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Detector Documentation
List of public detectors
- Check:
abiencoderv2-array - Severity:
High - Confidence:
High
solc versions 0.4.7-0.5.9 contain a compiler bug leading to incorrect ABI encoder usage.
contract A {
uint[2][3] bad_arr = [[1, 2], [3, 4], [5, 6]];
/* Array of arrays passed to abi.encode is vulnerable */
function bad() public {
bytes memory b = abi.encode(bad_arr);
}
}abi.encode(bad_arr) in a call to bad() will incorrectly encode the array as [[1, 2], [2, 3], [3, 4]] and lead to unintended behavior.
Use a compiler >= 0.5.10.
- Check:
arbitrary-send-erc20 - Severity:
High - Confidence:
High
Detect when msg.sender is not used as from in transferFrom.
function a(address from, address to, uint256 amount) public {
erc20.transferFrom(from, to, am);
}Alice approves this contract to spend her ERC20 tokens. Bob can call a and specify Alice's address as the from parameter in transferFrom, allowing him to transfer Alice's tokens to himself.
Use msg.sender as from in transferFrom.
- Check:
array-by-reference - Severity:
High - Confidence:
High
Detect arrays passed to a function that expects reference to a storage array
contract Memory {
uint[1] public x; // storage
function f() public {
f1(x); // update x
f2(x); // do not update x
}
function f1(uint[1] storage arr) internal { // by reference
arr[0] = 1;
}
function f2(uint[1] arr) internal { // by value
arr[0] = 2;
}
}Bob calls f(). Bob assumes that at the end of the call x[0] is 2, but it is 1.
As a result, Bob's usage of the contract is incorrect.
Ensure the correct usage of memory and storage in the function parameters. Make all the locations explicit.
- Check:
encode-packed-collision - Severity:
High - Confidence:
High
Detect collision due to dynamic type usages in abi.encodePacked
contract Sign {
function get_hash_for_signature(string name, string doc) external returns(bytes32) {
return keccak256(abi.encodePacked(name, doc));
}
}Bob calls get_hash_for_signature with (bob, This is the content). The hash returned is used as an ID.
Eve creates a collision with the ID using (bo, bThis is the content) and compromises the system.
Do not use more than one dynamic type in abi.encodePacked()
(see the Solidity documentation).
Use abi.encode(), preferably.
- Check:
incorrect-shift - Severity:
High - Confidence:
High
Detect if the values in a shift operation are reversed
contract C {
function f() internal returns (uint a) {
assembly {
a := shr(a, 8)
}
}
}The shift statement will right-shift the constant 8 by a bits
Swap the order of parameters.
- Check:
multiple-constructors - Severity:
High - Confidence:
High
Detect multiple constructor definitions in the same contract (using new and old schemes).
contract A {
uint x;
constructor() public {
x = 0;
}
function A() public {
x = 1;
}
function test() public returns(uint) {
return x;
}
}In Solidity 0.4.22, a contract with both constructor schemes will compile. The first constructor will take precedence over the second, which may be unintended.
Only declare one constructor, preferably using the new scheme constructor(...) instead of function <contractName>(...).
- Check:
name-reused - Severity:
High - Confidence:
High
If a codebase has two contracts the similar names, the compilation artifacts will not contain one of the contracts with the duplicate name.
Bob's truffle codebase has two contracts named ERC20.
When truffle compile runs, only one of the two contracts will generate artifacts in build/contracts.
As a result, the second contract cannot be analyzed.
Rename the contract.
- Check:
protected-vars - Severity:
High - Confidence:
High
Detect unprotected variable that are marked protected
contract Buggy{
/// @custom:security write-protection="onlyOwner()"
address owner;
function set_protected() public onlyOwner(){
owner = msg.sender;
}
function set_not_protected() public{
owner = msg.sender;
}
} owner must be always written by function using onlyOwner (write-protection="onlyOwner()"), however anyone can call set_not_protected.
Add access controls to the vulnerable function
- Check:
public-mappings-nested - Severity:
High - Confidence:
High
Prior to Solidity 0.5, a public mapping with nested structures returned incorrect values.
Bob interacts with a contract that has a public mapping with nested structures. The values returned by the mapping are incorrect, breaking Bob's usage
Do not use public mapping with nested structures.
- Check:
rtlo - Severity:
High - Confidence:
High
An attacker can manipulate the logic of the contract by using a right-to-left-override character (U+202E).
contract Token
{
address payable o; // owner
mapping(address => uint) tokens;
function withdraw() external returns(uint)
{
uint amount = tokens[msg.sender];
address payable d = msg.sender;
tokens[msg.sender] = 0;
_withdraw(/*owner/*noitanitsed*/ d, o/*
/*value */, amount);
}
function _withdraw(address payable fee_receiver, address payable destination, uint value) internal
{
fee_receiver.transfer(1);
destination.transfer(value);
}
}Token uses the right-to-left-override character when calling _withdraw. As a result, the fee is incorrectly sent to msg.sender, and the token balance is sent to the owner.
Special control characters must not be allowed.
- Check:
shadowing-state - Severity:
High - Confidence:
High
Detection of state variables shadowed.
contract BaseContract{
address owner;
modifier isOwner(){
require(owner == msg.sender);
_;
}
}
contract DerivedContract is BaseContract{
address owner;
constructor(){
owner = msg.sender;
}
function withdraw() isOwner() external{
msg.sender.transfer(this.balance);
}
}owner of BaseContract is never assigned and the modifier isOwner does not work.
Remove the state variable shadowing.
- Check:
suicidal - Severity:
High - Confidence:
High
Unprotected call to a function executing selfdestruct/suicide.
contract Suicidal{
function kill() public{
selfdestruct(msg.sender);
}
}Bob calls kill and destructs the contract.
