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29116 - [SC - Low] Using deposit results in more shares for the sa....md

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Using deposit results in more shares for the same amount of stETH

Submitted on Mar 7th 2024 at 13:58:54 UTC by @LokiThe5th for Boost | Puffer Finance

Report ID: #29116

Report type: Smart Contract

Report severity: Low

Target: https://etherscan.io/address/0xd9a442856c234a39a81a089c06451ebaa4306a72

Impacts:

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

Description

Brief/Intro

The PufferVault is an ERC4626 vault which mints shares to a user. However, the underlying asset is stETH, which always leaves some amount of shares over when using the normal transferFrom function. By using the standard ERC4626::deposit() and ERC4626:mint() functionality the user is always minted slightly more shares than expected.

Vulnerability Details

stETH has a known property that when using the transferFrom or transfer functions it always leaves some small amount of stETH behind.

In the PufferVault the deposit and mint functions which are inherited from ERC4626 both use the transferFrom method via safeTransferFrom within the ERC4626::_deposit function.

This has the effect that when a user deposits via the PufferVault there is always slightly less stETH transferred than expected, but the user is still minted shares according the initial amount.

This also allows a user to mint free shares for dust amounts. As the stETH amount left over rom transferFrom increases, so too does the amount of free shares the user receives. This extra amount can become larger as stETH/share rate grows. See there docs here: https://docs.lido.fi/guides/lido-tokens-integration-guide/#1-2-wei-corner-case.

Impact Details

A user can mint 1 share in exchange for the rounding amount (1-2 wei). This may become larger in the future.

In this case a user should always use deposit instead of mint, as the rounding differences means they will always receive slightly more shares when using deposit.

References

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

Proof of Concept

The below files should be placed within the puffEth repo.

https://drive.google.com/drive/folders/1-h_HIsiwwqv2L80TIOZd9x-HwvD0Yuwu?usp=sharing

Place the OverSharePoC.sol file in the test directory.

The OverSharePoc.sol file mimics stETH's property of leaving dust amounts after normal transferFrom calls.

Place the MockAccessManager.sol, stETHMockDust.sol, LidoWithdrawalQueueMockExpanded.sol and the stETHStrategyMock.solfiles in the test/mocks directory.

Run the test with forge test --match-contract OverSharePoC -vvv.

The console output:

[PASS] test_PoC_deposit() (gas: 281393)
Logs:
  Shares received:  1000000000000000001
  steth used:  999999999999999999

[PASS] test_PoC_mint() (gas: 281591)
Logs:
  Shares received:  1000000000000000000
  steth used:  999999999999999999

This shows Alice receiving slightly more shares when using deposit vs mint.