Reentrancy Attacks: Safeguarding Against the Dangerous Smart Contract Exploit

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By Connor
Estimated reading: 4mins
Reentrancy Attacks

Reentrancy attacks have drained millions from decentralized applications built on blockchain platforms like Ethereum. Learn what exactly reentrancy attacks are, how to guard smart contracts against them, and best practices for blockchain software development.

What is a Reentrancy Attack?

A reentrancy attack refers to a malicious smart contract exploiting a vulnerability in another contract to recursively call functions and drain funds. By repeatedly calling functions before the victim contract can update state, the attacker extracts more funds each round - like multiple withdrawals from a bank account before balances and limits can update.

These attacks target the core mechanisms that enable reentrancy in software - features like callbacks and recursive calls. The attacker contract leaves execution of the first call unfinished, starts a second call, and leverages the unfinished state to their advantage before state gets locked.

Reentrancy Attack Triggers Vulnerability in Smart Contracts

Smart contracts are especially vulnerable due to their inherent trust of other contracts and the irreversible nature of transactions. An attacker contract can call functions to siphon crypto funds little by little with the right exploit.

Once a reentrancy vulnerability exists, attackers can drain millions in funds before getting detected. This has happened in high profile cases like The DAO hack which led to $60 million in stolen Ethereum.

Such attacks expose logical flaws in code. Rather than a direct breach of infrastructure, the contract logic incorrectly assumes state will get updated instantly before another call can start. By exploiting this window, the attack contract recursively drains funds.

Guarding Against Reentrancy Attack Solidity Smart Contracts

As developers build ever more complex blockchain decentralised apps, guarding against reentrancy is crucial. Solidity is the most popular language for writing Ethereum smart contracts. How can Solidity developers prevent such attacks?

Follow Best Practices for State Locking

A key prevention mechanism is adding locks in the code to protect critical sections that manipulate state - like balances. By locking state changes before calling any external, untrusted contracts, developers prevent the changes from getting interrupted and exploited.

For Solidity, this involves using mutex locks around vulnerable code sections:

uint256 private guardCounter = 1; 

modifier noReentrancy() {

  require(guardCounter == 1, "Reentrancy");

  uint256 localCounter = guardCounter;

  guardCounter = 2; 

  _; // Execute function code

  guardCounter = localCounter;


Any functions updating state are marked noReentrancy to prevent external calls from interrupting them once executed. This ensures any transfers or balance updates cannot be recursively called.

reentrancy attacks smart contracts solidity
Limit External Calls in Critical Functions

Another best practice is to simply avoid making any external contract calls from within critical state-changing functions. Functions dealing with asset transfers or balance updates should not call out. This reduces the risk of exploitable entry points.

Delay State Changes Where Possible

For operations like withdrawals from a bank contract, add delays between steps so that balances get updated before next steps execute. This acts as a circuit breaker for recursive attacks.

By taking care around state locking, external calls and code flow, Solidity engineers can write reentrancy-proof contracts.

Guarding Against Reentrancy Attacks - An Example Walkthrough

Let's walk through a basic example attack flow, and how coded best practices can prevent it:

  1. Bank contract has withdraw() function that transfers Eth to user accounts.
  2. Attacking contract calls withdraw() asking for 1 Eth.
  3. withdraw() reduces calling contract's balances by 1 Eth after sending funds (state change #1).
  4. Before withdraw() finishes, attacking contract calls it again asking for 2 Eth this time.
  5. Because balances got updated in step #3, another 2 Eth gets sent while first call still pending.

This attack was possible because withdraw() state changes were not locked. We could prevent this with:

  1. Add a mutex around balance update in withdraw()
  2. Mark withdraw() as noReentrancy() modifier
  3. No external calls allowed within withdraw()

Now any external call cannot interrupt the intermediate state changes. Recursive theft is no longer possible!

While simple examples can be prevented easily, production scale decentralized apps require rigorous, proactive efforts to guard against reentrancy at every level.

Move Fast By Auditing Early and Often

With billions in crypto assets relying on smart contract code, the stakes are high for engineers to get reentrancy right. The solutions exist - what matters is rigorously applying them across codebases.

The work should start early, analyzing code paths during initial design, adding protection mechanisms upfront, and extensive testing of every interaction. Waiting until late in development cycles to audit contracts leads to expensive fixes.

Ongoing audits are needed as functionality evolves across expanding codebases. Consistent, layered defenses offer reliable protection against this dangerous attack vector.

By understanding exactly how reentrancy attacks unfold and combining disciplined best practices, blockchain engineers can confidently develop world class decentralized applications.

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Disclaimer:Please note that nothing on this website constitutes financial advice. Whilst every effort has been made to ensure that the information provided on this website is accurate, individuals must not rely on this information to make a financial or investment decision. Before making any decision, we strongly recommend you consult a qualified professional who should take into account your specific investment objectives, financial situation and individual needs.

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Connor is a US-based digital marketer and writer. He has a diverse military and academic background, but developed a passion over the years for blockchain and DeFi because of their potential to provide censorship resistance and financial freedom. Connor is dedicated to educating and inspiring others in the space, and is an active member and investor in the Ethereum, Hex, and PulseChain communities.

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