1. Brief Introduction
The 'Unemitted Transfer Event Issue', 'Unemitted Approval Event Issue', 'Fake Recharge Vulnerability' and 'Writing Errors of Constructed Function' are uniformly classified as 'Ethereum's smart contract specification problem' in 'Knownsec Ethereum Contract Audit Checklist' which sorted out by the Knownsec 404 Blockchain Security Research Team.
'Hao Tian' is an automation platform for monitoring, scanning, analysis and auditing blockchain smart contract. It is independently developed by the Knownsec 404 Blockchain Security Research Team. We use this platform to scan and analyze the smart contract codes publicly posted across the web for the above-mentioned "EthereumSmart Contract Specification".
2. Vulnerability Details
ERC20 is a standard of the token for the smart contract on the Ethereum blockchain. ERC20 defines a general rule that must be enforced by Ethereum. Exchanges can be integrated to implement token trading if the token issued at Ethereum reaches the ERC20 standard.
ERC20 stipulates that the transfer function must trigger a transfer event and return a Boolean value. It should also throw an error instead of returning the error simply when making a balance judgment. And the approve function must trigger an approve event.
1) Unemitted Transfer Event
function transfer(address _to, uint256 _value) public returns (bool success) {
require(balanceOf[msg.sender] >= _value);
require(balanceOf[_to] + _value >= balanceOf[_to]);
balanceOf[msg.sender] -= _value;
balanceOf[_to] += _value;
return true;
}
The above codes did not emit the Transfer event when the transaction occurred. Failure to comply with the ERC20 standard makes it difficult for developers to monitor contract transactions.
2) Unemitted Approval Event
function approve(address _spender, uint256 _value) public
returns (bool success) {
allowance[msg.sender][_spender] = _value;
return true;
}
The above codes did not emit the Approve event when the transaction occurred. Failure to comply with the ERC20 standard makes it difficult for developers to monitor contract transactions.
3) Fake Recharge Vulnerability
function transfer(address _to, uint256 _amount) returns (bool success) {
initialize(msg.sender);
if (balances[msg.sender] >= _amount
&& _amount > 0) {
initialize(_to);
if (balances[_to] + _amount > balances[_to]) {
balances[msg.sender] -= _amount;
balances[_to] += _amount;
Transfer(msg.sender, _to, _amount);
return true;
} else {
return false;
}
} else {
return false;
}
}
The above codes use the if statement to judge the balance. ERC20 stipulates the contract should throw an error to roll back the transaction, rather than returning the error simply when the balance is insufficient.
In this case, the transaction will still succeed even if there is no real transaction, which may affect the judgment of the trading platform and lead to false recharge.
On July 9, 2018, Slow Fog Safety Team issued a warning on the vulnerability of fake recharge.
On July 9, 2018, the Knownsec 404 Blockchain Security Research Team followed with the vulnerability and issued a vulnerability warning for the vulnerability. If the case of the constructor name doesn't match the contract, this function will still be treated as a normal function and can be called by any user.
4) Writing Error of Constructed Function
The compiler required that the constructor name should be consistent with the contract name before the Solidity version 0.4.22 released.
Improper use of constructors is introduced in Solidity 0.4.22. The constructor adds a function definition incorrectly, which causes the constructor can be called by any user and lead to more serious harm, such as the Owner's permission being stolen.
- Case Error
contract own(){
function Own() {
owner = msg.sender;
}
}
Capitalized constructor name incorrectly causes the constructor name doesn't match the contract name. In this case, the function is set as a normal public function. Anyone can modify themselves to the owner of the contract by this function. That's will lead to other serious results.
On June 22, 2018, the MorphToken contract token announced the update of an smart new contract, which fixed the constructor problem on case errors.
On June 22, 2018, the Knownsec 404 Blockchain Security Research Team followed with the emergency and released the "Ethereum's smart contract constructor coding errors leading to an illegal contract ownership transfer report."
- Coding Error
function constructor() public {
owner = msg.sender;
}
Using function as a decorate word for constructor function is incorrectly in the above codes. In this case, the function is set as a normal public function. Anyone can modify themselves to the owner of the contract by this function. That's will lead to other serious consequences.
On July 14, 2018, Link Safe Technology released details in their public account about the constructor function's writing errors.
On July 15, 2018, the Knownsec 404 Blockchain Security Research Team followed with the emergency and released the "Ethereum's smart contract constructor writing errors leading to an illegal contract ownership transfer report."
