ArroToken Audit

The Arro team asked us to audit their ERC20 ArroToken project. We look at the code and now we publish our results.

The audited code is located in the pjsimpkins/ArroERC20Code Github repository. The version used for this report is commit 1ca1cc85f2e7e046070c187031b75b3335ef57c3.

Update: Some of the issues listed below have been fixed by the Arro team in pull request #1. We have updated the corresponding issues with details about our review. While reviewing those fixes we have identified a few new issues. We have added them to this report with the mark “[introduced with first round of fixes]”. Our analysis of the mitigations assumes the pull request will be merged, but disregards all other unrelated changes to the code base.

Here are our assessment and recommendations, in order of importance.

Critical severity

No critical severity issues were found.

High Severity

[introduced with the first round of fixes] Stray tokens can be locked in ArroToken contract.

The transferAnyERC20Token function is intended to transfer any ERC20 tokens accidentally received by the ArroToken contract. However, this function calls the _transfer internal function passing the address of the stray ERC20 token as the sender. This means that instead of transferring stray tokens out of the ArroToken contract, this function tries to transfer the ArroTokens owned by the stray contract.

Consider calling the transfer function of the stray token contract, instead of the one from the ArroToken contract.

Not following good Smart Contract development practices

The ArroToken project lacks of a standard Solidity project structure. There are no unit tests and integration tests implemented, and there is no code coverage set up. All the contracts are defined in the same file, which doesn’t have the proper file extension. Additionally, the file is empty.

Consider using the OpenZeppelin SDK or Truffle for setting up the project environment, organizing the contracts, and adequately develop the project.
Consider using the OpenZeppelin Test Environment and OpenZepplin Test Helpers for writing automated Smart Contract tests.
Consider adding relevant content to the file to explain the project and its structure, and make it Standard Readme compliant.

Update: Partially fixed. Arro’s statement for this issue:

The project has been restructured to follow the standard Solidity project structure provided by the OpenZeppelin CLI. The code functionality relies heavily on the provided libraries and interfaces from the OpenZeppelin SDK, this provides test coverage over a majority of the functionality. There are plans in the future to add unit or integration tests on contract functions.
A relevant README has been added that provides general project information and installation and development instructions.

Note that the project is missing the .openzeppelin/project.json file from the OpenZeppelin CLI.

Medium severity

No allowance front-running mitigation

The ArroToken contract is vulnerable to the ERC20 approve and double spend front-running attack.
In this attack, a token owner authorizes another account to transfer a specific amount of tokens on their behalf, and in the case that the token owner decides to change that allowance amount, the spender could spend both allowances by front running the allowance-changing transaction.

Consider using the OpenZeppelin’s ERC20 implementation, and the decreaseAllowance and increaseAllowance functions to help mitigate this.

Update: Fixed in pull request #1.

Outdated solidity version in use

An outdated Solidity version, 0.4.24, is currently in use.

This can introduce vulnerabilities to the project that were fixed in newer solidity versions, such as the short address attack in the transfer and transferFrom functions.
In this attack, a user inputs a malformed address as destination for a token transfer in an exchange, which is misled to craft a transaction for a much bigger amount of tokens. This does not constitute an issue in the smart contract code per se, since it is ultimately the responsibility of the entity crafting the transaction to check its validity and integrity, but it should be taken into account when interacting with the token contract.

Consider bumping the project to the latest version supported by OpenZeppelin Contracts (presently 0.5.15).

Update: Fixed in pull request #1.

Missing docstrings

All the contracts and functions in Arro’s code base lack proper 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).

Update: Partially fixed in pull request #1. Arro’s statement for this issue:

The project structure has been changed to rely heavily on the OpenZeppelin ERC20 implementation. This provides a well-documented base of most of the contract functionality. The rest of the structure of the documentation has been changed to follow a consistent structure but will be updated in the future to follow the Ethereum Natural Specification Format (NatSpec).

Low severity

Lack of event emission OwnershipTransferred on contract construction

The OwnershipTransferred event is not being emitted when the ArroToken contract is initialized.

This means that there will be no record of who was the initial owner of the token in the OwnershipTransferred event logs.

Consider using OpenZeppelin’s Ownable contract which already covers this, or consider emitting an OwnershipTransferred after asigning the owner in the Owned‘s constructor.

Update: Fixed in pull request #1.

Lack of event emission after modifying newOwner variable

In line 90 and in line 96, the storage variable newOwner is being modifed, but no events are being triggered.

This means that there will be no record of the changes of the newOwner variable, and traceability of the potential new owners of the TokenArro will be lost.

Consider to define an event for tracking the newOwner variable value, and emit an event each time it is modified.

Update: Fixed in pull request #1.

owner and newOwner storage variables can be modified without events emission

The owner and newOwner storage variables in the Owned contract are defined as public.
Even though they are not being modified within the ArroToken contract, there are no explicit validations for avoiding to assign them directly, which will bypass the emission of the OwnershipTransferred event.

Consider using OpenZeppelin’s Ownable contract which already covers this, or consider declaring owner and newOwner variables as private, and add getter functions for accessing them.

Update: Fixed in pull request #1.

