Withdrawals of rebasing tokens can lead to insolvency and unfair distribution of protocol reserves

criticalCode4rena

Lines of code

https://github.com/code-423n4/2024-04-renzo/blob/main/contracts/Withdraw/WithdrawQueue.sol#L229-L232 https://github.com/code-423n4/2024-04-renzo/blob/main/contracts/Withdraw/WithdrawQueue.sol#L305-L308

Vulnerability details

The WithdrawQueue contract allows users to withdraw their funds in various tokens, including liquid staking derivatives (LSDs) such as stETH. The withdraw() function calculates the amount of the specified _assetOut token equivalent to the ezETH being withdrawn using the renzoOracle.lookupTokenAmountFromValue() function. This amount is then stored in the amountToRedeem field of a new WithdrawRequest struct, which is added to the user's withdrawRequests array and the token's claimReserve.

When the user later calls claim(), the contract transfers the amountToRedeem to the user via the IERC20.transfer() function.

However, this implementation does not properly handle rebasing tokens like stETH. The stETH balance of the WithdrawQueue can change between the time a withdrawal is recorded and when it is claimed, even though the contract's stETH shares remain constant.

If the stETH balance decreases during this period due to a rebasing event (e.g., a slashing of the staked ETH), the amountToRedeem stored in the WithdrawRequest may exceed the contract's actual stETH balance at the time of claiming. As a result, the withdrawal can fail or result in the user receiving a larger share of the total protocol reserves than intended.

The issue can be illustrated by comparing the behavior of withdrawals for non-rebasing and rebasing LSDs:

1. Non-rebasing LSD (e.g., wBETH):

   • User A requests a withdrawal of 10 wBETH (worth 10 ETH) from the protocol.
   • While the withdrawal is pending, wBETH's underlying staked ETH suffers a 50% slashing event.
   • The price of wBETH drops to 0.5 ETH per token due to the slashing.
   • When User A claims their withdrawal, they receive 10 wBETH, which is now worth only 5 ETH.
   • User A bears the loss from the slashing event.

2. Rebasing LSD (e.g., stETH):

   • User B requests a withdrawal of 10 stETH (worth 10 ETH) from the protocol.
   • While the withdrawal is pending, the underlying staked ETH suffers a 50% slashing event.
   • Everyone's stETH balances are rebased to maintain the ETH peg, so the protocol's stETH balance is halved.
   • When User B claims their withdrawal, they receive the original 10 stETH (still worth 10 ETH) as recorded in the withdrawal request.
   • The protocol bears the loss from the slashing event, as it has sent out more than its fair share of the rebased stETH balance.

Impact

The current withdrawal mechanism for rebasing tokens like stETH can lead to:

• Unfair distribution of funds: Users who claim their withdrawals after a rebasing event that decreases the contract's balance will receive a larger share of the reserves than intended, at the expense of other users.
• Withdrawal failures: If the contract's balance falls below the total amountToRedeem of all pending withdrawals due to rebasing, users will face transaction failures when attempting to claim their withdrawals.

Proof of Concept

We can validate the vulnerability through a Foundry test case POC. This test case will simulate the exploit scenario and confirm the issue by performing the following actions: 2. Alice and Bob initiate withdrawals of their ezETH shares for stETH. 3. Simulate a negative rebasing event by transferring 10% of stETH balance from the withdrawQueue contract. 4. Alice claims her withdrawal successfully, receiving her original stETH amount. 5. Bob's attempt to claim his withdrawal fails due to insufficient stETH balance. 6. Verify that ezETH supply remains unchanged while TVL is significantly reduced, demonstrating ezETH becoming uncollateralized.

The PoC can be run in Foundry by using the setup and mock infra provided here -> https://gist.github.com/3docSec/a4bc6254f709a6218907a3de370ae84e

solidity📋
pragma solidity ^0.8.19;

import "contracts/Errors/Errors.sol";
import "./Setup.sol";

contract H6 is Setup {

    function testH6() public {
        // we set the buffer to something reasonably high
        WithdrawQueueStorageV1.TokenWithdrawBuffer[] memory buffers = new WithdrawQueueStorageV1.TokenWithdrawBuffer[](2);

        buffers[0] = WithdrawQueueStorageV1.TokenWithdrawBuffer(address(stETH), 100e18 - 1);
        buffers[1] = WithdrawQueueStorageV1.TokenWithdrawBuffer(address(cbETH), 100e18 - 1);

        vm.startPrank(OWNER);
        withdrawQueue.updateWithdrawBufferTarget(buffers);

