29099 - [SC - Insight] Actual amount of stETH deposited is less than t....md

Actual amount of stETH deposited is less than the specified amount parameter to be deposited

Submitted on Mar 7th 2024 at 02:56:41 UTC by @kaysoft for Boost | Puffer Finance

Report ID: #29099

Report type: Smart Contract

Report severity: Insight

Target: https://etherscan.io/address/0x7276925e42f9c4054afa2fad80fa79520c453d6a

Impacts:

• Contract fails to deliver promised returns, but doesn't lose value

Description

Brief/Intro

Due to stETH's 1-2 corner case the actual amount transfered from user is when the depositStETH() function is less than the specified amount in the parameter.

Vulnerability Details

In the function below, permitData.amount of stETH is transfered from the msg.sender to the the PufferDepositor contract. The issue is that the actual amount transferred from msg.sender to the contract is less than the specified permitData.amount.

The reason for this is described in 1-2 wei stETH corner case

File: PufferDeposit.sol
function depositStETH(Permit calldata permitData) external restricted returns (uint256 pufETHAmount) {
        try ERC20Permit(address(_ST_ETH)).permit({
            owner: msg.sender,
            spender: address(this),
            value: permitData.amount,
            deadline: permitData.deadline,
            v: permitData.v,
            s: permitData.s,
            r: permitData.r
        }) { } catch { }

192:        SafeERC20.safeTransferFrom(IERC20(address(_ST_ETH)), msg.sender, address(this), permitData.amount);

        return PUFFER_VAULT.deposit(permitData.amount, msg.sender);//@audit use transfershares: https://docs.lido.fi/guides/lido-tokens-integration-guide/#1-2-wei-corner-case
    }

For example is you specify permitData.amount as 1000000000000000000001 the actual amount that will be pulled from the user is 1000000000000000000000.

This leaves 1 wei of stETH that is not pulled from msg.sender.

Why this is an issue is that there are 2 transfers:

1. First from msg.sender to the PufferDepositor contract
2. And secondly from the PufferDepositor to the PufferVault contract.

1 wei of stETH difference may seem small but the doc further stated that:

The same thing can actually happen at any transfer or deposit transaction. >In the future, when the stETH/share rate will be greater, the error can >become a bit bigger.

Impact Details

Possible Denial of service due to transfer amount difference.

Recommendation

Short Term: Consider using the transferShares() function to transfer stETH as recommended by Lido docs

Long Term: Instead of stEth, consider integration the non-rebasable value-accruing counterpart wstETH as recommeded by the Lido docs

References

https://docs.lido.fi/guides/lido-tokens-integration-guide/#1-2-wei-corner-case

Proof of Concept

1. Create a file in the test/integration directory and name is POC.t.sol
2. Copy and paste the code below to the new file: POC.t.sol.
3. Run forge test --match-path test/Integration/POC.t.sol -vvv
4. The test should fail with some logs of amounts
5. This fails because depositAmount is not equal to actualAmount.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.8.0 <0.9.0;

import { TestHelper } from "../TestHelper.sol";
import { Permit } from "../../src/structs/Permit.sol";
import { IERC20 } from "openzeppelin/token/ERC20/IERC20.sol";
import "forge-std/console.sol";


contract PufferDepositorV2ForkTest is TestHelper {
    
    function test_stETH_deposit_pulls_less_stETH()
        public
        giveToken(BLAST_DEPOSIT, address(stETH), alice, 35000 ether)
        withCaller(alice)
    {

        uint256 depositAmount = 1000000000000000000001;
        Permit memory permit = _signPermit(
            _testTemps(
                "alice",
                address(pufferDepositor),
                depositAmount,
                block.timestamp,
                hex"260e7e1a220ea89b9454cbcdc1fcc44087325df199a3986e560d75db18b2e253"
            )
        );

        uint256 balanceBefore = stETH.balanceOf(alice);
        pufferDepositor.depositStETH(permit, alice);
        uint256 balanceAfter = stETH.balanceOf(alice);
        

        console.log("Alice's Deposit Amount:::", depositAmount);
        uint256 actualTransfer = balanceBefore - balanceAfter;
        console.log("Actual stETH transfered from Alice:::", actualTransfer );
        console.log("Amount of stETh not transfered:::", depositAmount - actualTransfer);
        
        // This will revert since not all depositAmount was transfered from Alice.
        // assertEq(actualTransfer, depositAmount, "despositAmount and actual transfer should be same");
    }
}