Thunder Loan
AI First Flight #7
Beginner FriendlyFoundryDeFiOracle
EXP
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Submission Details
Severity: low
Valid

getCalculatedFee rounds to zero for small borrows, giving free flash loans

dngr2

Description

ThunderLoan::getCalculatedFee divides twice by s_feePrecision (1e18). For small borrow amounts the integer division truncates the fee to zero, so small flash loans are free.

uint256 valueOfBorrowedToken = (amount * getPriceInWeth(address(token))) / s_feePrecision;
@> fee = (valueOfBorrowedToken * s_flashLoanFee) / s_feePrecision; // truncates to 0 for small amounts

Risk

Likelihood: Medium

  • Any borrow below roughly s_feePrecision / s_flashLoanFee tokens pays no fee; trivially reproducible.

Impact: Low

  • Fee revenue leaks on small loans; negligible per call, but a free-loan primitive and a precision-correctness defect.

Proof of Concept

Borrowing 300 wei of an 18-decimal token whose price is 1e18 yields (300 * 1e18 / 1e18) * 3e15 / 1e18 = 0, so the fee truncates to zero and the loan is free:

function test_G4_smallLoanHasZeroFee() public allowTokenA {
assertEq(thunderLoan.getCalculatedFee(tokenA, 300), 0); // fee rounds to zero
}

Recommended Mitigation

Restructure the fee math to avoid the double division by s_feePrecision (or enforce a minimum fee) so small borrows cannot round down to a zero fee.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[L-01] getCalculatedFee can be 0

## Description getCalculatedFee can be as low as 0 ## Vulnerability Details Any value up to 333 for "amount" can result in 0 fee based on calculation ``` function testFuzzGetCalculatedFee() public { AssetToken asset = thunderLoan.getAssetFromToken(tokenA); uint256 calculatedFee = thunderLoan.getCalculatedFee( tokenA, 333 ); assertEq(calculatedFee ,0); console.log(calculatedFee); } ``` ## Impact Low as this amount is really small ## Recommendations A minimum fee can be used to offset the calculation, though it is not that important.

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