Protect access to all sensitive functions.
- Check:
uninitialized-state - Severity:
High - Confidence:
High
Uninitialized state variables.
contract Uninitialized{
address destination;
function transfer() payable public{
destination.transfer(msg.value);
}
}Bob calls transfer. As a result, the Ether are sent to the address 0x0 and are lost.
Initialize all the variables. If a variable is meant to be initialized to zero, explicitly set it to zero to improve code readability.
- Check:
uninitialized-storage - Severity:
High - Confidence:
High
An uninitialized storage variable will act as a reference to the first state variable, and can override a critical variable.
contract Uninitialized{
address owner = msg.sender;
struct St{
uint a;
}
function func() {
St st;
st.a = 0x0;
}
}Bob calls func. As a result, owner is overridden to 0.
Initialize all storage variables.
- Check:
unprotected-upgrade - Severity:
High - Confidence:
High
Detects logic contract that can be destructed.
contract Buggy is Initializable{
address payable owner;
function initialize() external initializer{
require(owner == address(0));
owner = msg.sender;
}
function kill() external{
require(msg.sender == owner);
selfdestruct(owner);
}
}Buggy is an upgradeable contract. Anyone can call initialize on the logic contract, and destruct the contract.
Add a constructor to ensure initialize cannot be called on the logic contract.
- Check:
arbitrary-send-erc20-permit - Severity:
High - Confidence:
Medium
Detect when msg.sender is not used as from in transferFrom and permit is used.
function bad(address from, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s, address to) public {
erc20.permit(from, address(this), value, deadline, v, r, s);
erc20.transferFrom(from, to, value);
}If an ERC20 token does not implement permit and has a fallback function e.g. WETH, transferFrom allows an attacker to transfer all tokens approved for this contract.
Ensure that the underlying ERC20 token correctly implements a permit function.
- Check:
arbitrary-send-eth - Severity:
High - Confidence:
Medium
Unprotected call to a function sending Ether to an arbitrary address.
contract ArbitrarySendEth{
address destination;
function setDestination(){
destination = msg.sender;
}
function withdraw() public{
destination.transfer(this.balance);
}
}Bob calls setDestination and withdraw. As a result he withdraws the contract's balance.
Ensure that an arbitrary user cannot withdraw unauthorized funds.
- Check:
controlled-array-length - Severity:
High - Confidence:
Medium
Detects the direct assignment of an array's length.
contract A {
uint[] testArray; // dynamic size array
function f(uint usersCount) public {
// ...
testArray.length = usersCount;
// ...
}
function g(uint userIndex, uint val) public {
// ...
testArray[userIndex] = val;
// ...
}
}Contract storage/state-variables are indexed by a 256-bit integer.
The user can set the array length to 2**256-1 in order to index all storage slots.
In the example above, one could call the function f to set the array length, then call the function g to control any storage slot desired.
Note that storage slots here are indexed via a hash of the indexers; nonetheless, all storage will still be accessible and could be controlled by the attacker.
Do not allow array lengths to be set directly set; instead, opt to add values as needed. Otherwise, thoroughly review the contract to ensure a user-controlled variable cannot reach an array length assignment.
- Check:
controlled-delegatecall - Severity:
High - Confidence:
Medium
Delegatecall or callcode to an address controlled by the user.
contract Delegatecall{
function delegate(address to, bytes data){
to.delegatecall(data);
}
}Bob calls delegate and delegates the execution to his malicious contract. As a result, Bob withdraws the funds of the contract and destructs it.
Avoid using delegatecall. Use only trusted destinations.
- Check:
delegatecall-loop - Severity:
High - Confidence:
Medium
Detect the use of delegatecall inside a loop in a payable function.
contract DelegatecallInLoop{
mapping (address => uint256) balances;
function bad(address[] memory receivers) public payable {
for (uint256 i = 0; i < receivers.length; i++) {
address(this).delegatecall(abi.encodeWithSignature("addBalance(address)", receivers[i]));
}
}
function addBalance(address a) public payable {
balances[a] += msg.value;
}
}When calling bad the same msg.value amount will be accredited multiple times.
Carefully check that the function called by delegatecall is not payable/doesn't use msg.value.
- Check:
incorrect-exp - Severity:
High - Confidence:
Medium
Detect use of bitwise xor ^ instead of exponential **
contract Bug{
uint UINT_MAX = 2^256 - 1;
...
}Alice deploys a contract in which UINT_MAX incorrectly uses ^ operator instead of ** for exponentiation
Use the correct operator ** for exponentiation.
- Check:
incorrect-return - Severity:
High - Confidence:
Medium
Detect if return in an assembly block halts unexpectedly the execution.
contract C {
function f() internal returns (uint a, uint b) {
assembly {
return (5, 6)
}
}
function g() returns (bool){
f();
return true;
}
}The return statement in f will cause execution in g to halt.
The function will return 6 bytes starting from offset 5, instead of returning a boolean.
Use the leave statement.
- Check:
msg-value-loop - Severity:
High - Confidence:
Medium
Detect the use of msg.value inside a loop.
contract MsgValueInLoop{
mapping (address => uint256) balances;
function bad(address[] memory receivers) public payable {
for (uint256 i=0; i < receivers.length; i++) {
balances[receivers[i]] += msg.value;
}
}
}Provide an explicit array of amounts alongside the receivers array, and check that the sum of all amounts matches msg.value.
- Check:
reentrancy-eth - Severity:
High - Confidence:
Medium
Detection of the reentrancy bug.
