McAfeeDex smart contract security audit report performed by Callisto Security Department
Commit hash 5db8389ce23ce11fb273c9573772287aa75ce43b.
In total, 20 issues were reported including:
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1 high severity issue.
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3 medium severity issues.
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6 low severity issues.
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7 notes.
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3 owner privileges (the ability of an owner to manipulate contract).
No critical security issues were found.
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Double withdrawal attack is possible. More details here.
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Lack of transaction handling mechanism. WARNING! This is a very common issue, and it already caused millions of dollars losses for lots of token users! More details here.
Add the following code to the transfer(_to address, ...) function:
require( _to != address(this) );
From ERC-20 specification:
The function SHOULD throw if the _from account balance does not have enough tokens to spend.
In the implementation of McAfeeDEX a function returns false instead. This can lead to serious consequences for 3d party developers who work with this contract.
For example, an external contract may use this token contract as:
AdChainToken.transferFrom(recipient, this, value);
points[recipient] += value;
In this case, the recipient will get the increase of points, but the transfer of tokens will not happen. According to the definition of ERC20 standard the transaction must be interrupted by the call of throw at the token contract, however in case of the audited smart-contract the code will successfully complete the transaction.
According to the ERC20 standard, a transfer event must be generated when the token contract is initialized, if any token value is set to any given address.
Transfer & transferFrom functions do not prevent from sending tokens to address 0x0.
Add zero address checking.
require(to != address(0));Both transfer and transferFrom can not process transfers of 0 tokens.
The condition balances[_to] + _value > balances[_to] means that value cannot be equal to zero.
Please note that "Transfers of 0 values MUST be treated as normal transfers and fire the Transfer event" following ERC20 standard. This issue can create compatibility issues with contracts that rely on ERC20 standard definition.
Please refer to the rfc definition of "MUST" and "SHOULD" to correctly implement ERC20.
Warning! This problem will lead to huge financial losses for McAfeeDEX customers.
There is a critical flaw in the design of ERC20 token standard. ERC20 tokens lack event emitting/handling.
This makes it impossible for the recipient contract to reject incorrect transactions of ERC20 tokens. For the McAfeeDEX contract, it means that users can (and will) deposit their tokens via the transfer function, and the transaction will be completed successfully. Tokens will not be credited to the customer's account. These tokens will be trapped inside the contract forever without the possibility of their recovery.
Knowing this, it is necessary to implement a function that will allow to withdraw "trapped" tokens from the contract and send them back to users.
It is necessary to track the amount of tokens that were deposited properly. In this case, the developer can compare the amount of tokens that the McAfeeDEX contract owns to the amount of tokens that were recorded as "properly deposited". Then the developer can extract the excess of tokens to send them back to users manually.
contract SwitchDex is SafeMath {
mapping (address => uint256) public total_deposited;
...
function depositToken(address token, uint amount) {
//remember to call Token(address).approve(this, amount) or this contract will not be able to do the transfer on your behalf.
if (token==0) throw;
if (!Token(token).transferFrom(msg.sender, this, amount)) throw;
tokens[token][msg.sender] = safeAdd(tokens[token][msg.sender], amount);
Deposit(token, msg.sender, amount, tokens[token][msg.sender]);
// Record tokens as properly deposited
total_deposited[token] = safeAdd( total_deposited[token], amount );
}
function extractERC20(address _token) {
if (msg.sender != admin) throw;
uint256 actual_balance = StandardToken( _token ).balanceOf( this );
StandardToken( _token ).transfer( msg.sender, safeSub( actual_balance, total_deposited[_token] );
}
}You could also use an alternative smart-contract development platform that is not prone to these problems to help your users.
When testTrade is used the tokens[tokenGet][sender] is checked to be higher or equal to amount, which is wrong since taker fees are also substructed from msg.sender token balance or ether balance. This means that in order for the test to be more correct, it is necessary to calculate the fee, add it to the amount and check against tokens[tokenGet][sender].
This issue will lead to the fact that the transaction of a user who wants to exchange the entire balance of one asset for another will be thrown if the amount is equal to their total balance, since they will not have enough tokens to cover the exchange cost.
Please note that this issue is applicable for users with accountLevels set to false.
The testTrade function is used in the UI to prevent users from making incorrect trades. However, as explained above, this can allow users to accept the offer and fail to execute it, causing them to lose all transaction gas since revert is not used.
Incoming addresses should be checked for an empty value(0x0 address) to avoid loss of funds or to block some functionality.
Admin address reset is handled by changeAdmin function. Multiple issues can occur if the input address is wrong:
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If
admin_address is set to zero accidentally then it will lock out the contract administrator forever. It is recommended to add a zero address check before setting the address. -
Smart-contract developer can use two-step address verification to avoid setting the wrong address. This means that in the second step, the address that will be set as the administrator must call a function confirming that it was not an incorrect input.
Please note that setting a wrong admin address will result in disabling an important feature of McAfeeDex, which eliminates the taker fees for future whitelisted users.
A call method has no gas limit, and it is possible to make reentrancy from another contract. There is no danger in this implementation but this can pose a potential threat.
This is not a problem with this contract, but it can still lead to financial losses for its customers.
Ethereum addresses are not confirmed on-chain. This means that an abstract address is valid for any Ethereum sisterchain. For the McAfeeDEX contract this means that users can successfully send their funds into the address of the contract at any sisterchain (for example at ETC chain) and the transaction will succeed because there is no contract at ETC chain at the same address and the transaction will not be reverted.
