Malicious user can drain PeUSDMainnetStableVision via executeFlashloan()

[code423n4]

Lines of code

https://github.com/code-423n4/2023-06-lybra/blob/main/contracts/lybra/token/PeUSDMainnetStableVision.sol#L129
https://github.com/code-423n4/2023-06-lybra/blob/main/contracts/lybra/token/PeUSDMainnetStableVision.sol#L132

Vulnerability details

Impact

Maliciuos user can use executeFlashloan() and steal all available EUSD tokens.

Proof of Concept

Here is a vulnerable function:

    function executeFlashloan(FlashBorrower receiver, uint256 eusdAmount, bytes calldata data) public payable {
        uint256 shareAmount = EUSD.getSharesByMintedEUSD(eusdAmount);
        EUSD.transferShares(address(receiver), shareAmount);
        receiver.onFlashLoan(shareAmount, data);
        bool success = EUSD.transferFrom(address(receiver), address(this), 
        EUSD.getMintedEUSDByShares(shareAmount));
        require(success, "TF");

        uint256 burnShare = getFee(shareAmount);
        EUSD.burnShares(address(receiver), burnShare);
        emit Flashloaned(receiver, eusdAmount, burnShare);
    }

It allows to pass an arbitrary bytes calldata data as one of the arguments. Later it calls receiver, that can be PeUSDMainnetStableVision contract itself, with a malicious data.

...
receiver.onFlashLoan(shareAmount, data);
...

So malicious user can pass next arguments:

executeFlashloan( PeUSDMainnetStableVision , 1, abi.encodeWithSignature("approve(address,uint256)", attacker, amount));

Step by step:

1. Hacker pass malicious arguments;
2. PeUSDMainnetStableVision calculates the share amount and transfer it to itself;
3. Call an approve function in malicious data;
4. Transfer loaned token back to itself again, calculate fee and burn it;

Right after that hacker will be approved and be able to transfer all EUSD tokens from the contract.

Sorry for no tests included. I was little bit confused with setting up all params.

P.S. The same issue was in Damn Vulnerably DeFi Challenges (Truster).

Tools Used

Manual review.

Recommended Mitigation Steps

You can provide an additional checks to prohibit calls for approve functions or receiver should not be the same as PeUSDMainnetStableVision contract.

Assessed type

Other

[c4-pre-sort]

JeffCX marked the issue as primary issue

[c4-sponsor]

LybraFinance marked the issue as sponsor confirmed

[c4-judge]

0xean marked the issue as satisfactory

[c4-judge]

0xean marked issue #769 as primary and marked this issue as a duplicate of 769