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  • Writer's pictureJeffrey Scholz

Smart Contract Security

Updated: 3 days ago

smart contract security

This article serves as a mini course on smart contract security and provides an extensive list of the issues and vulnerabilities that tend to recur in Solidity smart contracts.

A security issue in Solidity boils down to smart contracts not behaving the way they were intended to. This can fall into four broad categories:

  • Funds getting stolen

  • Funds getting locked up or frozen inside a contract

  • People receive less rewards than anticipated (rewards are delayed or reduced)

  • People receive more rewards than anticipated (leading to inflation and devaluation)

It isn’t possible to make a comprehensive list of everything that can go wrong. However, just as traditional software engineering has common themes of vulnerabilities such as SQL injection, buffer overruns, and cross site scripting, smart contracts have recurring anti-patterns that can be documented.

Smart contract hacks and vulnerabilities

Think of this guide as more of a reference. It isn’t possible to discuss every concept in detail without turning this into a book (fair warning: this article is 10k+ words long, so feel free to bookmark it and read it in chunks). However, it serves as a list of what to look out for and what to study. If a topic feels unfamiliar, that should serve as an indicator that it is worth putting time into practicing identifying that class of vulnerability.


This article assumes basic proficiency in Solidity. If you are new to Solidity, please see our free Solidity tutorial.


We’ve written extensively on smart contract reentrancy so we won’t repeat it here. But here is a quick summary:

Whenever a smart contract calls the function of another smart contract, sends Ether to it, or transfers a token to it, then there is a possibility of re-entrancy.

  • When Ether is transferred, the receiving contract’s fallback or receive function is called. This hands control over to the receiver.

  • Some token protocols alert the receiving smart contract that they have received the token by calling a predetermined function. This hands the control flow over to that function.

  • When an attacking contract receives control, it doesn’t have to call the same function that handed over control. It could call a different function in the victim smart contract (cross-function reentrancy) or even a different contract (cross-contract reentrancy)

  • Read-only reentrancy happens when a view function is accessed while the contract is in an intermediate state.

Despite reentrancy likely being the most well known smart contract vulnerability, it only makes up a small percentage of hacks that happen in the wild. Security researcher Pascal Caversaccio (pcaveraccio) keeps an up-to-date github list of reentrancy attacks. As of April 2023, 46 reentrancy attacks have been documented in that repository.

Access Control

It seems like a simple mistake, but forgetting to place restrictions on who can call a sensitive function (like withdrawing ether or changing ownership) happens surprisingly often.

Even if a modifier is in place, there have been cases where the modifier was not implemented correctly, such as in the example below where the require statement is missing.

modifier onlyMinter {
    minters[msg.sender] == true_;

This above code is a real example from this audit:

Here is another way access control can go wrong

function claimAirdrop(bytes32 calldata proof[]) {

    bool verified = MerkleProof.verifyCalldata(proof, merkleRoot, keccak256(abi.encode(msg.sender)));
    require(verified, "not verified");
    require(alreadyClaimed[msg.sender], "already claimed");

    _transfer(msg.sender, AIRDROP_AMOUNT);

In this case, “alreadyClaimed” is never set to true, so the claimant can issue call the function multiple times.

Real life example: Trader bot exploited

A fairly recent example of insufficient access control was an unprotected function to receive flashloans by a trading bot (which went by the name 0xbad, as the address started with that sequence). It racked up over a million dollars in profit until one day an attacker noticed any address could call the flashloan receive function, not just the flashloan provider.

As is usually the case with trading bots, the smart contract code to execute the trades was not verified, but the attacker discovered the weakness anyway. More info in the rekt news coverage.

Improper Input Validation

If access control is about controlling who calls a function, input validation is about controlling what they call the contract with.

This usually comes down to forgetting to put the proper require statements in place.