Do not report reentrancies that don't involve Ether (see reentrancy-no-eth)
function withdrawBalance(){
// send userBalance[msg.sender] Ether to msg.sender
// if msg.sender is a contract, it will call its fallback function
if( ! (msg.sender.call.value(userBalance[msg.sender])() ) ){
throw;
}
userBalance[msg.sender] = 0;
}Bob uses the re-entrancy bug to call withdrawBalance two times, and withdraw more than its initial deposit to the contract.
Apply the check-effects-interactions pattern.
- Check:
return-leave - Severity:
High - Confidence:
Medium
Detect if a return is used where a leave should be used.
contract C {
function f() internal returns (uint a, uint b) {
assembly {
return (5, 6)
}
}
}The function will halt the execution, instead of returning a two uint.
Use the leave statement.
- Check:
storage-array - Severity:
High - Confidence:
Medium
solc versions 0.4.7-0.5.9 contain a compiler bug
leading to incorrect values in signed integer arrays.
contract A {
int[3] ether_balances; // storage signed integer array
function bad0() private {
// ...
ether_balances = [-1, -1, -1];
// ...
}
}bad0() uses a (storage-allocated) signed integer array state variable to store the ether balances of three accounts.
-1 is supposed to indicate uninitialized values but the Solidity bug makes these as 1, which could be exploited by the accounts.
Use a compiler version >= 0.5.10.
- Check:
unchecked-transfer - Severity:
High - Confidence:
Medium
The return value of an external transfer/transferFrom call is not checked
contract Token {
function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);
}
contract MyBank{
mapping(address => uint) balances;
Token token;
function deposit(uint amount) public{
token.transferFrom(msg.sender, address(this), amount);
balances[msg.sender] += amount;
}
}Several tokens do not revert in case of failure and return false. If one of these tokens is used in MyBank, deposit will not revert if the transfer fails, and an attacker can call deposit for free..
Use SafeERC20, or ensure that the transfer/transferFrom return value is checked.
- Check:
weak-prng - Severity:
High - Confidence:
Medium
Weak PRNG due to a modulo on block.timestamp, now or blockhash. These can be influenced by miners to some extent so they should be avoided.
contract Game {
uint reward_determining_number;
function guessing() external{
reward_determining_number = uint256(block.blockhash(10000)) % 10;
}
}Eve is a miner. Eve calls guessing and re-orders the block containing the transaction.
As a result, Eve wins the game.
Do not use block.timestamp, now or blockhash as a source of randomness
- Check:
codex - Severity:
High - Confidence:
Low
Use codex to find vulnerabilities
N/A
Review codex's message.
- Check:
domain-separator-collision - Severity:
Medium - Confidence:
High
An ERC20 token has a function whose signature collides with EIP-2612's DOMAIN_SEPARATOR(), causing unanticipated behavior for contracts using permit functionality.
contract Contract{
function some_collisions() external() {}
}some_collision clashes with EIP-2612's DOMAIN_SEPARATOR() and will interfere with contract's using permit.
Remove or rename the function that collides with DOMAIN_SEPARATOR().
- Check:
enum-conversion - Severity:
Medium - Confidence:
High
Detect out-of-range enum conversion (solc < 0.4.5).
pragma solidity 0.4.2;
contract Test{
enum E{a}
function bug(uint a) public returns(E){
return E(a);
}
}Attackers can trigger unexpected behaviour by calling bug(1).
Use a recent compiler version. If solc <0.4.5 is required, check the enum conversion range.
- Check:
erc20-interface - Severity:
Medium - Confidence:
High
Incorrect return values for ERC20 functions. A contract compiled with Solidity > 0.4.22 interacting with these functions will fail to execute them, as the return value is missing.
contract Token{
function transfer(address to, uint value) external;
//...
}Token.transfer does not return a boolean. Bob deploys the token. Alice creates a contract that interacts with it but assumes a correct ERC20 interface implementation. Alice's contract is unable to interact with Bob's contract.
Set the appropriate return values and types for the defined ERC20 functions.
- Check:
erc721-interface - Severity:
Medium - Confidence:
High
Incorrect return values for ERC721 functions. A contract compiled with solidity > 0.4.22 interacting with these functions will fail to execute them, as the return value is missing.
contract Token{
function ownerOf(uint256 _tokenId) external view returns (bool);
//...
}Token.ownerOf does not return an address like ERC721 expects. Bob deploys the token. Alice creates a contract that interacts with it but assumes a correct ERC721 interface implementation. Alice's contract is unable to interact with Bob's contract.
Set the appropriate return values and vtypes for the defined ERC721 functions.
- Check:
incorrect-equality - Severity:
Medium - Confidence:
High
Use of strict equalities that can be easily manipulated by an attacker.
contract Crowdsale{
function fund_reached() public returns(bool){
return this.balance == 100 ether;
}Crowdsale relies on fund_reached to know when to stop the sale of tokens.
Crowdsale reaches 100 Ether. Bob sends 0.1 Ether. As a result, fund_reached is always false and the crowdsale never ends.
Don't use strict equality to determine if an account has enough Ether or tokens.
- Check:
locked-ether - Severity:
Medium - Confidence:
High
Contract with a payable function, but without a withdrawal capacity.
pragma solidity 0.4.24;
contract Locked{
function receive() payable public{
}
}Every Ether sent to Locked will be lost.
Remove the payable attribute or add a withdraw function.
- Check:
mapping-deletion - Severity:
Medium - Confidence:
High
A deletion in a structure containing a mapping will not delete the mapping (see the Solidity documentation). The remaining data may be used to compromise the contract.
struct BalancesStruct{
address owner;
mapping(address => uint) balances;
}
mapping(address => BalancesStruct) public stackBalance;
function remove() internal{
delete stackBalance[msg.sender];
}remove deletes an item of stackBalance.
