YieldManager's distributeYield can be subject to sandwich attacks

[code423n4]

Lines of code

https://github.com/code-423n4/2022-05-sturdy/blob/78f51a7a74ebe8adfd055bdbaedfddc05632566f/smart-contracts/YieldManager.sol#L178-L187

Vulnerability details

distributeYield uses Uniswap swaps via _convertAssetToExchangeToken and Curve swaps via _convertToStableCoin. UniswapAdapter and CurveswapAdapter do use Oracle for price estimation, but distributeYield calls use hard coded 5% SLIPPAGE, which is wide enough to make sandwich attacks economically viable, specifically for Uniswap case, where price manipulation can be less costly.

As a result trades converting assets to _exchangeToken can happen at a manipulated price and end up receiving fewer _exchangeTokens than current market price dictates.

Placing severity to medium as impact here is a partial fund loss conditional only on big enough asset amount to be swapped: sandwich attacks are common and can be counted to happen almost always as long as economic viability is present.

Proof of Concept

_convertAssetToExchangeToken and _convertToStableCoin use SLIPPAGE for DEX output control:

https://github.com/code-423n4/2022-05-sturdy/blob/78f51a7a74ebe8adfd055bdbaedfddc05632566f/smart-contracts/YieldManager.sol#L178-L187

  function _convertAssetToExchangeToken(address asset, uint256 amount) internal {
    UniswapAdapter.swapExactTokensForTokens(
      _addressesProvider,
      asset,
      _exchangeToken,
      amount,
      UNISWAP_FEE,
      SLIPPAGE
    );
  }

https://github.com/code-423n4/2022-05-sturdy/blob/78f51a7a74ebe8adfd055bdbaedfddc05632566f/smart-contracts/YieldManager.sol#L204-L211

    receivedAmount = CurveswapAdapter.swapExactTokensForTokens(
      _addressesProvider,
      _pool,
      _exchangeToken,
      _tokenOut,
      _amount,
      SLIPPAGE
    );

SLIPPAGE is hard coded to be 5%, which is excessively wide and can be exploited whenever the asset amount is big enough:

https://github.com/code-423n4/2022-05-sturdy/blob/78f51a7a74ebe8adfd055bdbaedfddc05632566f/smart-contracts/YieldManager.sol#L49

  uint256 public constant SLIPPAGE = 500; // 5%

Recommended Mitigation Steps

Consider adding slippage argument to the distributeYield, so it can be tuned each time accordingly to the current market conditions.

[sforman2000]

Duplicate of #133 (high risk)

[HickupHH3]

Wrong duplicate identifier. It's a duplicate of #61