approve return values not checked & unsafe

[code423n4]

Handle

cmichel

Vulnerability details

The ERC20.approve() function returns a boolean value indicating success.
This parameter needs to be checked for success.
Some tokens do not revert if the transfer failed but return false instead.

In addition, some tokens (like USDT L199) 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.

IERC20(token).safeApprove(address(operator), 0);
IERC20(token).safeApprove(address(operator), amount);

This issue exists for example in AuctionParticipant.purchaseArbitrageTokens:

auctionRewardToken.approve(address(auction), balance);

As well as in UniswapHandler.buyMalt:

rewardToken.approve(address(router), rewardBalance);

Impact

Tokens that don't correctly implement the latest EIP20 spec, by either returning false on failure or reverting if approved from a non-zero value, will be unusable in the protocol as they revert the transaction because of the missing return value.

Recommended Mitigation Steps

We recommend using OpenZeppelin’s SafeERC20 versions with the safeApprove(0) functions that handle the return value check as well as non-standard-compliant tokens.

[GalloDaSballo]

Agree with the finding, not sure if this should be raised to medium severity, either way the finding is valid