The mapping balances is never deleted, so remove does not work as intended.
Use a lock mechanism instead of a deletion to disable structure containing a mapping.
- Check:
shadowing-abstract - Severity:
Medium - Confidence:
High
Detection of state variables shadowed from abstract contracts.
contract BaseContract{
address owner;
}
contract DerivedContract is BaseContract{
address owner;
}owner of BaseContract is shadowed in DerivedContract.
Remove the state variable shadowing.
- Check:
tautological-compare - Severity:
Medium - Confidence:
High
A variable compared to itself is probably an error as it will always return true for ==, >=, <= and always false for <, > and !=.
function check(uint a) external returns(bool){
return (a >= a);
}check always return true.
Remove comparison or compare to different value.
- Check:
tautology - Severity:
Medium - Confidence:
High
Detects expressions that are tautologies or contradictions.
contract A {
function f(uint x) public {
// ...
if (x >= 0) { // bad -- always true
// ...
}
// ...
}
function g(uint8 y) public returns (bool) {
// ...
return (y < 512); // bad!
// ...
}
}x is a uint256, so x >= 0 will be always true.
y is a uint8, so y <512 will be always true.
Fix the incorrect comparison by changing the value type or the comparison.
- Check:
write-after-write - Severity:
Medium - Confidence:
High
Detects variables that are written but never read and written again.
```solidity
contract Buggy{
function my_func() external initializer{
// ...
a = b;
a = c;
// ..
}
}
```
`a` is first asigned to `b`, and then to `c`. As a result the first write does nothing.
Fix or remove the writes.
- Check:
boolean-cst - Severity:
Medium - Confidence:
Medium
Detects the misuse of a Boolean constant.
contract A {
function f(uint x) public {
// ...
if (false) { // bad!
// ...
}
// ...
}
function g(bool b) public returns (bool) {
// ...
return (b || true); // bad!
// ...
}
}Boolean constants in code have only a few legitimate uses. Other uses (in complex expressions, as conditionals) indicate either an error or, most likely, the persistence of faulty code.
Verify and simplify the condition.
- Check:
constant-function-asm - Severity:
Medium - Confidence:
Medium
Functions declared as constant/pure/view using assembly code.
constant/pure/view was not enforced prior to Solidity 0.5.
Starting from Solidity 0.5, a call to a constant/pure/view function uses the STATICCALL opcode, which reverts in case of state modification.
As a result, a call to an incorrectly labeled function may trap a contract compiled with Solidity 0.5.
contract Constant{
uint counter;
function get() public view returns(uint){
counter = counter +1;
return counter
}
}Constant was deployed with Solidity 0.4.25. Bob writes a smart contract that interacts with Constant in Solidity 0.5.0.
All the calls to get revert, breaking Bob's smart contract execution.
Ensure the attributes of contracts compiled prior to Solidity 0.5.0 are correct.
- Check:
constant-function-state - Severity:
Medium - Confidence:
Medium
Functions declared as constant/pure/view change the state.
constant/pure/view was not enforced prior to Solidity 0.5.
Starting from Solidity 0.5, a call to a constant/pure/view function uses the STATICCALL opcode, which reverts in case of state modification.
As a result, a call to an incorrectly labeled function may trap a contract compiled with Solidity 0.5.
contract Constant{
uint counter;
function get() public view returns(uint){
counter = counter +1;
return counter
}
}Constant was deployed with Solidity 0.4.25. Bob writes a smart contract that interacts with Constant in Solidity 0.5.0.
All the calls to get revert, breaking Bob's smart contract execution.
Ensure that attributes of contracts compiled prior to Solidity 0.5.0 are correct.
- Check:
divide-before-multiply - Severity:
Medium - Confidence:
Medium
Solidity's integer division truncates. Thus, performing division before multiplication can lead to precision loss.
contract A {
function f(uint n) public {
coins = (oldSupply / n) * interest;
}
}If n is greater than oldSupply, coins will be zero. For example, with oldSupply = 5; n = 10, interest = 2, coins will be zero.
If (oldSupply * interest / n) was used, coins would have been 1.
In general, it's usually a good idea to re-arrange arithmetic to perform multiplication before division, unless the limit of a smaller type makes this dangerous.
Consider ordering multiplication before division.
- Check:
out-of-order-retryable - Severity:
Medium - Confidence:
Medium
Out-of-order retryable transactions
contract L1 {
function doStuffOnL2() external {
// Retryable A
IInbox(inbox).createRetryableTicket({
to: l2contract,
l2CallValue: 0,
maxSubmissionCost: maxSubmissionCost,
excessFeeRefundAddress: msg.sender,
callValueRefundAddress: msg.sender,
gasLimit: gasLimit,
maxFeePerGas: maxFeePerGas,
data: abi.encodeCall(l2contract.claim_rewards, ())
});
// Retryable B
IInbox(inbox).createRetryableTicket({
to: l2contract,
l2CallValue: 0,
maxSubmissionCost: maxSubmissionCost,
excessFeeRefundAddress: msg.sender,
callValueRefundAddress: msg.sender,
gasLimit: gas,
maxFeePerGas: maxFeePerGas,
data: abi.encodeCall(l2contract.unstake, ())
});
}
}
contract L2 {
function claim_rewards() public {
// rewards is computed based on balance and staking period
uint unclaimed_rewards = _compute_and_update_rewards();
token.safeTransfer(msg.sender, unclaimed_rewards);
}
// Call claim_rewards before unstaking, otherwise you lose your rewards
function unstake() public {
_free_rewards(); // clean up rewards related variables
balance = balance[msg.sender];
balance[msg.sender] = 0;
staked_token.safeTransfer(msg.sender, balance);
}
}Bob calls doStuffOnL2 but the first retryable ticket calling claim_rewards fails. The second retryable ticket calling unstake is executed successfully. As a result, Bob loses his rewards.
