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Introduction

A time-boxed security review of the Mugen protocol was done by pashov, with a focus on the security aspects of the application's implementation.

Disclaimer

A smart contract security review can never verify the complete absence of vulnerabilities. This is a time, resource and expertise bound effort where I try to find as many vulnerabilities as possible. I can not guarantee 100% security after the review or even if the review will find any problems with your smart contracts.

Protocol Overview

The protocol is a DEX/swap adapter that allows complex transactions, for example multiple tokens in to multiple/single tokens out and vice versa on several DEXes (Uniswap, Sushiswap, 3xcalibur). The protocol integrates the native adapters provided by the DEXes to ensure flawless transfers of tokens. It also allows cross-chain swaps as it is integrated with LayerZero's Stargate protocol.

Severity classification

Severity Impact: High Impact: Medium Impact: Low
Likelihood: High Critical High Medium
Likelihood: Medium High Medium Low
Likelihood: Low Medium Low Low

Security Assessment Summary

review commit hash - 61564a3e1eac743cb9b89976cbefbcb0fd15f38f

Scope

The following smart contracts were in scope of the audit:

  • ArbitrumSwaps
  • StargateArbitrum

The following number of issues were found, categorized by their severity:

  • High: 2 issues
  • Medium: 1 issue
  • Low: 3 issues
  • Informational: 10 issues

Findings Summary

ID Title Severity
[H-01] Anyone can use or steal ArbitrumSwaps native asset balance High
[H-02] Malicious user can easily make the protocol revert on every USDT swap on Uniswap High
[M-01] Use quoteLayerZeroFee instead of sending all native asset balance as gas fee for swap call Medium
[L-01] Check array arguments have the same length Low
[L-02] A require check can easily be bypassed Low
[L-03] The gasLeft() after gas-limited external call might not be enough to complete the transaction Low
[I-01] Prefer battle-tested code over reimplementing common patterns Informational
[I-02] Use an enum for the "step" types in ArbitrumSwaps Informational
[I-03] Move code to bring cohesion up Informational
[I-04] Use x != 0 to get positive-only uint values Informational
[I-05] Remove not needed custom error Informational
[I-06] Solidity safe pragma best practices are not used Informational
[I-07] External method missing a NatSpec Informational
[I-08] Mismatch between contract and file names Informational
[I-09] Missing override keyword Informational
[I-10] Typos in comments Informational

Detailed Findings

[H-01] Anyone can use or steal ArbitrumSwaps native asset balance

Severity

Likelihood: High, because this can easily be noticed and exploited

Impact: Medium, because value can be stolen, but it should be limited to gas refunds

Description

An attacker can steal the ArbitrumSwaps native asset balance by doing a call to the arbitrumSwaps method with steps WETH_DEPOSIT and WETH_WITHDRAW - this will send over the whole contract balance to a caller-supplied address. This shouldn't be a problem, because the contract is a "swap router" and is not expected to hold any native asset balance at any time. Well this assumption does not hold, because in the stargateSwap method the _refundAddress argument of the swap method call to the stargateRouter is address(this). This means that all of the native asset that is refunded will be held by the ArbitrumSwaps contract and an attacker can back-run this refund and steal the balance.

Recommendations

The refund address should be msg.sender and not address(this). This way the protocol won't be expected to receive native assets, so they can be stolen only if someone mistakenly sends them to the ArbitrumSwaps contract which is an expected risk.

[H-02] Malicious user can easily make the protocol revert on every USDT swap on Uniswap

Severity

Likelihood: High, attack can easily be done and it exploits a well-known attack vector of USDT

Impact: Medium, because the protocol will not work with only one ERC20 token, but it is a widely used one

Description

A malicious user can get the ArbitrumSwaps contract to revert on each USDT swap on Uniswap, because of a well-known attack vector of the token implementation. The problem is in the following code from UniswapAdapter.sol and is present in both swapExactInputSingle and swapExactInputMultihop

        TransferHelper.safeApprove(
            swapParams.token1, address(swapRouter), IERC20(swapParams.token1).balanceOf(address(this))
        );

Here is how the attack can be done:

  1. Malicious user transfers manually 1 wei of USDT to ArbitrumSwaps
  2. Now he calls ArbitrumSwaps::arbitrumSwaps with step == UNI_SINGLE to swap 1 USDT
  3. Now swapExactInputSingle approves 1 + 1e-18 USDT, because of the balanceOf call
  4. Transaction will complete successfully, but next time anyone wants to swap USDT on Uniswap the transaction will revert, because of USDT approval race condition - to do an approve call the allowance should be either 0 or type(uint256).max, which is not the case because allowance is 1 wei

Recommendation

Instead of using the IERC20(multiParams.token1).balanceOf(address(this)) as the approved allowance, use the amountIn parameter.

