ERC4626YieldBackend.withdrawSurplus() uses convertToAssets() which doesn't account for fees

Summary

ERC4626YieldBackend.withdrawSurplus() uses convertToAssets() to calculate the surplus, which does not account for withdrawal fees, potentially causing the function to revert or withdraw more than the actual surplus.

Vulnerability Detail

The withdrawSurplus() function calculates the vault's asset value using convertToAssets():

uint256 vaultAssets = VAULT.convertToAssets(
    VAULT.balanceOf(address(this))
);

Per EIP-4626, convertToAssets() returns the amount of assets that would be exchanged for the given shares, but it does not account for withdrawal fees. For vaults that charge withdrawal fees, the actual redeemable amount is less than what convertToAssets() reports.

This causes the calculated surplusAmount to be higher than the actual withdrawable surplus. When VAULT.withdraw(surplusAmount, ...) is called, it may:

1. Revert if the vault cannot fulfill the inflated withdrawal amount
2. Withdraw assets that belong to SuperToken holders (not actual surplus)

Impact

For ERC4626 vaults with withdrawal fees, withdrawSurplus() either reverts (DoSing surplus withdrawal) or incorrectly withdraws funds that belong to SuperToken holders, causing a loss for users.

Code Snippet

https://github.com/sherlock-audit/2026-01-superfluid-update-jan-13th/blob/main/protocol-monorepo/packages/ethereum-contracts/contracts/superfluid/ERC4626YieldBackend.sol#L45-L55

function withdrawSurplus(uint256 totalSupply) external {
    (uint256 normalizedTotalSupply, ) = ISuperToken(address(this))
        .toUnderlyingAmount(totalSupply);

    uint256 vaultAssets = VAULT.convertToAssets(
        VAULT.balanceOf(address(this))
    );

    uint256 surplusAmount = vaultAssets + ASSET_TOKEN.balanceOf(address(this)) - normalizedTotalSupply;
    VAULT.withdraw(surplusAmount, SURPLUS_RECEIVER, address(this));
}

Tool Used

Manual Review

Recommendation

Use previewRedeem() instead of convertToAssets() to get an accurate estimate that includes fees, or use maxWithdraw() to get the maximum withdrawable amount:

function withdrawSurplus(uint256 totalSupply) external {
    (uint256 normalizedTotalSupply, ) = ISuperToken(address(this))
        .toUnderlyingAmount(totalSupply);

    // Use maxWithdraw to account for fees and other constraints
    uint256 vaultAssets = VAULT.maxWithdraw(address(this));

    uint256 surplusAmount = vaultAssets + ASSET_TOKEN.balanceOf(address(this)) - normalizedTotalSupply;
    VAULT.withdraw(surplusAmount, SURPLUS_RECEIVER, address(this));
}