Submission Details
Severity:
`DebtToken.sol#mint()` function calculates `balanceIncrease` wrongly.
Summary
DebtToken.sol#mint() function calculates balanceIncrease wrongly.
Vulnerability Details
DebtToken.sol#mint() function is as follows.
function mint(
address user,
address onBehalfOf,
uint256 amount,
uint256 index
) external override onlyReservePool returns (bool, uint256, uint256) {
if (user == address(0) || onBehalfOf == address(0)) revert InvalidAddress();
if (amount == 0) {
return (false, 0, totalSupply());
}
uint256 amountScaled = amount.rayDiv(index);
if (amountScaled == 0) revert InvalidAmount();
150@> uint256 scaledBalance = balanceOf(onBehalfOf);
bool isFirstMint = scaledBalance == 0;
uint256 balanceIncrease = 0;
if (_userState[onBehalfOf].index != 0 && _userState[onBehalfOf].index < index) {
155@> balanceIncrease = scaledBalance.rayMul(index) - scaledBalance.rayMul(_userState[onBehalfOf].index);
}
_userState[onBehalfOf].index = index.toUint128();
@> uint256 amountToMint = amount + balanceIncrease;
_mint(onBehalfOf, amountToMint.toUint128());
emit Transfer(address(0), onBehalfOf, amountToMint);
@> emit Mint(user, onBehalfOf, amountToMint, balanceIncrease, index);
return (scaledBalance == 0, amountToMint, totalSupply());
}
As we can see above, on L150 scaledBalance is value which is scaled up by usageIndex.
But on L155 it is scaled up by usageIndex recursively.
Impact
This leads to wrong balanceIncrease which is used to record log and calculating amountToMint.
Tools Used
Manual review
Recommendations
Modify DebtToken.sol#mint() function as follows.
function mint(
address user,
address onBehalfOf,
uint256 amount,
uint256 index
) external override onlyReservePool returns (bool, uint256, uint256) {
if (user == address(0) || onBehalfOf == address(0)) revert InvalidAddress();
if (amount == 0) {
return (false, 0, totalSupply());
}
uint256 amountScaled = amount.rayDiv(index);
if (amountScaled == 0) revert InvalidAmount();
-- uint256 scaledBalance = balanceOf(onBehalfOf);
++ uint256 scaledBalance = scaledBalanceOf(onBehalfOf);
bool isFirstMint = scaledBalance == 0;
uint256 balanceIncrease = 0;
if (_userState[onBehalfOf].index != 0 && _userState[onBehalfOf].index < index) {
balanceIncrease = scaledBalance.rayMul(index) - scaledBalance.rayMul(_userState[onBehalfOf].index);
}
...
return (scaledBalance == 0, amountToMint, totalSupply());
}
Updates
Lead Judging Commences
inallhonesty 7 months ago
Submission Judgement Published Assigned finding tags:
DebtToken::mint miscalculates debt by applying interest twice, inflating borrow amounts and risking premature liquidations
Prize pool breakdown
Total prize
77,280 USDC
nSLOC
3,86420
USDC / LOC
74,000
3,280
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