        // we'll be using stETH and cbETH with unitary price for simplicity
        stEthPriceOracle.setAnswer(1e18);
        cbEthPriceOracle.setAnswer(1e18);

        // and we start with 0 TVL
        (, , uint tvl) = restakeManager.calculateTVLs();
        assertEq(0, tvl);

        // let's then imagine that Alice and Bob hold 90 and 10 ezETH each
        address alice = address(1234567890);
        address bob = address(1234567891);
        stETH.mint(alice, 100e18);
        vm.startPrank(alice);
        stETH.approve(address(restakeManager), 100e18);
        restakeManager.deposit(IERC20(address(stETH)), 100e18);
        ezETH.transfer(bob, 10e18);

        // ✅ TVL and balance are as expected
        (, , tvl) = restakeManager.calculateTVLs();
        assertEq(100e18, tvl);
        assertEq(90e18, ezETH.balanceOf(alice));
        assertEq(10e18, ezETH.balanceOf(bob));

        // Now Bob initiates withdrawal of their shares
        vm.startPrank(bob);
        ezETH.approve(address(withdrawQueue), 10e18);
        withdrawQueue.withdraw(10e18, address(stETH));

        // Alice, too, initiates withdrawal of their shares
        vm.startPrank(alice);
        ezETH.approve(address(withdrawQueue), 90e18 - 1);
        withdrawQueue.withdraw(90e18 - 1, address(stETH));

        // ☢️ time passes, and an stETH negative rebasing happens, wiping
        // 10% of the balance
        vm.startPrank(address(withdrawQueue));
        stETH.transfer(address(1), 10e18);

        vm.warp(block.timestamp + 10 days);

        // 🚨 now, since WithdrawQueue checked availability at withdrawal initiation
        // only and didn not account for the possibility of rebases, the 10% loss
        // has been completely dodged by Alice and is attributed to the last
        // user exiting.
        vm.startPrank(alice);
        withdrawQueue.claim(0);
        assertEq(90e18 - 1, stETH.balanceOf(alice));

        // 🚨 not only Bob can't withdraw
        vm.startPrank(bob);
        vm.expectRevert();
        withdrawQueue.claim(0);

        // 🚨 but ezETH as a whole also became completely uncollateralized
        assertEq(10e18 + 1, ezETH.totalSupply());
        (, , tvl) = restakeManager.calculateTVLs();
        assertEq(1, tvl);
    }
}

Tools Used

Manual review

Recommended Mitigation Steps

To address the issue of unfair distribution of funds when withdrawing rebasing tokens like stETH, the WithdrawQueue contract should store and transfer the user's withdrawal as stETH shares instead of a fixed stETH amount.

When a user initiates a withdrawal with stETH as the _assetOut, the contract should convert the calculated amountToRedeem to stETH shares using the stETH.getSharesByPooledEth() function:

uint256 sharesAmount = IStETH(stETHAddress).getSharesByPooledEth(amountToRedeem);

The resulting sharesAmount should be stored in the WithdrawRequest struct instead of the amountToRedeem.

When the user calls claim(), the contract should transfer the stETH shares directly to the user using the stETH.transferShares() function:

IStETH(stETHAddress).transferShares(msg.sender, sharesAmount);

By storing and transferring stETH shares instead of a fixed stETH amount, the contract ensures that each user receives their fair share of the stETH balance, regardless of any rebasing events that occur between the time of the withdrawal request and the claim.

To implement this mitigation, the contract should:

1. Check if the _assetOut is stETH when processing a withdrawal request.
2. If so, convert the amountToRedeem to stETH shares using stETH.getSharesByPooledEth() and store the shares amount in the WithdrawRequest struct.
3. Update the claim() function to check if the withdrawal is in stETH and, if so, transfer the shares directly using stETH.transferShares() instead of using the standard IERC20.transfer() function.

Note that this mitigation is specific to stETH and may need to be adapted for other rebasing tokens that use a similar shares-based system.

Furthermore, the claimReserve and withdrawalBufferTarget for stETH would also need to be stored in shares and converted to underlying in TVL and withdraw buffer calculations, respectively.

Assessed type

Other