Lines of code

Vulnerability details

Summary

Deposits to SafEth and VotiumStrategy coming from rewards lack slippage control, making them susceptible to sandwich attacks by MEV bots, which can result in a loss of funds for the protocol.

Impact

Rewards coming from the VotiumStrategy contract are compounded and deposited back to the protocol using the depositRewards() function:

<https://github.com/code-423n4/2023-09-asymmetry/blob/main/contracts/AfEth.sol#L272-L293&gt;

272:     function depositRewards(uint256 _amount) public payable {
273:         IVotiumStrategy votiumStrategy = IVotiumStrategy(vEthAddress);
274:         uint256 feeAmount = (_amount * protocolFee) / 1e18;
275:         if (feeAmount > 0) {
276:             // solhint-disable-next-line
277:             (bool sent, ) = feeAddress.call{value: feeAmount}("");
278:             if (!sent) revert FailedToSend();
279:         }
280:         uint256 amount = _amount - feeAmount;
281:         uint256 safEthTvl = (ISafEth(SAF_ETH_ADDRESS).approxPrice(true) *
282:             safEthBalanceMinusPending()) / 1e18;
283:         uint256 votiumTvl = ((votiumStrategy.cvxPerVotium() *
284:             votiumStrategy.ethPerCvx(true)) *
285:             IERC20(vEthAddress).balanceOf(address(this))) / 1e36;
286:         uint256 totalTvl = (safEthTvl + votiumTvl);
287:         uint256 safEthRatio = (safEthTvl * 1e18) / totalTvl;
288:         if (safEthRatio < ratio) {
289:             ISafEth(SAF_ETH_ADDRESS).stake{value: amount}(0);
290:         } else {
291:             votiumStrategy.depositRewards{value: amount}(amount);
292:         }
293:     }

As we can see in lines 289 and 291, depending on the current TVL ratio between both collaterals, rewards will be either staked in SafEth or deposited back to VotiumStrategy.

Both scenarios experience the same lack of slippage control. The call to stake() sets zero as the _minOut argument in SafEth. Similarly, in the depositRewards() function of VotiumStrategy, the process involves using a Curve Pool to exchange ETH for CVX tokens with a hardcoded min_dy parameter set to zero.

In any case, the reward compounding transaction could be sandwiched by a MEV bot, causing less output received and a loss of funds to the protocol in general.

Recommendation

Add slippage control to the deposit rewards flow in order to allow the rewarded role to specify a minimum output of assets when rewards are either staked in SafEth, or swapped for CVX in VotiumStrategy.

Assessed type

MEV

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