Submission Details
Severity:
incorrect debt token mint amount when borrowing from lending pool
Summary
DebtToken contract mints incorrect amount of tokens when mint() method is called
Vulnerability Details
DebtToken is used for record debt when borrowing happens in lending pool. The minted amount is calculated as borrow amount devided by real time liquidity index.
DebtToken::mint implements a incorrect logis where minting amount is added up by an unneed value:balanceIncrease.
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);
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);
}
_userState[onBehalfOf].index = index.toUint128();
uint256 amountToMint = amount + balanceIncrease; //@audit
_mint(onBehalfOf, amountToMint.toUint128());
emit Transfer(address(0), onBehalfOf, amountToMint);
emit Mint(user, onBehalfOf, amountToMint, balanceIncrease, index);
return (scaledBalance == 0, amountToMint, totalSupply());
}
The right implementation should be :
function mint(
address user,
address onBehalfOf,
uint256 amount,
uint256 index
) external override onlyReservePool returns (bool, uint256, uint256) {
// ...
_mint(onBehalfOf, amount.toUint128());
emit Transfer(address(0), onBehalfOf, amountToMint);
emit Mint(user, onBehalfOf, amountToMint, balanceIncrease, index);
return (scaledBalance == 0, amountToMint, totalSupply());
}
since _update function handles the minted debtToken amount related to newly borrowed amount.
function _update(address from, address to, uint256 amount) internal virtual override {
if (from != address(0) && to != address(0)) {
revert TransfersNotAllowed();
}
uint256 scaledAmount = amount.rayDiv(ILendingPool(_reservePool).getNormalizedDebt());
super._update(from, to, scaledAmount);
emit Transfer(from, to, amount);
}
Impact
borrower are minted more debtTokens than they expected
Tools Used
manual
Recommendations
consider modifying minted amount
Updates
Lead Judging Commences
inallhonesty about 2 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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