Do not rely on the order or successful execution of retryable tickets.
- Check:
reentrancy-no-eth - Severity:
Medium - Confidence:
Medium
Detection of the reentrancy bug.
Do not report reentrancies that involve Ether (see reentrancy-eth).
function bug(){
require(not_called);
if( ! (msg.sender.call() ) ){
throw;
}
not_called = False;
} Apply the check-effects-interactions pattern.
- Check:
reused-constructor - Severity:
Medium - Confidence:
Medium
Detects if the same base constructor is called with arguments from two different locations in the same inheritance hierarchy.
pragma solidity ^0.4.0;
contract A{
uint num = 5;
constructor(uint x) public{
num += x;
}
}
contract B is A{
constructor() A(2) public { /* ... */ }
}
contract C is A {
constructor() A(3) public { /* ... */ }
}
contract D is B, C {
constructor() public { /* ... */ }
}
contract E is B {
constructor() A(1) public { /* ... */ }
}The constructor of A is called multiple times in D and E:
-
Dinherits fromBandC, both of which constructA. -
Eonly inherits fromB, butBandEconstructA. .
Remove the duplicate constructor call.
- Check:
tx-origin - Severity:
Medium - Confidence:
Medium
tx.origin-based protection can be abused by a malicious contract if a legitimate user interacts with the malicious contract.
contract TxOrigin {
address owner = msg.sender;
function bug() {
require(tx.origin == owner);
}Bob is the owner of TxOrigin. Bob calls Eve's contract. Eve's contract calls TxOrigin and bypasses the tx.origin protection.
Do not use tx.origin for authorization.
- Check:
unchecked-lowlevel - Severity:
Medium - Confidence:
Medium
The return value of a low-level call is not checked.
contract MyConc{
function my_func(address payable dst) public payable{
dst.call.value(msg.value)("");
}
}The return value of the low-level call is not checked, so if the call fails, the Ether will be locked in the contract. If the low level is used to prevent blocking operations, consider logging failed calls.
Ensure that the return value of a low-level call is checked or logged.
- Check:
unchecked-send - Severity:
Medium - Confidence:
Medium
The return value of a send is not checked.
contract MyConc{
function my_func(address payable dst) public payable{
dst.send(msg.value);
}
}The return value of send is not checked, so if the send fails, the Ether will be locked in the contract.
If send is used to prevent blocking operations, consider logging the failed send.
Ensure that the return value of send is checked or logged.
- Check:
uninitialized-local - Severity:
Medium - Confidence:
Medium
Uninitialized local variables.
contract Uninitialized is Owner{
function withdraw() payable public onlyOwner{
address to;
to.transfer(this.balance)
}
}Bob calls transfer. As a result, all Ether is sent to the address 0x0 and is lost.
Initialize all the variables. If a variable is meant to be initialized to zero, explicitly set it to zero to improve code readability.
- Check:
unused-return - Severity:
Medium - Confidence:
Medium
The return value of an external call is not stored in a local or state variable.
contract MyConc{
using SafeMath for uint;
function my_func(uint a, uint b) public{
a.add(b);
}
}MyConc calls add of SafeMath, but does not store the result in a. As a result, the computation has no effect.
Ensure that all the return values of the function calls are used.
- Check:
incorrect-modifier - Severity:
Low - Confidence:
High
If a modifier does not execute _ or revert, the execution of the function will return the default value, which can be misleading for the caller.
modidfier myModif(){
if(..){
_;
}
}
function get() myModif returns(uint){
}If the condition in myModif is false, the execution of get() will return 0.
All the paths in a modifier must execute _ or revert.
- Check:
shadowing-builtin - Severity:
Low - Confidence:
High
Detection of shadowing built-in symbols using local variables, state variables, functions, modifiers, or events.
pragma solidity ^0.4.24;
contract Bug {
uint now; // Overshadows current time stamp.
function assert(bool condition) public {
// Overshadows built-in symbol for providing assertions.
}
function get_next_expiration(uint earlier_time) private returns (uint) {
return now + 259200; // References overshadowed timestamp.
}
}now is defined as a state variable, and shadows with the built-in symbol now. The function assert overshadows the built-in assert function. Any use of either of these built-in symbols may lead to unexpected results.
Rename the local variables, state variables, functions, modifiers, and events that shadow a builtin symbol.
- Check:
shadowing-local - Severity:
Low - Confidence:
High
Detection of shadowing using local variables.
pragma solidity ^0.4.24;
contract Bug {
uint owner;
function sensitive_function(address owner) public {
// ...
require(owner == msg.sender);
}
function alternate_sensitive_function() public {
address owner = msg.sender;
// ...
require(owner == msg.sender);
}
}sensitive_function.owner shadows Bug.owner. As a result, the use of owner in sensitive_function might be incorrect.
Rename the local variables that shadow another component.
- Check:
uninitialized-fptr-cst - Severity:
Low - Confidence:
High
solc versions 0.4.5-0.4.26 and 0.5.0-0.5.8 contain a compiler bug leading to unexpected behavior when calling uninitialized function pointers in constructors.
contract bad0 {
constructor() public {
/* Uninitialized function pointer */
function(uint256) internal returns(uint256) a;
a(10);
}
}The call to a(10) will lead to unexpected behavior because function pointer a is not initialized in the constructor.
Initialize function pointers before calling. Avoid function pointers if possible.
