Basic Attention Token (BAT) Security Audit Report

Basic Attention Token (BAT) security audit, conducted by the Callisto Network Security Department in April 2019.

Basic Attention Token (BAT) Specificities

Audit Request

Audit Top 200 CoinMarketCap tokens.

Symbol      : BAT
Name        : Basic Attention Token
Circulating supply: 1,244,766,783
Total supply: 1,500,000,000

https://basicattentiontoken.org/

Deployed at:

https://etherscan.io/address/0x0d8775f648430679a709e98d2b0cb6250d2887ef#code

Source Code:

https://gist.github.com/yuriy77k/9f8fcc78745cf81e3aff92da724b3d5c

Disclosure polic:

Public.

Platform:

ETH.

Number of lines:

129.

Basic Attention Token (BAT) Smart Contract Security Audit Report

Are Your Funds Safe?

1. In scope

- BAToken.sol

2. Findings

In total, 4 issues were reported including:

1 medium severity issues.
3 low severity issues.

No critical security issues were found.

2.1. Known vulnerabilities of ERC-20 token

Severity: low.

Description:

It is possible to double withdrawal attack. More details here.
Lack of transaction handling mechanism issue. WARNING! This is a very common issue and it already caused millions of dollars losses for lots of token users! More details here.

Recommendation:

Add the following code to the transfer(_to address, ...) function:
require( _to != address(this) );

2.2. ERC20 Compliance — transfer should throw

Severity: medium.

Description:

From ERC-20 specification:

The function SHOULD throw if the _from account balance does not have enough tokens to spend.

But in this implementation of transfer and transferFrom it just returns false. This can lead to serious consequences. Because checking the return value of this function is rare.
For example, external contract may use this token contract as:
```
BatToken.transferFrom(recipient, this, value);
points[recipient] += value;
```
In this case recipient can get any value of points, but he may not have enough money and the code will succeed.

Code snippet:

https://gist.github.com/yuriy77k/9f8fcc78745cf81e3aff92da724b3d5c#file-batoken-sol-L54

https://gist.github.com/yuriy77k/9f8fcc78745cf81e3aff92da724b3d5c#file-batoken-sol-L66

2.3. ERC20 Compliance — zero-value transfers rejecting

Severity: low.

Description:

EIP20 says that:

Transfers of 0 values MUST be treated as normal transfers and fire the Transfer event.
But in this contract, function transfer and transferFrom has a condition:

if (balances[msg.sender] >= _value && _value > 0) {
    // ...
}

Code snippet:

https://gist.github.com/yuriy77k/9f8fcc78745cf81e3aff92da724b3d5c#file-batoken-sol-L48

https://gist.github.com/yuriy77k/9f8fcc78745cf81e3aff92da724b3d5c#file-batoken-sol-L59

2.4. ERC20 Compliance — event missing

Severity: low.

Description:

According to ERC20 standard when coins are minted a Transfer event should be emitted.
The createTokens function also should emit the Transfer event.

Code snippet:

https://gist.github.com/yuriy77k/9f8fcc78745cf81e3aff92da724b3d5c#file-batoken-sol-L127

https://gist.github.com/yuriy77k/9f8fcc78745cf81e3aff92da724b3d5c#file-batoken-sol-L145

3. Conclusion

The audited smart contract has some issues with ERC20 Compliance that could cause losing the money in a particular situation. We recommend fixing these issues.