1inch Exchange Audit

The 1inch Exchange is a DEX aggregator that allows users to perform complex trades using multiple decentralized exchanges. We reviewed part of the on-chain infrastructure used to manage the external calls and redeem CHI tokens as desired.

The review was undertaken by two auditors over one week. The audited commit is 72f2812837fdd73ec2d32c8988811df361e80985 and the scope included the following contracts:
OneInchExchange.sol
RevertReasonParser.sol
UniERC20.sol

All other components of the system were assumed to behave as documented. Importantly, this includes arbitrary calls to external contracts that are expected to perform the token transfers.

Summary

Overall, we are happy with the structure and implementation of the code. Our main recommendation is to introduce thorough documentation to better communicate the expected behavior.

System Overview

The main exchange contract is simply a scaffold around a set of arbitrary calls, which are used to interact with various decentralized exchange contracts to perform the desired token swaps. It provides safety checks to ensure the overall effect matches the criteria set by the caller. It also provides a mechanism to redeem CHI tokens after the call, thereby reducing the gas costs.

We also reviewed two helper contracts. One parses and adds context to the error messages associated with failed transactions, and the other provides a unifying interface for ERC20 tokens and native ETH operations.

Privileged Roles

The OneInchExchange.sol contract has an owner address, controlled by the 1inch team, that can pause the exchange. It can also withdraw any funds that were mistakenly sent to the contract itself.

Here we present our findings.

Critical severity

None.

High severity

None.

Medium severity

[M01] Cannot unpause exchange

The OneInchExchange.sol contract exposes a mechanism for the owner to pause the contract. This disables the swap functionality. However, there is no corresponding mechanism to unpause the contract.

Consider introducing a mechanism for the owner to unpause the contract. Alternatively, if the current behavior is expected, consider renaming the pause function to shutdown or something similar that implies the contract will be permanently disabled.

Update: Fixed in commit 0b89110a. The pause function has been renamed to shutdown.

[M02] Cannot decrease allowance to non-zero value

The uniApprove function of the UniERC20 library can be used to change the token allowance granted to a specified address. However, if the new value is non-zero and the existing allowance is greater than the new value, the request is discarded. This means the token allowance cannot be decreased to a non-zero value, which differs from the typical ERC20 approve behavior. Consider introducing a decreaseAllowance function to handle this case, or documenting the new behavior and its rationale in the function comments.

This issue is related to [ M03 ] Incorrect safeApprove usage and any mitigation should consider both simultaneously.

Update: Fixed in commit bdbda2c7. The function replicates standard approve functionality, whether or not the token prevents changing allowances between non-zero values. The code intentionally disregards the possible frontrunning attack in the interest of universal applicability.

[M03] Incorrect safeApprove usage

The safeApprove function of the OpenZeppelin SafeERC20 library prevents changing an allowance between non-zero values to mitigate a possible front-running attack. Instead, the safeIncreaseAllowance and safeDecreaseAllowance functions should be used. However, the UniERC20 library simply bypasses this restriction by first setting the allowance to zero. The reintroduces the front-running attack and undermines the value of the safeApprove function. Consider introducing an increaseAllowance function to handle this case.

This issue is related to [ M02 ] Cannot decrease allowance to non-zero value and any mitigation should consider both simultaneously.

Update: Fixed in commit bdbda2c7. The function replicates standard approve functionality, whether or not the token prevents changing allowances between non-zero values. The code intentionally disregards the possible frontrunning attack in the interest of universal applicability.

Low severity

[L01] Transfer may fail

The UniERC20 contract performs a low-level transfer to send ETH. This has some notable shortcomings when the recipient is a smart contract. Specifically, the transfer will fail when:

  • the withdrawer smart contract does not implement a payable fallback or receive function
  • the smart contract implements a payable fallback function that uses more than 2300 gas, potentially due to a proxy that increases the call’s gas usage.

To prevent unexpected behavior, consider explicitly warning users about these shortcomings. Additionally, note that the sendValue function available in OpenZeppelin Contract’s Address library can be used to transfer Ether without being limited to 2300 gas units. Risks of reentrancy stemming from the use of this function can be mitigated by tightly following the “Check-effects-interactions” pattern and using OpenZeppelin Contract’s ReentrancyGuard contract. For further reference on why using Solidity’s transfer is no longer recommended, refer to these articles:

Notes & Additional Information

[N01] Missing docstrings and references

The contracts and functions in the reviewed code base lack documentation. This hinders reviewers’ understanding of the code’s intention, which is fundamental to correctly assess not only security, but also correctness. Additionally, docstrings improve readability and ease maintenance. They should explicitly explain the purpose or intention of the functions, the scenarios under which they can fail, the roles allowed to call them, the values returned and the events emitted.

Consider thoroughly documenting all functions (and their parameters) that are part of the contracts’ public API. Functions implementing sensitive functionality, even if not public, should be clearly documented as well. When writing docstrings, consider following the Ethereum Natural Specification Format (NatSpec).

Furthermore, the code comments should reference any relevant standards. In particular:

These should be explicitly noted.

[N02] Mixing roles

The OneInchExchange contract has a mechanism for the owner to withdraw funds held by the contract to their own account. However, the role of the owner and the funds recipient are conceptually distinct and implicitly combining them can introduce additional complexity if the owner role is transferred to a governance mechanism. Consider allowing the owner to specify a recipient address when withdrawing funds from the contract.

[N03] Safety check is manipulable

The swap function of the OneInchExchange contract allows users to perform a sequence of arbitrary calls and respond to potential failures in order to execute a complicated token swap across multiple decentralized exchanges. However, there are many ways that this function can be abused to subvert its intended effect. Some examples include:

  • the caller can set the _SHOULD_CLAIM flag to false and perform swaps without transferring in the expected amount of tokens.
  • the caller could use the arbitrary calls to transfer tokens in or out of the destination address, manipulating the calculation of the returned tokens.
  • the caller could use the arbitrary calls to reenter the swap function so the balance change calculations will be performed on multiple swaps.

This should not pose a safety risk because the function is intended to provide consistency checks for the caller, so these manipulations only undermine that goal. All security considerations around the token swaps themselves should be handled by the external exchanges. However, this does imply that the emitted event may not accurately reflect the executed operation. If this is undesirable, consider validating the consistency of the parameters and the external calls.

Conclusions

No critical or high severity issues were found. Some changes were proposed to follow best practices, reduce the potential attack surface, and enhance overall code quality.