SDAIPriceFeed is vulnerable to donation attack via manipulatable vault rate

Summary

The SDAIPriceFeed uses the instantaneous vault rate from convertToAssets() which can be manipulated through donation attacks, allowing attackers to drain the protocol.

Vulnerability Detail

The SDAIPriceFeed calculates the sDAI price using the ERC4626 vault's convertToAssets() function, which returns the current exchange rate between sDAI shares and DAI assets. This rate can be manipulated by donating assets directly to the vault.

Attack scenario:

1. Attacker deposits 500 sDAI when vault has 500 sDAI shares and 500 DAI (1:1 rate)
2. Attacker donates 9000 DAI directly to the vault
3. Now 1000 sDAI shares = 10000 DAI (1:10 rate)
4. Attacker opens a trove with 1000 sDAI and borrows 10000 EVRO at the inflated rate
5. Attacker redeems the 10000 EVRO against other collaterals, draining the protocol

Impact

An attacker can manipulate the sDAI price to borrow significantly more EVRO than the collateral is worth, then redeem against other collaterals to drain the protocol.

Code Snippet

https://github.com/sherlock-audit/2025-12-evro-finance-dec-26th/blob/main/evro/contracts/src/PriceFeeds/SDAIPriceFeed.sol#L47

function _fetchPricePrimary() internal returns (uint256, bool) {
    assert(priceSource == PriceSource.primary);

    (uint256 daiUSDPrice, bool daiEurOracleDown) = _getOracleAnswer(ethUsdOracle);
    (uint256 eurUsdPrice, bool eurUsdOracleDown) = _getOracleAnswer(eurUsdOracle);

    // Get the DAI rate of the SDAI vault
    uint256 sdaiDaiRate = sdai.convertToAssets(1e18);  // @audit - manipulatable!

    // ...

    uint256 sdaiUSDPrice = FixedPointMathLib.mulWadUp(daiUSDPrice, sdaiDaiRate);
    lastGoodPrice = FixedPointMathLib.divWad(sdaiUSDPrice, eurUsdPrice);

    return (lastGoodPrice, false);
}

Tool Used

Manual Review

Recommendation

Use the max price between an oracle rate and the instantaneous rate for redemptions, and the min price for calculating ICR when opening/adjusting troves:

function _fetchPricePrimary(bool _isRedemption) internal returns (uint256, bool) {
    uint256 sdaiDaiRate = sdai.convertToAssets(1e18);
    (uint256 oracleRate, bool oracleDown) = _getOracleAnswer(sdaiOracle);

    uint256 rateToUse;
    if (_isRedemption) {
        rateToUse = LiquityMath._max(sdaiDaiRate, oracleRate);
    } else {
        rateToUse = LiquityMath._min(sdaiDaiRate, oracleRate);
    }
    // ... rest of calculation
}