Here is a rudimentary example

contract UnsafeBank {
    mapping(address => uint256) public balances;

    // allow depositing on other's behalf
    function deposit(address for) public payable {
        balances += msg.value;

    function withdraw(address from, uint256 amount) public {
        require(balances[from] <= amount, "insufficient balance");

        balances[from] -= amount;{value: amout}("");

The contract above does check that you aren’t withdrawing more than you have in your account, but it doesn’t stop you from withdrawing from an arbitrary account.

Real life example: Sushiswap

Sushiswap experienced a hack of this type due to one of the parameters of an external function not being sanitized.

Sushiswap hack

What is the difference between improper access control and improper input validation?

Improper access control means msg.sender does not have adequate restrictions. Improper input validation means the arguments to the function are not sufficiently sanitized. There is also an inverse to this anti-pattern: placing too much restriction on a function call.

Excessive function restriction

Excessive validation probably means funds won’t get stolen, but it could mean funds get locked into the contract. Having too many safeguards in place is not a good thing either.

Real life example: Akutars NFT

One of the most high-profile incidents was the Akutars NFT which ended up with 34 million dollars worth of Eth stuck inside the smart contract and unwithdrawable.

The contract had a well-intentioned mechanism to prevent the owner of the contract from withdrawing until all refunds from paying above the dutch auction price had been given. But due to a bug documented in the Twitter thread linked below, the owner was unable to withdraw the funds.

Akutars NFT vulnerability

Getting the balance right

Sushiswap gave too much power to untrusted users, and the Akutars NFT gave too little power to the admin. When designing smart contracts, a subjective judgement about how much liberty each class of users must be made, and this decision cannot be left to automated testing and tooling. There are significant tradeoffs with decentralization, security, and UX that must be considered.

For the smart contract programmer, explicitly writing out what users should and should not be able to do with certain functions is an important part of the development process.

We will revisit the topic of overpowered admins later.

Security often boils down to managing the way money exits the contract

As stated in the introduction, there are four primary ways smart contracts get hacked:

  • Money stolen

  • Money frozen

  • Insufficient rewards

  • Excessive rewards

“Money” here means anything of value, such as tokens, not just cryptocurrency. When coding or auditing a smart contract, the developer must be conscientious of the intended ways value is to flow in and out of the contract. The issues listed above are the primary ways smart contracts get hacked, but there are a lot of other root causes that can cascade into major issues, which are documented below.

Double voting or msg.sender spoofing

Using vanilla ERC20 tokens or NFTs as tickets to weigh vote is unsafe because attackers can vote with one address, transfer the tokens to another address, and vote again from that address.

Here is a minimal example

// A malicious voter can simply transfer their tokens to
// another address and vote again.
contract UnsafeBallot {

    uint256 public proposal1VoteCount;
    uint256 public proposal2VoteCount;

    IERC20 immutable private governanceToken;

    constructor(IERC20 _governanceToken) {
        governanceToken = _governanceToken;
    function voteFor1() external notAlreadyVoted {
        proposal1VoteCount += governanceToken.balanceOf(msg.sender);

    function voteFor2() external notAlreadyVoted {
        proposal2VoteCount += governanceToken.balanceOf(msg.sender);

    // prevent the same address from voting twice,
    // however the attacker can simply
    // transfer to a new address
    modifier notAlreadyVoted {
        require(!alreadyVoted[msg.sender], "already voted");
        alreadyVoted[msg.sender] = true;

To prevent this attack, ERC20 Snapshot or ERC20 Votes should be used. By snapshotting a point of time in the past, the current token balances cannot be manipulated to gain illicit voting power.

Flashloan Governance Attacks

However, using an ERC20 token with a snapshot or vote capability doesn’t fully solve the problem if someone can take a flashloan to temporarily increase their balance, then take a snapshot of their balance in the same transaction. If that snapshot is used for voting, they will have an unreasonably large amount of votes at their disposal.