[M-01] Use quoteLayerZeroFee instead of sending all native asset balance as gas fee for swap call

Severity

Likelihood: High, because the wrong value will be sent always

Impact: Low, because the swap function has a gas refund mechanism

Description

Currently in StargateArbitrum::stargateSwap when doing a call to the swap method of stargateRouter, all of the contract's native asset balance is sent to it so it can be used to pay the gas fee. The Stargate docs show that there is a proper way to calculate the fee and it is by utilizing the quoteLayerZeroFee method of stargateRouter.

Recommendations

Follow the documentation to calculate the fee correctly instead of always sending the whole contract's balance as a fee, even though there is a refund mechanism.

[L-01] Check array arguments have the same length

In both ArbitrumSwaps::arbitrumSwaps and in StargateArbitrum::stargateSwap you have multiple array-type arguments. Validate in both places that the arguments have the same length so you do not get unexpected errors if they don't.

[L-02] A require check can easily be bypassed

In StargateArbitrum::stargateSwap we have the following code

if (msg.value <= 0) revert MustBeGt0();

This check can easily be bypassed by just sending 1 wei. Remove the check completely, since msg.value is not used in the method anyway.

[L-03] The gasLeft() after gas-limited external call might not be enough to complete the transaction

In StargateArbitrum::sgReceive we have the following piece of code

try IArbitrumSwaps(payable(address(this))).arbitrumSwaps{gas: 200000}(steps, data) {}
        catch (bytes memory) {
            IERC20(_token).safeTransfer(to, amountLD);
            failed = true;
        }

Now if the arbitrumSwaps call took up all of the gas it is possible that there is not enough gas left for the safeTransfer call, as well for the code below it. Consider a different approach, that will check gasleft() and make sure that there will be enough, something like in this method

[I-01] Prefer battle-tested code over reimplementing common patterns

Replace the locked modifier in ArbitrumSwaps with the nonReentrant from OpenZeppelin, since it is well tested and optimized.

[I-02] Use an enum for the "step" types in ArbitrumSwaps

Currently the step types are handled by constants that have an integer value and are not sequential (numbers 7 to 11 are missing). This is a great use case for an enum, where you will have sequential numbering and proper naming. Remove the constants and use an enum.

[I-03] Move code to bring cohesion up

The event FeePaid and the calculateFee method should both be in ArbitrumSwaps instead of in StargateArbitrum since they have nothing to do with the Stargate logic, but are used in some swap/transfer scenarios.

[I-04] Use x != 0 to get positive-only uint values

The "positive uint" checks in the code are not done in the best possible way, one example is _amount <= 0 - if a number is expected to be of a uint type, then you can check that it is positive by doing x != 0 since uint can never be a negative number. Replace all x <= 0 occurrences with x != 0 when x is a uint.

[I-05] Remove not needed custom error

The MoreThanZero custom error in ArbitrumSwaps is badly named and also duplicates the inherited from StargateArbitrum custom error MustBeGt0 - prefer using the latter and remove the former.

[I-06] Solidity safe pragma best practices are not used

Always use a stable pragma to be certain that you deterministically compile the Solidity code to the same bytecode every time. Also IStargateReceiver and IStargateRouter interfaces are using an old compiler version - upgrade it to a newer version, use the same pragma statement throughout the whole codebase.

[I-07] External method missing a NatSpec

The arbitrumSwaps method in ArbitrumSwaps is missing a NatSpec doc, add one to improve the code technical documentation.

[I-08] Mismatch between contract and file names

The ArbitrumSwaps contract inherits from SushiLegacyAdapter which is imported from SushiAdapter.sol. Use the same name for the smart contract and the file.

[I-09] Missing override keyword

The method arbitrumSwaps in ArbitrumSwaps is inheriting the method from IArbitrumSwaps but is missing the override keyword which should be there.

[I-10] Typos in comments

In StargateArbitrum you wrote arrat -> array