Address library can be used to transfer the withdrawn Ether without being limited to 2300 gas units. Risks of reentrancy stemming from the use of this function can be mitigated by tightly following the "Check-effects-interactions" pattern and using OpenZeppelin Contract’s ReentrancyGuard contract. For further reference on why using Solidity’s transfer is no longer recommended, refer to these articles:

Proof of Concept

strategies/BkdEthCvx.sol#L77\ strategies/BkdEthCvx.sol#L93\ strategies/BkdEthCvx.sol#L117\ pool/EthPool.sol#L30\

Note: If receiver is ETHVault contract and receive() function in ETHVault has additional logic, it could DoS

vault/EthVault.sol#L29\ vault/EthVault.sol#L37

Tools Used

Manual review

Recommended mitigation steps

Use Solidity's low-level call() function or the sendValue function available in OpenZeppelin Contract’s Address library to send Ether.

code-423n4 / 2022-04-backd-findings