A flashloan lends a large amount of Ether or token to an address, but reverts if the money is not repaid in the same transaction.

contract SimpleFlashloan {

    function borrowERC20Tokens() public {
        uint256 before = token.balanceOf(address(this));

        // send tokens to the borrower
        token.transfer(msg.sender, amount);

        // hand control back to the borrower to 
        // let them do something

        // require that the tokens got returned
        require(token.balanceOf(address(this) >= before);

An attacker can use a flashloan to suddenly gain a lot of votes to swing proposals in their favor and/or do something malicious.

Flashloan Price Attacks

This is arguably the most common (or at least most high-profile) attack on DeFi, accounting for hundreds of millions of dollars lost. Here is a list of high profile ones.

The price of an asset on the blockchain is often calculated as the current exchange rate between assets. For example, if a contract is currently trading 1 USDC for 100 k9coin, then you could say k9coin has a price of 0.01 USDC. However, prices generally move in response to buying and selling pressure, and flash loans can create massive buying and selling pressure.

When querying another smart contract about the price of an asset, the developer needs to be very careful because they are assuming the smart contract they are calling is immune to flash loan manipulation.

Bypassing the contract check

You can “check” if an address is a smart contract by looking at it’s bytecode size. Externally owned accounts (regular wallets) don’t have any bytecode. Here are a few ways of doing it

import "@openzeppelin/contracts/utils/Address.sol"
contract CheckIfContract {

    using Address for address;

    function addressIsContractV1(address _a) {
        return _a.code.length == 0;

    function addressIsContractV2(address _a) {

        // use the openzeppelin libraryreturn _a.isContract();

However, this has a few limitations

  • If a contract makes an external call from a constructor, then it’s apparent bytecode size will be zero because the smart contract deployment code hasn’t returned the runtime code yet

  • The space might be empty now, but an attacker might know they can deploy a smart contract there in the future using create2

In general checking if an address is a contract is usually (but not always) an antipattern. Multisignature wallets are smart contracts themselves, and doing anything that might break multisignature wallets breaks composability.

The exception to this is checking if the target is a smart contract before calling a transfer hook. More on this later.


There is rarely a good reason to use tx.origin. If tx.origin is used to identify the sender, then a man-in-the-middle attack is possible. If the user is tricked into calling a malicious smart contract, then the smart contract can use all the authority tx.origin has to wreak havoc.

Consider this following exercise, and the comments above the code.

contract Phish {
    function phishingFunction() public {

        // this fails, because this contract does not have approval/allowance
        token.transferFrom(msg.sender, address(this), token.balanceOf(msg.sender));

        // this also fails, because this creates approval for the contract,// not the wallet calling this phishing function
        token.approve(address(this), type(uint256).max);

This does not mean you are safe calling arbitrary smart contracts. But there is a layer of safety built into most protocols that will be bypassed if tx.origin is used for authentication.

Sometimes, you might see code that looks like this:

require(msg.sender == tx.origin, "no contracts");

When a smart contract calls another smart contract, msg.sender will be the smart contract and tx.origin will be the user’s wallet, thus giving a reliable indication that the incoming call is from a smart contract. This is true even if the call happens from the constructor.

Most of the time, this design pattern not a good idea. Multisignature wallets and Wallets from EIP 4337 won’t be able to interact with a function that has this code. This pattern can commonly be seen in NFT mints, where it’s reasonable to expect most users are using a traditional wallet. But as account abstraction becomes more popular, this pattern will hinder more than it helps.

Gas Griefing or Denial of Service

A griefing attack means the hacker is trying to "cause grief" for other people, even if they don't gain economically from doing so.