- Check:
variable-scope - Severity:
Low - Confidence:
High
Detects the possible usage of a variable before the declaration is stepped over (either because it is later declared, or declared in another scope).
contract C {
function f(uint z) public returns (uint) {
uint y = x + 9 + z; // 'x' is used pre-declaration
uint x = 7;
if (z % 2 == 0) {
uint max = 5;
// ...
}
// 'max' was intended to be 5, but it was mistakenly declared in a scope and not assigned (so it is zero).
for (uint i = 0; i < max; i++) {
x += 1;
}
return x;
}
}In the case above, the variable x is used before its declaration, which may result in unintended consequences.
Additionally, the for-loop uses the variable max, which is declared in a previous scope that may not always be reached. This could lead to unintended consequences if the user mistakenly uses a variable prior to any intended declaration assignment. It also may indicate that the user intended to reference a different variable.
Move all variable declarations prior to any usage of the variable, and ensure that reaching a variable declaration does not depend on some conditional if it is used unconditionally.
- Check:
void-cst - Severity:
Low - Confidence:
High
Detect the call to a constructor that is not implemented
contract A{}
contract B is A{
constructor() public A(){}
}When reading B's constructor definition, we might assume that A() initiates the contract, but no code is executed.
Remove the constructor call.
- Check:
calls-loop - Severity:
Low - Confidence:
Medium
Calls inside a loop might lead to a denial-of-service attack.
contract CallsInLoop{
address[] destinations;
constructor(address[] newDestinations) public{
destinations = newDestinations;
}
function bad() external{
for (uint i=0; i < destinations.length; i++){
destinations[i].transfer(i);
}
}
}If one of the destinations has a fallback function that reverts, bad will always revert.
Favor pull over push strategy for external calls.
- Check:
events-access - Severity:
Low - Confidence:
Medium
Detect missing events for critical access control parameters
contract C {
modifier onlyAdmin {
if (msg.sender != owner) throw;
_;
}
function updateOwner(address newOwner) onlyAdmin external {
owner = newOwner;
}
}updateOwner() has no event, so it is difficult to track off-chain owner changes.
Emit an event for critical parameter changes.
- Check:
events-maths - Severity:
Low - Confidence:
Medium
Detect missing events for critical arithmetic parameters.
contract C {
modifier onlyOwner {
if (msg.sender != owner) throw;
_;
}
function setBuyPrice(uint256 newBuyPrice) onlyOwner public {
buyPrice = newBuyPrice;
}
function buy() external {
... // buyPrice is used to determine the number of tokens purchased
}
}setBuyPrice() does not emit an event, so it is difficult to track changes in the value of buyPrice off-chain.
Emit an event for critical parameter changes.
- Check:
incorrect-unary - Severity:
Low - Confidence:
Medium
Unary expressions such as x=+1 probably typos.
contract Bug{
uint public counter;
function increase() public returns(uint){
counter=+1;
return counter;
}
}increase() uses =+ instead of +=, so counter will never exceed 1.
Remove the unary expression.
- Check:
missing-zero-check - Severity:
Low - Confidence:
Medium
Detect missing zero address validation.
contract C {
modifier onlyAdmin {
if (msg.sender != owner) throw;
_;
}
function updateOwner(address newOwner) onlyAdmin external {
owner = newOwner;
}
}Bob calls updateOwner without specifying the newOwner, so Bob loses ownership of the contract.
Check that the address is not zero.
- Check:
reentrancy-benign - Severity:
Low - Confidence:
Medium
Detection of the reentrancy bug.
Only report reentrancy that acts as a double call (see reentrancy-eth, reentrancy-no-eth).
function callme(){
if( ! (msg.sender.call()() ) ){
throw;
}
counter += 1
} callme contains a reentrancy. The reentrancy is benign because it's exploitation would have the same effect as two consecutive calls.
Apply the check-effects-interactions pattern.
- Check:
reentrancy-events - Severity:
Low - Confidence:
Medium
Detects reentrancies that allow manipulation of the order or value of events.
contract ReentrantContract {
function f() external {
if (BugReentrancyEvents(msg.sender).counter() == 1) {
BugReentrancyEvents(msg.sender).count(this);
}
}
}
contract Counter {
uint public counter;
event Counter(uint);
}
contract BugReentrancyEvents is Counter {
function count(ReentrantContract d) external {
counter += 1;
d.f();
emit Counter(counter);
}
}
contract NoReentrancyEvents is Counter {
function count(ReentrantContract d) external {
counter += 1;
emit Counter(counter);
d.f();
}
}If the external call d.f() re-enters BugReentrancyEvents, the Counter events will be incorrect (Counter(2), Counter(2)) whereas NoReentrancyEvents will correctly emit
(Counter(1), Counter(2)). This may cause issues for offchain components that rely on the values of events e.g. checking for the amount deposited to a bridge.
Apply the check-effects-interactions pattern.
- Check:
return-bomb - Severity:
Low - Confidence:
Medium
A low level callee may consume all callers gas unexpectedly.
//Modified from https://github.com/nomad-xyz/ExcessivelySafeCall
contract BadGuy {
function youveActivateMyTrapCard() external pure returns (bytes memory) {
assembly{
revert(0, 1000000)
}
}
}
contract Mark {
function oops(address badGuy) public{
bool success;
bytes memory ret;
// Mark pays a lot of gas for this copy
//(success, ret) = badGuy.call{gas:10000}(
(success, ret) = badGuy.call(
abi.encodeWithSelector(
BadGuy.youveActivateMyTrapCard.selector
)
);
// Mark may OOG here, preventing local state changes
//importantCleanup();
}
}
After Mark calls BadGuy bytes are copied from returndata to memory, the memory expansion cost is paid. This means that when using a standard solidity call, the callee can "returnbomb" the caller, imposing an arbitrary gas cost. Callee unexpectedly makes the caller OOG.
