The Wings team asked us to review and audit their new Token contract code. We looked at their code and now publish our results.

The audited contract is at their contracts GitHub repo. The version used for this report is commit 1b308105a31c5b005c21fbbda3b1ff5b3fac9bae. The main contract file is Token.sol.

Code quality is good. We’re very happy to audit a project using OpenZeppelin.

Here’s our assessment and recommendations, in order of importance:

Update: The Wings team implement most recommendations in their master branch.

Severe

We haven’t found any severe security problems with the code.

Potential problems

Be careful with types

Avoid declaring variables using var if possible. The type-deduction system can bring some surprises if you don’t think about types very carefully. For example, for ints, it will choose the smallest type that is required to hold assigned constants. This can lead to endless loops and gas depletion. Better use the explicit type uint for no surprises and higher limits. Consider replacing all occurrences of var with a specific type.

Fixed: https://github.com/WingsDao/contracts/blob/master/contracts/Token.sol#L222

Timestamp usage

There’s a problem with using timestamps and now (alias for block.timestamp) for contract logic, based on the fact that miners can perform some manipulation. In general, it’s better not to rely on timestamps for contract logic. The solutions is to use block.number instead, and approximate dates with expected block heights and time periods with expected block amounts.

The Token.sol contract uses timestamps in line 212. The risk of miner manipulation, though, is really low. The potential damage is also limited: miners could only slightly manipulate when each preallocation can be done. We recommend the team to consider the potential risk of this manipulation and switch to block.number if necessary.

For more info on this topic, see this stack exchange question.

Warnings

Usage of magic constants

There are several magic constants in the contract code. Some examples are:

Use of magic constants reduces code readability and makes it harder to understand code intention. We recommend extracting magic constants into contract constants.

Fix: https://github.com/WingsDao/contracts/blob/master/contracts/Token.sol#L53

Bug Bounty

Formal security audits are not enough to be safe. We recommend implementing an automated contract-based bug bounty and setting a period of time where security researchers from around the globe can try to break the contract’s invariants. For more info on how to implement automated bug bounties with OpenZeppelin, see this guide.

Avoid duplicated code

Duplicate code makes it harder to understand the code’s intention and thus, auditing the code correctly. It also increases the risk of introducing hidden bugs when modifying one of the copies of some code and not the others.

The logic in transfer, transferFrom, and approve is very similar to the provided methods in StandardToken and could be refactored to avoid repetition. There is no need to rewrite the parent’s contract logic: consider using the super keyword to access StandardToken implementation of methods, like so:

function transfer(address _to, uint _value) whenAllocation(false) returns (bool success) {

return super.transfer(_to, _value);

}

Fixed in https://github.com/WingsDao/contracts/blob/master/contracts/Token.sol#L131

Naming suggestions

All references to premine (Preminer, PreminerAdded, PremineAllocationAdded, PremineRelease, etc.) are confusing. There is no mining of these tokens. Consider using another name like “initial assignment”.

checkUserAllocation and checkPreminerAllocation are not very good names, as they don’t clearly indicate what they are supposed to do. Consider renaming it for example to whenHasntAllocated(user) .

and are not very good names, as they don’t clearly indicate what they are supposed to do. Consider renaming it for example to . monthlyPayment uint field of Preminer struct is confusing. It’s name implies it’s a monthly payment but there’s nothing enforcing the payment is only done monthly. Consider renaming the field or adding time checks.

Fixed in

https://github.com/WingsDao/contracts/blob/master/contracts/Token.sol#L86

Use latest version of Solidity

Current code is written for old versions of solc (0.4.2). With the storage overwriting vulnerability found on versions of solc prior to 0.4.4, there’s a risk this code can be compiled with vulnerable versions. We recommend changing the solidity version pragma for the latest version ( pragma solidity ^0.4.10; ) to enforce latest compiler version to be used.

Fixed in https://github.com/WingsDao/contracts/blob/master/contracts/Token.sol#L1

Additional Information and Notes

Conclusions

No severe security issues were found. Some changes were recommended to follow best practices and reduce potential attack surface.

Code quality is good. We’re very happy to audit a project using OpenZeppelin.

Update: The Wings team implement most recommendations in their master branch.