3. The Scope within Vulnerability Impact
The smart contract audit function of the Haotian platform can scan for this type of the problem accurately.
We scanned a total of 39,548 contract codes for the entire network based on the Haotian platform's smart contract auditing rules. It included a total of 14,978 contracts involving such issues.
1) Unemitted Transfer Event
As of August 10, 2018, we found 4604 contract codes that didn't reach the standard of ERC20 and didn't emit the Transfer event. 10 contracts with the highest transaction volume are as follows:
2) Unemitted Approval Event
As of August 10, 2018, we found 4604 contract codes that didn't reach the standard of ERC20 and didn't emit the Approval event. 10 contracts with the highest transaction volume are as follows:
3) Fake Recharge Vulnerability
On July 9, 2018, we scanned contract codes for the entire network when the Knownsec 404 Blockchain Security Research Team followed and responded urgently with the Fake Recharge Vulnerability. At that time, over 3141 contract codes with fake recharge issue we were found. The 10 of them with the highest transaction volume are as follows:
As of August 10, 2018, over 5027 contract codes with fake recharge issue we were found. The 10 of them with the highest transaction volume are as follows:
4) Writing Error of Constructed Function
- Case Error
On June 22, 2018, the Knownsec 404 Blockchain Security Research Team followed and responded urgently with the Fake Recharge Vulnerability, there are about 16 contracts with this problem in the whole network.
As of August 10, 2018, 90 contract codes with case errors on the constructor we were found. The 10 of them with the highest transaction volume are as follows:
- Coding Error
As of August 10, 2018, 24 contract codess with writing errors on the constructor we found, only one more contract than the emergency response to this vulnerability on July 14, 2018. 10 contracts with the highest transaction volume are as follows:
4. Repair
1) Transfer function emit Transfer event
function transfer(address _to, uint256 _value) public returns (bool) {
require(_value <= balances[msg.sender]);
require(_to != address(0));
balances[msg.sender] = balances[msg.sender].sub(_value);
balances[_to] = balances[_to].add(_value);
emit Transfer(msg.sender, _to, _value);
return true;
}
2) Approve function emit Approve event
function approve(address _spender, uint256 _value) public returns (bool) {
allowed[msg.sender][_spender] = _value;
emit Approval(msg.sender, _spender, _value);
return true;
}
3) Transfer balance verification should use require to throw an error
function transfer(address _to, uint256 _value) public returns (bool) {
require(_value <= balances[msg.sender]);
require(_to != address(0));
balances[msg.sender] = balances[msg.sender].sub(_value);
balances[_to] = balances[_to].add(_value);
emit Transfer(msg.sender, _to, _value);
return true;
}
4) The constructor should be consistent with the contract name before the 0.4.22 version
contract ownable {
function ownable() public {
owner = msg.sender;
}
5) The constructor should not be decorated with function after the 0.4.22 version.
constructor() public {
owner = msg.sender;
}
5. Reflection
These questions are a type of problem that I often find in reviewing historical vulnerabilities. They are caused by developers not complying with the ERC20 standard. Because of these out-standard problems, many problems in the maintenance of contract tokens will appear later. Although these will not cause the emergence of contract vulnerabilities directly.
If not emit the corresponding events on the transfer and approve function, developers need more complicated ways to monitor contract transaction. There are not even enough logs to provide a rollback once a large-scale stolen time occurs.
The error that is not thrown at the time of transfer brings the fake recharge vulnerability to occur. The platform benefits will be compromised if it's judged by the transaction status when checks the transaction result.
If the developer does not pay attention to different versions of the compiler standard when constructing the function, it may cause contract ownership can be stolen easily, and further bring more serious problems such as the steal of currency.
It's easy to find that there is a large number of developers have not noticed these problems. When we scan and monitor the open contract codes of the whole network even low-level errors such as constructor writing errors are still happening after the vulnerability warning. Considering that most of the private contract codes, there may still many developers develop without complying on standards and there are many potential issues to consider.
In order to avoid unnecessary hassles and security issues, we recommend that all developers review their contract codes and check they comply with the REC20 contract standard whether or not.
6. Reference
[1] ERC standard
https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
[2] Morpheus official announcement
https://medium.com/@themorpheus/new-morpheus-network-token-smart-contract-91 b80dbc7655
[3] Constructor writing problem vulnerability details
https://mp.weixin.qq.com/s/xPwhanev-cjHhc104Wmpug
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