Summary

Approval may revert if current allowance is non-zero

Impact

Some tokens (like USDT) do not work when changing the allowance from an existing non-zero allowance value. They must first be approved by zero and then the actual allowance must be approved.

Tools Used

Manual review

Recommendations

Add an approval with amount 0 before approving the desired amount, or use safeApprove from OpenZeppelin's SafeERC20 (https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/utils/SafeERC20.sol) library.