A smart contract can maliciously use up all the gas forwarded to it by going into an infinite loop. Consider the following example:

contract Mal {

    fallback() external payable {

        // infinite loop uses up all the gas
        while (true) {


If another contract distributes ether to a list of addresses such as follows:

contract Distribute {
    funtion distribute(uint256 total) public nonReentrant {
        for (uint i; i < addresses.length; ) {

            (bool ok, ){value: total / addresses.length}("");
            // ignore ok, if it reverts we move on
            // traditional gas saving trick for for loops
            unchecked {

Then the function will revert when it sends ether to Mal. The call in the code above forwards 63 / 64 of the gas available (read more about this rule in our article on EIP 150), so there likely won’t be enough gas to complete the operation with only 1/64 of the gas left.

A smart contract can return a large memory array that consumes a lot of gas

Consider the following example

function largeReturn() public {

    // result might be extremely long!
    (book ok, bytes memory result) =    "foo()"));
    require(ok, "call failed");

Memory arrays use up quadratic amount of gas after 724 bytes, so a carefully chosen return data size can grief the caller.

Even if the variable result is not used, it is still copied to memory. If you want to restrict the return size to a certain amount, you can use assembly

function largeReturn() public {
    assembly {
        let ok := call(gas(), destinationAddress, value, dataOffset, dataSize, 0x00, 0x00);
        // nothing is copied to memory until you 
        // use returndatacopy()

Deleting arrays that others can add to is also an denial of service vector

Although erasing storage is a gas-efficient operation, it still has a net cost. If an array becomes too long, it becomes impossible to delete. Here is a minimal example

contract VulnerableArray {

    address[] public stuff;

    function addSomething(address something) public {

    // if stuff is too long, this will become undeletable due to
    // the gas cost
    function deleteEverything() public onlyOwner {
        delete stuff;

ERC777, ERC721, and ERC1155 can also be griefing vectors

If a smart contract transfers tokens that have transfer hooks, an attacker can set up a contract that does not accept the token (it either does not have an onReceive function or programs the function to revert). This will make the token untransferable and cause the entire transaction to revert.

Before using safeTransfer or transfer, consider the possibility that the receiver might force the transaction to revert.

contract Mal is IERC721Receiver, IERC1155Receiver, IERC777Receiver {

    // this will intercept any transfer hook
    fallback() external payable {

        // infinite loop uses up all the gaswhile (true) {


    // we could also selectively deny transactions
    function onERC721Received(address operator,
        address from,
        uint256 tokenId,
        bytes calldata data
    ) external returns (bytes4) {

        if (wakeUpChooseViolence()) {
        else {
            return IERC721Receiver.onERC721Received.selector;

Insecure Randomness

It is currently not possible to generate randomness securely with a single transaction on the blockchain. Blockchains need to be fully deterministic, otherwise distributed nodes would not be able to reach a consensus about the state. Because they are fully deterministic, any “random” number can be predicted. The following dice rolling function can be exploited.

contract UnsafeDice {
    function randomness() internal returns (uint256) {
        return keccak256(abi.encode(msg.sender, tx.origin, block.timestamp, tx.gasprice, blockhash(block.number - 1);

    // our dice can land on one of {0,1,2,3,4,5}function rollDice() public payable {
        require(msg.value == 1 ether);

        if (randomness() % 6) == 5) {
  {value: 2 ether}("");

contract ExploitDice {
    function randomness() internal returns (uint256) {
        return keccak256(abi.encode(msg.sender, tx.origin, block.timestamp, tx.gasprice, blockhash(block.number - 1);

    function betSafely(IUnsafeDice game) public payable {
        if (randomness % 6) == 5)) {
            game.betSafely{value: 1 ether}()

        // else don't do anything

It doesn’t matter how you generate randomness because an attacker can replicate it exactly. Throwing in more sources of “entropy” such as the msg.sender, timestamp, etc won’t have any effect because the smart contract can measure it too.

Using the Chainlink Randomness Oracle Wrong

Chainlink is a popular solution to get secure random numbers. It does it in two steps. First, the smart contracts sends a randomness request to the oracle, then some blocks later, the oracle responds with a random number.

Since an attacker can’t predict the future, they can’t predict the random number.

Unless the smart contract uses the oracle wrong.