Tether USD (USDT)

[carlossampol]

Audit request

Audit Top 200 CoinMarketCap tokens.

Tether USD (USDT) Ethereum Token

Deployed at https://etherscan.io/address/0xdac17f958d2ee523a2206206994597c13d831ec7#contracts

Source code

https://etherscan.io/address/0xdac17f958d2ee523a2206206994597c13d831ec7#contracts

Disclosure policy

public

Platform

Ethereum

Number of lines:

252

[MrCrambo]

Auditing time 1 day

[yuriy77k]

@MrCrambo assigned

[MrCrambo]

My report is finished

[RideSolo]

Auditing time: 1 day

[yuriy77k]

@RideSolo assigned

[danbogd]

Auditing time: 1 day.

[danbogd]

My report is finished.

[yuriy77k]

@danbogd assigned

[yuriy77k]

Tether USD (USDT) Security Audit Report

1. Summary

Tether USD (USDT) smart contract security audit report performed by Callisto Security Audit Department

2. In scope

• USDT solidity contract .

3. Findings

In total, 7 issues were reported including:

• 3 low severity issues.

• 4 owner privileges (the ability of an owner to manipulate contract, may be risky for investors).

No critical security issues were found.

3.1. Owner Privileges

Severity: Owner Privileges

Description

1. Pause/unpause transfer and transferFrom.
2. Blacklist users addresses individualy from using transfer and transferFrom through addBlackList and removeBlackList. And burn blacklisted users assets (please note that the assets are supposed to be backed in a 1:1 ratio) using destroyBlackFunds function.
3. A maximal fee percentage of 20/10000 can be applied per transaction with a maximal value of 50 USDT, check here.
4. Owner can upgrade contract using deprecate and implement any logic in the new contract. And even if the new contract will be audited, at any time possible to change the address of the new contract again to not audited and insecure.

3.2. Transfer to Address(0)

Severity: low

Description

transfer and transferFrom allow the destination address to be equal to zero, meaning that users fund can be lost if sent to it by mistake.

Code snippet

    function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32) {
        uint fee = (_value.mul(basisPointsRate)).div(10000);
        if (fee > maximumFee) {
            fee = maximumFee;
        }
        uint sendAmount = _value.sub(fee);
        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(sendAmount);
        if (fee > 0) {
            balances[owner] = balances[owner].add(fee);
            Transfer(msg.sender, owner, fee);
        }
        Transfer(msg.sender, _to, sendAmount);
    }

    function transferFrom(address _from, address _to, uint _value) public onlyPayloadSize(3 * 32) {
        var _allowance = allowed[_from][msg.sender];

        uint fee = (_value.mul(basisPointsRate)).div(10000);
        if (fee > maximumFee) {
            fee = maximumFee;
        }
        if (_allowance < MAX_UINT) {
            allowed[_from][msg.sender] = _allowance.sub(_value);
        }
        uint sendAmount = _value.sub(fee);
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(sendAmount);
        if (fee > 0) {
            balances[owner] = balances[owner].add(fee);
            Transfer(_from, owner, fee);
        }
        Transfer(_from, _to, sendAmount);
    }

3.3. Not Emitted Transfer Event

Severity: low

Description

When issuing or redeeming tokens a transfer event should be emitted back and forth to address(0). ERC20 standard: "A token contract which creates new tokens SHOULD trigger a Transfer event with the _from address set to 0x0 when tokens are created". the same can be deducted when redeeming or burning tokens.

Same issue is applicable for TetherToken constructor.

Code snippet

https://gist.github.com/RideSolo/24c79eb34b565ade477ec89c2af49a5b#file-usdt-sol-L406

https://gist.github.com/RideSolo/24c79eb34b565ade477ec89c2af49a5b#file-usdt-sol-L420

3.4. Known vulnerabilities of ERC-20 token

Severity: low

Description

1. It is possible to double withdrawal attack. More details here
2. 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 into the function transfer(address _to, ... ) and transferFrom the following code, to avoid transfers to the contract address:

   require( _to != address(this) );

4. Conclusion

The audited smart contract can be deployed. Only low severity issues were found during the audit.

5. Revealing audit reports

https://gist.github.com/yuriy77k/49b74a164bccac9b2554de9b25ffae8b

https://gist.github.com/yuriy77k/476b9556f4895d32867890af4e4199ba

https://gist.github.com/yuriy77k/d75e9365b6ec7eefd46a237d09e673bc

[MillianoConti]

Reddit Announced

[HawaiiToastNo1]

0xdac17f958d2ee523a2206206994597c13d831ec7 - this smart contract has stolen my 50k USDT funds from Trust Wallet!

[kostapsimoulis]

@HawaiiToastNo1 were you ever able to get it back?