While this is not an issue of this contract, I would recommend to deploy a dummy contract (with ERC20 extraction function) at the most popular Ethereum forks to save users from possible mistakes.
This is not just a theoretical issue! Here is the real example of fund loss.
I would recommend deploying a dummy contract at the same address at Ethereum fork-chains. The address of the contract is determined by the creator's address and the transaction nonce. So, a developer can use the same address that was used to create the contract at the Ethereum mainnet. It is necessary to increase the nonce at the forkchain to (deployment nonce - 1).
Then you need to deploy the dummy contract. This will cause the dummy contract to be deployed to the same address as the main McAfeeDEX contract.
It is advised to design contracts so that it is possible to migrate/upgrade them in the future.
In some specific circumstances, it may be worth to stop users from using the previous version of the contract if some issues were detected. I would recommend implementing a freeze function that can stop users from depositing their funds into the contract.
This is possible to implement it so that the function will be disabled after a certain period of time, for example, one year.
A throw was used multiple times inside the audited contract. Please note that it is deprecated in favor of revert, require, or assert since solidity 0.4.13 version.
Please note that, using require in some specific cases will allow returning the users' remaining gas after transaction execution. This will also leave a return message that provides readable reasons about the throw.
Refer to Solidity docs for more details.
Developers can use the latest solidity version for contract development.
https://github.com/RideSolo/MCAFEEDEXCONTRACT/blob/master/contract.sol#L21#L23
https://github.com/RideSolo/MCAFEEDEXCONTRACT/blob/master/contract.sol#L119
https://github.com/RideSolo/MCAFEEDEXCONTRACT/blob/master/contract.sol#L124#L125
https://github.com/RideSolo/MCAFEEDEXCONTRACT/blob/master/contract.sol#L177
https://github.com/RideSolo/MCAFEEDEXCONTRACT/blob/master/contract.sol#L181
feeRebateXfer is set to zero and should be removed from tradeBalances function since it adds extra complexity and gas usage.
AccountLevels and AccountLevelsTest contracts can be removed since only two-level user logic is implemented. It is possible to use accountLevels mapping instead.
The AccountLevelsTest contract member function setAccountLevel allow to set any level to any account without owner restriction.
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accountLevelsAddris not used anywhere atSwitchDexcontract. The functionality ofAccountLevelscontract is not used as well. -
feeRebatevariable is not used properly.
Contract owner allows himself to:
The audited smart contract must not be deployed. Reported issues must be fixed prior to the usage of this contract.
https://gist.github.com/danbogd/0d69629206d1a4473523edee33fd6cdf
https://gist.github.com/Dexaran/7c170cefa4f06f8dc5cb8c00c2a920ab
https://gist.github.com/gorbunovperm/d8bae42e359f2de5551f3bfc312c964e
https://gist.github.com/MrCrambo/f5b5c3f6fb934bd944202e917d7e5561
https://gist.github.com/RideSolo/4bd626b544ae8f38400046dee67716f6
5.1 Notes about danbogd report.
3.8. Bug - rounding error. the amount will have a bigger size with decimals as in your example. Therefore no problem in the calculation. Not a security issue.
3.9. Truncated division the amount is in amountGet terms. Therefore it will be truncated the only amount that is less than the lowest decimal. Not a security issue.
5.2 Notes about Dexaran report.
3.1 Lack of ERC20 extraction functions. McAfeeDex has a user interface and does not require direct blockchain transaction with the contract. Also, the McAfeeDex contract address is not published on the mcafeedex.com website. If a user intentionally commits an incorrect transaction that is not provided for by the user interface, then this is not a smart contract error. This issue can be Medium severity in maximum. This is a variation of "Lack of transaction handling mechanism issue." that is low severity.
5.3 Notes about MrCrambo report.
3. Anyone can set account level pointed function is a member of AccountLevelsTest contract, which is unused. This is Note severity.
Example 1: lost tokens in contract
Example 2: lost tokens in contract
Example 3: lost tokens in contract
Example 4: lost tokens in contract
Example 5: lost tokens in contract
Example 6: lost tokens in contract
Example 7: lost tokens in contract
Example 8: lost tokens in contract
Example 9: lost tokens in contract
Example 10: ENS smart-contract caused financial losses for token users
Example 10 is of decisive importance here. ENS smart-contract has UI also. However it did not prevent the described issue. I can conclude that we can not assume that in case of McAfeeDEX it is something different and UI will solve the problem that it could not solve in the above mentioned situations.
What if user has committed this action unintentionally?
Users tend to make mistakes and we must keep this in mind when designing a fault-tolerant financial system. Our goal as a security auditor is to help the project solve these problems or prevent potential problems in advance.
User error is not a smart contract error, and I recommend that you read my description of the problem again. I never said that a user error was caused by the smart contract. I stated that the inability to reject a transaction in the event of a user error is a flaw of the smart contract that will have disastrous consequences. Thats why this must be solved prior to using this contract.
"Lack of transaction handling mechanism issue" is a problem of ERC20 token but not of this contract. The described problem is low severity for the ERC20 contract because it is mostly a problem of SERVICES THAT WILL WORK WITH TOKEN but not of the contract itself.
McAfeeDEX however is exactly the SERVICE THAT WILL WORK WITH TOKENS - thats why it is so important for the exchange smart-contract. This issue can occur at any time and it can be caused by anyone and it may have huge impact. We have real example of how this caused losses of more than $3,200,000 for smart-contract projects. Thats why the issue was assigned High severity.