Avoid unlimited implicit decoding of returndata.
- Check:
timestamp - Severity:
Low - Confidence:
Medium
Dangerous usage of block.timestamp. block.timestamp can be manipulated by miners.
"Bob's contract relies on block.timestamp for its randomness. Eve is a miner and manipulates block.timestamp to exploit Bob's contract.
Avoid relying on block.timestamp.
- Check:
assembly - Severity:
Informational - Confidence:
High
The use of assembly is error-prone and should be avoided.
Do not use evm assembly.
- Check:
assert-state-change - Severity:
Informational - Confidence:
High
Incorrect use of assert(). See Solidity best practices.
contract A {
uint s_a;
function bad() public {
assert((s_a += 1) > 10);
}
}The assert in bad() increments the state variable s_a while checking for the condition.
Use require for invariants modifying the state.
- Check:
boolean-equal - Severity:
Informational - Confidence:
High
Detects the comparison to boolean constants.
contract A {
function f(bool x) public {
// ...
if (x == true) { // bad!
// ...
}
// ...
}
}Boolean constants can be used directly and do not need to be compare to true or false.
Remove the equality to the boolean constant.
- Check:
cyclomatic-complexity - Severity:
Informational - Confidence:
High
Detects functions with high (> 11) cyclomatic complexity.
Reduce cyclomatic complexity by splitting the function into several smaller subroutines.
- Check:
deprecated-standards - Severity:
Informational - Confidence:
High
Detect the usage of deprecated standards.
contract ContractWithDeprecatedReferences {
// Deprecated: Change block.blockhash() -> blockhash()
bytes32 globalBlockHash = block.blockhash(0);
// Deprecated: Change constant -> view
function functionWithDeprecatedThrow() public constant {
// Deprecated: Change msg.gas -> gasleft()
if(msg.gas == msg.value) {
// Deprecated: Change throw -> revert()
throw;
}
}
// Deprecated: Change constant -> view
function functionWithDeprecatedReferences() public constant {
// Deprecated: Change sha3() -> keccak256()
bytes32 sha3Result = sha3("test deprecated sha3 usage");
// Deprecated: Change callcode() -> delegatecall()
address(this).callcode();
// Deprecated: Change suicide() -> selfdestruct()
suicide(address(0));
}
}Replace all uses of deprecated symbols.
- Check:
erc20-indexed - Severity:
Informational - Confidence:
High
Detects whether events defined by the ERC20 specification that should have some parameters as indexed are missing the indexed keyword.
contract ERC20Bad {
// ...
event Transfer(address from, address to, uint value);
event Approval(address owner, address spender, uint value);
// ...
}Transfer and Approval events should have the 'indexed' keyword on their two first parameters, as defined by the ERC20 specification.
Failure to include these keywords will exclude the parameter data in the transaction/block's bloom filter, so external tooling searching for these parameters may overlook them and fail to index logs from this token contract.
Add the indexed keyword to event parameters that should include it, according to the ERC20 specification.
- Check:
function-init-state - Severity:
Informational - Confidence:
High
Detects the immediate initialization of state variables through function calls that are not pure/constant, or that use non-constant state variable.
contract StateVarInitFromFunction {
uint public v = set(); // Initialize from function (sets to 77)
uint public w = 5;
uint public x = set(); // Initialize from function (sets to 88)
address public shouldntBeReported = address(8);
constructor(){
// The constructor is run after all state variables are initialized.
}
function set() public returns(uint) {
// If this function is being used to initialize a state variable declared
// before w, w will be zero. If it is declared after w, w will be set.
if(w == 0) {
return 77;
}
return 88;
}
}In this case, users might intend a function to return a value a state variable can initialize with, without realizing the context for the contract is not fully initialized. In the example above, the same function sets two different values for state variables because it checks a state variable that is not yet initialized in one case, and is initialized in the other. Special care must be taken when initializing state variables from an immediate function call so as not to incorrectly assume the state is initialized.
Remove any initialization of state variables via non-constant state variables or function calls. If variables must be set upon contract deployment, locate initialization in the constructor instead.
- Check:
incorrect-using-for - Severity:
Informational - Confidence:
High
In Solidity, it is possible to use libraries for certain types, by the using-for statement (using <library> for <type>). However, the Solidity compiler doesn't check whether a given library has at least one function matching a given type. If it doesn't, such a statement has no effect and may be confusing.
```solidity
library L {
function f(bool) public pure {}
}
using L for uint;
```
Such a code will compile despite the fact that `L` has no function with `uint` as its first argument.
Make sure that the libraries used in using-for statements have at least one function matching a type used in these statements.
- Check:
low-level-calls - Severity:
Informational - Confidence:
High
The use of low-level calls is error-prone. Low-level calls do not check for code existence or call success.
Avoid low-level calls. Check the call success. If the call is meant for a contract, check for code existence.
- Check:
missing-inheritance - Severity:
Informational - Confidence:
High
Detect missing inheritance.
interface ISomething {
function f1() external returns(uint);
}
contract Something {
function f1() external returns(uint){
return 42;
}
}Something should inherit from ISomething.
Inherit from the missing interface or contract.
- Check:
naming-convention - Severity:
Informational - Confidence:
High
Solidity defines a naming convention that should be followed.
- Allow constant variable name/symbol/decimals to be lowercase (
ERC20). - Allow
_at the beginning of themixed_casematch for private variables and unused parameters.
