[Lines of code](https://github.com/code-423n4/2022-10-inverse/blob/main/src/Oracle.sol#L78-L105) # Vulnerability details ## Impact Calling the Oracle contract's viewPrice or getPrice function executes uint price = feeds[token].feed.latestAnswer() and require(price > 0, "Invalid feed price"). Besides that Chainlink's latestAnswer function is deprecated, only verifying that price > 0 is true is also not enough to guarantee that the returned price is not stale. Using a stale price can cause the calculations for the credit and withdrawal limits to be inaccurate, which, for example, can mistakenly consider a user's debt to be under water and unexpectedly allow the user's debt to be liquidated. To avoid using a stale answer returned by the Chainlink oracle data feed, according to [Chainlink's documentation](): 1. The latestRoundData function can be used instead of the deprecated latestAnswer function. 2. roundId and answeredInRound are also returned. "You can check answeredInRound against the current roundId. If answeredInRound is less than roundId, the answer is being carried over. If answeredInRound is equal to roundId, then the answer is fresh." 3. "A read can revert if the caller is requesting the details of a round that was invalid or has not yet been answered. If you are deriving a round ID without having observed it before, the round might not be complete. To check the round, validate that the timestamp on that round is not 0." function viewPrice(address token, uint collateralFactorBps) external view returns (uint) { if(fixedPrices[token] > 0) return fixedPrices[token]; if(feeds[token].feed != IChainlinkFeed(address(0))) { // get price from feed uint price = feeds[token].feed.latestAnswer(); require(price > 0, "Invalid feed price"); // normalize price uint8 feedDecimals = feeds[token].feed.decimals(); uint8 tokenDecimals = feeds[token].tokenDecimals; uint8 decimals = 36 - feedDecimals - tokenDecimals; uint normalizedPrice = price * (10 ** decimals); uint day = block.timestamp / 1 days; // get today's low uint todaysLow = dailyLows[token][day]; // get yesterday's low uint yesterdaysLow = dailyLows[token][day - 1]; // calculate new borrowing power based on collateral factor uint newBorrowingPower = normalizedPrice * collateralFactorBps / 10000; uint twoDayLow = todaysLow > yesterdaysLow && yesterdaysLow > 0 ? yesterdaysLow : todaysLow; if(twoDayLow > 0 && newBorrowingPower > twoDayLow) { uint dampenedPrice = twoDayLow * 10000 / collateralFactorBps; return dampenedPrice < normalizedPrice ? dampenedPrice: normalizedPrice; } return normalizedPrice; } revert("Price not found"); } function getPrice(address token, uint collateralFactorBps) external returns (uint) { if(fixedPrices[token] > 0) return fixedPrices[token]; if(feeds[token].feed != IChainlinkFeed(address(0))) { // get price from feed uint price = feeds[token].feed.latestAnswer(); require(price > 0, "Invalid feed price"); // normalize price uint8 feedDecimals = feeds[token].feed.decimals(); uint8 tokenDecimals = feeds[token].tokenDecimals; uint8 decimals = 36 - feedDecimals - tokenDecimals; uint normalizedPrice = price * (10 ** decimals); // potentially store price as today's low uint day = block.timestamp / 1 days; uint todaysLow = dailyLows[token][day]; if(todaysLow == 0 || normalizedPrice < todaysLow) { dailyLows[token][day] = normalizedPrice; todaysLow = normalizedPrice; emit RecordDailyLow(token, normalizedPrice); } // get yesterday's low uint yesterdaysLow = dailyLows[token][day - 1]; // calculate new borrowing power based on collateral factor uint newBorrowingPower = normalizedPrice * collateralFactorBps / 10000; uint twoDayLow = todaysLow > yesterdaysLow && yesterdaysLow > 0 ? yesterdaysLow : todaysLow; if(twoDayLow > 0 && newBorrowingPower > twoDayLow) { uint dampenedPrice = twoDayLow * 10000 / collateralFactorBps; return dampenedPrice < normalizedPrice ? dampenedPrice: normalizedPrice; } return normalizedPrice; } revert("Price not found"); } function getCreditLimitInternal(address user) internal returns (uint) { uint collateralValue = getCollateralValueInternal(user); return collateralValue * collateralFactorBps / 10000; } function getCollateralValueInternal(address user) internal returns (uint) { IEscrow escrow = predictEscrow(user); uint collateralBalance = escrow.balance(); return collateralBalance * oracle.getPrice(address(collateral), collateralFactorBps) / 1 ether; } function getWithdrawalLimitInternal(address user) internal returns (uint) { IEscrow escrow = predictEscrow(user); uint collateralBalance = escrow.balance(); if(collateralBalance == 0) return 0; uint debt = debts[user]; if(debt == 0) return collateralBalance; if(collateralFactorBps == 0) return 0; uint minimumCollateral = debt * 1 ether / oracle.getPrice(address(collateral), collateralFactorBps) * 10000 / collateralFactorBps; if(collateralBalance <= minimumCollateral) return 0; return collateralBalance - minimumCollateral; } ## Proof of Concept The following steps can occur for the described scenario. 1. Alice calls the depositAndBorrow function to deposit some WETH as the collateral and borrows some DOLA against the collateral. 2. Bob calls the liquidate function for trying to liquidate Alice's debt. Because the Chainlink oracle data feed returns an up-to-date price at this moment, the getCreditLimitInternal function calculates Alice's credit limit accurately, which does not cause Alice's debt to be under water. Hence, Bob's liquidate transaction reverts. 3. After some time, Bob calls the liquidate function again for trying to liquidate Alice's debt. This time, because the Chainlink oracle data feed returns a positive but stale price, the getCreditLimitInternal function calculates Alice's credit limit inaccurately, which mistakenly causes Alice's debt to be under water. 4. Bob's liquidate transaction is executed successfully so he gains some of Alice's WETH collateral. Alice loses such WETH collateral amount unexpectedly because her debt should not be considered as under water if the stale price was not used. ## Tools Used VSCode #[Recommended Mitigation Steps](https://github.com/code-423n4/2022-10-inverse/blob/main/src/Oracle.sol#L82-L83> and = roundId, "answer is stale"); require(updatedAt > 0, "round is incomplete"); require(answer > 0, "Invalid feed answer"); uint256 price = uint256(answer); --- The text was updated successfully, but these errors were encountered: 👀 1 neumoxx reacted with eyes emoji All reactions * 👀 1 reaction