[M-02] transferring LESS funds from lender than needed

[code423n4]

Lines of code

https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L247-L305

Vulnerability details

Impact

Wrong accounting calculation may result in malfunctioning and loss of funds for the project.

Proof of Concept

https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L281-L283
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L297

• As seen in the lines above, the amount transferred from the lender is after subtracting the fee he actually needs to pay
• On the other hand, the amount swapped on 'swivel' is the original (but should be the subtracted amount - fee) and this takes out from lender.sol more than received.
• On other lend functions the accounting is done as I described, the opposite of the current code.

Recommended Mitigation Steps

I recommend doing the accounting like this:

uint256 amount = a[i];
lent += amount; 
uint256 fee = calculateFee(amount);
a[i] -= fee  // correct the amount vector
amountToMint += a[i]  // amount minus the fee

Then

• transferFrom with corrected lent (including the fees)
• initiate swivel with the corrected amounts vector a
• mint only amountToMint to the lender (sum of amounts minus the fees)

[KenzoAgada]

Duplicate of #201