Duplicate code in transfer and transferFrom functions

The transfer and transferFrom functions share very similar code.
Duplicate code is more difficult to maintain, as it is longer and needs to be updated in different sections of the codebase.

Consider moving the duplicated code into another function and call it from the transfer and transferFrom functions.

Update: Fixed in pull request #1.

Wrong usage of inheritance with Safemath contract

A SafeMath contract is implemented in order to manage arithmetic overflows and underflows.

In this contract, all its functions are defined as public, which means that the ArroToken contract will expose them as part of its API, as it inherits from SafeMath. This extends the functionality that should be exposed by an ERC20 token. Additionally, the gas costs of the deploy will be higher as opposed to using SafeMath as a library.

Given that the functionality of the SafeMath contract can be exposed as a library, consider using OpenZeppelin’s SafeMath

Update: Fixed in pull request #1.

The TokenArro contract can be transferred to the zero address

The transferOwnership function in the Owned contract does not prevent to transfer the ownership of the contract to the zero address.

Consider using OpenZeppelin’s Ownable contract which already covers this, or consider restricting the new owner to non-zero addresses.

Update: Fixed in pull request #1.

Tokens can be transferred to the zero address

The transfer and the transferFrom functions in the ArroToken contract do not prevent to transfer tokens to the zero address.

Consider using the OpenZeppelin’s ERC20 implementation which already covers this, or consider restricting the to parameter to non-zero addresses.

Update: Fixed in pull request #1.

The totalSupply function does not return the real total supply

The totalSupply function returns the difference between the token’s total supply defined in the constructor and the tokens owned by the zero address, which is not strictly the total supply amount.

This could lead to a misinterpretation of the real total supply of the token when querying it.

Consider modifying the the totalSupply function implementation so it returns the real total supply value, and consider using the OpenZeppelin’s ERC20 implementation that implements the _burn and _burnFrom functions to keep the total supply variable up-to-date.

Update: Fixed in pull request #1.

Notes & Additional information

[introduced with the first round of fixes] Unnecessary inheritance and imports.

The ArroToken contract inherits from the OwnableUpgradeSafe, Initializable, and ContextUpgradeSafe contracts. Because the OwnableUpgradeSafe contract inherits from the Initializable and ContextUpgradeSafe contracts, the declaration to inherit from these two contracts is redundant.

Consider simplifying the code by removing the redundant inheritance declarations.

[introduced with the first round of fixes] initialize function missing the initializer modifier

The initialize function of the ArroToken contract is not using the initializer modifier. Currently this is not an issue because the functions called by initialize have the initializer modifier and will prevent it to be called more than once. However, it is safer to use the initializer modifier to make the intention clearer and safer in case the code is modified.

Consider adding the initializer modifier in the definition of the initialize function.

[introduced with the first round of fixes] Unnecessary calls to initializers

The initialize function of the ArroToken contract calls __Context_init_unchained(), __Pausable_init_unchained(), and __ERC20Pausable_init_unchained(). The __ERC20Pausable_init() calls the same three initializers, so they can be replaced by this single call.

Consider calling __ERC20Pausable_init() to simplify the code.

Not following a consistent coding style

The code base deviates from the Solidity Style Guide. A consistent coding style helps with the readability of the project. Consider enforcing a standard coding style with help of linter tools such as Solhint.

Update: Fixed in pull request #1.

Multiple variables declared as uint

There are several variable declarations of uint variables throughout the project.

To favor explicitness, all instances of uint should be declared as uint256. See for example lines 25 and 29, 49, 109 and 111.

Update: Partially fixed in pull request #1. In line 46 of ArroToken.sol there is still a uint declaration.

No explicit visibility definition of variables

In line 111 and line 112, the balances and allowed variables are not explicitly declared as public.

To favor explicitness and readability, consider adding the visibility in all variable declarations.

Update: Fixed in pull request #1.

Link reference to the ERC20 EIP is deprecated

The links to the ERC20 EIP in line 46 and in line 161 references to an old ERC20 document.

Consider referencing the official ERC20 EIP site instead.

Update: Fixed in pull request #1.

Unused variables defined in function definitions

Named return variables are declared and unused. For example, in lines 139, 149, 165 and 181.

Consider removing all declared but unused return variables, or explicitly declaring them as local variables, and adding the necessary return statements when appropriate.

Update: Fixed in pull request #1.

Missing error messages in require and revert statements

There are require and revert statements without an error message specified in line 85, line 93, and line 216.

Consider adding an appropriate message describing the validation or revert reason.

Update: Fixed in pull request #1.

Inconsistent variable naming

Variable naming is inconsistent. For example, in line 109 _totalSupply has a leading underscore, while the other storage variables within the same contract do not.

Consider defining a naming convention based on Solidity Style Guide and follow this convention throughout the project.

Update: Fixed in pull request #1.

Non-standard initialization of ERC20

In the ArroToken constructor, the variables of the token are being hardcoded.

Consider using OpenZeppelin’s ERC20Detailed contract’s constructor for initializing the name, symbol, and decimals, and consider sending totalSupply as a parameter to the ArroToken constructor.

Update: Fixed in pull request #1.


No critical and one high severity issue was found. Some changes were proposed to follow best programming practices and reduce the potential attack surface.

Update: Changes in pull request #1 have introduced one high severity issue.