How can you secure an Ethereum smart contract from denial-of-service attacks?

1 Avoid loops and arrays

One of the main sources of DoS attacks on smart contracts is the use of loops and arrays that consume a lot of gas and can be exploited by attackers to drain your contract's balance or block its execution. For example, if your contract has a loop that iterates over a large array of users or tokens, an attacker can add more elements to the array and make the loop run out of gas or exceed the block gas limit. To avoid this, you should use mappings instead of arrays, limit the size of your loops, and use pagination or batching techniques to split your operations into smaller chunks.

2 Use pull over push

Another common DoS attack vector is the use of push payments, where your contract sends funds to multiple recipients in one transaction. This can be problematic if one of the recipients is a malicious contract that rejects or reverts the payment, causing the whole transaction to fail and preventing the other recipients from receiving their funds. To avoid this, you should use the pull payment pattern, where your contract only records the balances of the recipients and lets them withdraw their funds individually. This way, you reduce the risk of failed transactions and save gas costs.

3 Implement circuit breakers

A circuit breaker is a mechanism that allows you to pause or stop the execution of your contract in case of an emergency or a bug. For example, if your contract is under a DoS attack or has a security breach, you can use a circuit breaker to halt its functions and prevent further damage. A circuit breaker can be implemented by using a modifier or a state variable that checks the contract's status before executing any function. You should also have a way to resume or update your contract after fixing the issue, such as by using an owner or an administrator role.

4 Use timeouts and limits

Another way to protect your contract from DoS attacks is to use timeouts and limits for your functions and transactions. For example, if your contract has a function that requires a user to confirm or reject an action within a certain period of time, you can use a timeout to automatically execute the default option if the user does not respond. This way, you prevent an attacker from blocking your contract by not confirming or rejecting the action. Similarly, you can use limits to restrict the number or frequency of transactions or requests that your contract can process, preventing an attacker from flooding your contract with spam.

5 Test and audit your code

Finally, the best way to secure your contract from DoS attacks is to test and audit your code thoroughly before deploying it to the mainnet. You should use tools and frameworks such as Truffle, Hardhat, or Remix to test your contract's functionality, performance, and gas consumption. You should also use code analysis tools such as Slither, MythX, or Securify to detect and fix any vulnerabilities or bugs in your code. Moreover, you should hire a professional auditor or a security expert to review your code and verify its quality and security.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?