Follow the Solidity naming convention.
- Check:
pragma - Severity:
Informational - Confidence:
High
Detect whether different Solidity versions are used.
Use one Solidity version.
- Check:
redundant-statements - Severity:
Informational - Confidence:
High
Detect the usage of redundant statements that have no effect.
contract RedundantStatementsContract {
constructor() public {
uint; // Elementary Type Name
bool; // Elementary Type Name
RedundantStatementsContract; // Identifier
}
function test() public returns (uint) {
uint; // Elementary Type Name
assert; // Identifier
test; // Identifier
return 777;
}
}Each commented line references types/identifiers, but performs no action with them, so no code will be generated for such statements and they can be removed.
Remove redundant statements if they congest code but offer no value.
- Check:
solc-version - Severity:
Informational - Confidence:
High
solc frequently releases new compiler versions. Using an old version prevents access to new Solidity security checks.
We also recommend avoiding complex pragma statement.
Deploy with a recent version of Solidity (at least 0.8.0) with no known severe issues.
Use a simple pragma version that allows any of these versions. Consider using the latest version of Solidity for testing.
- Check:
unimplemented-functions - Severity:
Informational - Confidence:
High
Detect functions that are not implemented on derived-most contracts.
interface BaseInterface {
function f1() external returns(uint);
function f2() external returns(uint);
}
interface BaseInterface2 {
function f3() external returns(uint);
}
contract DerivedContract is BaseInterface, BaseInterface2 {
function f1() external returns(uint){
return 42;
}
}DerivedContract does not implement BaseInterface.f2 or BaseInterface2.f3.
As a result, the contract will not properly compile.
All unimplemented functions must be implemented on a contract that is meant to be used.
Implement all unimplemented functions in any contract you intend to use directly (not simply inherit from).
- Check:
unused-import - Severity:
Informational - Confidence:
High
Importing a file that is not used in the contract likely indicates a mistake. The import should be removed until it is needed.
```solidity
import {A} from "./A.sol";
contract B {}
```
B either should import from A and it was forgotten or the import is not needed and should be removed.
Remove the unused import. If the import is needed later, it can be added back.
- Check:
unused-state - Severity:
Informational - Confidence:
High
Unused state variable.
Remove unused state variables.
- Check:
costly-loop - Severity:
Informational - Confidence:
Medium
Costly operations inside a loop might waste gas, so optimizations are justified.
contract CostlyOperationsInLoop{
uint loop_count = 100;
uint state_variable=0;
function bad() external{
for (uint i=0; i < loop_count; i++){
state_variable++;
}
}
function good() external{
uint local_variable = state_variable;
for (uint i=0; i < loop_count; i++){
local_variable++;
}
state_variable = local_variable;
}
}Incrementing state_variable in a loop incurs a lot of gas because of expensive SSTOREs, which might lead to an out-of-gas.
Use a local variable to hold the loop computation result.
- Check:
dead-code - Severity:
Informational - Confidence:
Medium
Functions that are not sued.
contract Contract{
function dead_code() internal() {}
}dead_code is not used in the contract, and make the code's review more difficult.
Remove unused functions.
- Check:
reentrancy-unlimited-gas - Severity:
Informational - Confidence:
Medium
Detection of the reentrancy bug.
Only report reentrancy that is based on transfer or send.
function callme(){
msg.sender.transfer(balances[msg.sender]):
balances[msg.sender] = 0;
} send and transfer do not protect from reentrancies in case of gas price changes.
Apply the check-effects-interactions pattern.
- Check:
similar-names - Severity:
Informational - Confidence:
Medium
Detect variables with names that are too similar.
Bob uses several variables with similar names. As a result, his code is difficult to review.
Prevent variables from having similar names.
- Check:
too-many-digits - Severity:
Informational - Confidence:
Medium
Literals with many digits are difficult to read and review. Use scientific notation or suffixes to make the code more readable.
contract MyContract{
uint 1_ether = 10000000000000000000;
}While 1_ether looks like 1 ether, it is 10 ether. As a result, it's likely to be used incorrectly.
Use:
- Check:
cache-array-length - Severity:
Optimization - Confidence:
High
Detects for loops that use length member of some storage array in their loop condition and don't modify it.
contract C
{
uint[] array;
function f() public
{
for (uint i = 0; i < array.length; i++)
{
// code that does not modify length of `array`
}
}
}Since the for loop in f doesn't modify array.length, it is more gas efficient to cache it in some local variable and use that variable instead, like in the following example:
contract C
{
uint[] array;
function f() public
{
uint array_length = array.length;
for (uint i = 0; i < array_length; i++)
{
// code that does not modify length of `array`
}
}
}Cache the lengths of storage arrays if they are used and not modified in for loops.
- Check:
constable-states - Severity:
Optimization - Confidence:
High
State variables that are not updated following deployment should be declared constant to save gas.
Add the constant attribute to state variables that never change.
- Check:
external-function - Severity:
Optimization - Confidence:
High
public functions that are never called by the contract should be declared external, and its immutable parameters should be located in calldata to save gas.
Use the external attribute for functions never called from the contract, and change the location of immutable parameters to calldata to save gas.
- Check:
immutable-states - Severity:
Optimization - Confidence:
High
State variables that are not updated following deployment should be declared immutable to save gas.
Add the immutable attribute to state variables that never change or are set only in the constructor.
- Check:
var-read-using-this - Severity:
Optimization - Confidence:
High
The contract reads its own variable using this, adding overhead of an unnecessary STATICCALL.
contract C {
mapping(uint => address) public myMap;
function test(uint x) external returns(address) {
return this.myMap(x);
}
}Read the variable directly from storage instead of calling the contract.