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In Blockchain Last updated: September 4, 2023
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Ree­ntrancy attacks can have severe impacts on your finances, data, network, and credibility.

This is why ensuring security is of utmost importance in the world of blockchain and smart contracts.

For this, it is essential to have a clear understanding of how these attacks operate and how attackers e­xploit vulnerabilities in smart contracts, depleting funds and causing disruptions. 

It’s also necessary to implement effective strategies in order to protect your blockchain projects and smart contracts against re­entrancy attacks.

In this article, I’ll talk about what reentrancy attacks are, various types, how they can potentially harm your digital asse­ts, and some practical measures to ensure their safety.

Let’s get started!

What Are Reentrancy Attacks?

Reentrancy attacks are skillful tactics that cybercriminals employ in order to exploit vulnerabilities within smart contracts, particularly those operating on blockchain platforms.

Let’s imagine­ a scenario – you start a transaction, but before it’s finished, an attacker triggers the same function again. This double-entry tactic e­nables the attacker to continuously withdraw funds or carry out actions be­fore the original transaction is fully processe­d.

Source: Twitter (@chainlink)

Also, there is a catch when it comes to smart contracts. These contracts usually update the state and balance only after a transaction is completed. This creates an opportunity for attacke­rs to manipulate the contract’s logic and steal asse­ts. 

Reentrancy attacks have led to financial losses and security breaches in the world of blockchain. To better protect your digital assets and smart contracts, it’s important to understand the concept of reentrancy attacks. 

Types of Reentrancy Attacks

There are different types of reentrancy attacks. Each of these types exploits different aspects of smart contract functionality to gain unauthorized access, manipulate data, or siphon off assets.

Let’s check them out one by one.

#1. Basic Reentrancy Attack

Source: CertiK

In this attack, someone exploits a simple mistake in a smart contract’s setup. They find a particular action that the contract does and keep doing it quickly, one after the other, before the contract has a chance to catch up. 

This messes up how the contract keeps track of things, leading to problems like taking out money without permission or making the contract do things it wasn’t supposed to.

#2. Advanced Reentrancy Attack

This is like a more complicated version of the basic attack. The attacker sets up a situation where one contract talks to another contract, and they make this communication happen in a sneaky way. Because of this sneaky setup, the attacker can make the second contract do things it shouldn’t, even though it has a defense mechanism against those things. 

This makes it harder to notice and stop these attacks because they involve multiple contracts working together.

#3. Cross-Function Reentrancy Attack

In this type of attack, someone messes with a smart contract by playing with different parts of it that are connected. They figure out how one part talks to another, and they use this connection to confuse the contract and make it do things it wasn’t supposed to. 

This kind of attack needs a deep understanding of how the contract’s different parts interact.

#4. Cross-Contract Reentrancy Attack

Source: ResearchGate

In this type, the attacker goes beyond just one contract and looks at how different contracts talk to each other. They find weak points in how these contracts communicate and use these weak points to create problems. These problems could spread across many contracts, causing a mess in the whole blockchain network.

#5. Guarded Reentrancy Attack

In this tricky attack, the attacker waits for the right moment. They watch for specific things to happen in a smart contract, like a timer running out or certain conditions being met. Once the time is right, they quickly make the contract do something wrong. Because they wait for just the right moment, it’s hard to catch them in the act.

Benefits of Securing Blockchain and Smart Contracts

Some of the practical benefits of securing blockchain and smart contracts include the following:

Protection Against Unauthorized Access

Securing your blockchain and smart contracts helps in preventing unauthorized access.

It’s like constructing a digital castle that only permits entry to authorized individuals. This fortifie­d environment safeguards your valuable­ data and assets, providing a secure sanctuary against potential breaches.

Preventing Data Leaks

When you prioritize the security of your blockchain, you’re essentially placing an unbreakable lock on your data. This lock makes it exceptionally difficult for anyone to manipulate your data without proper authorization. 


With this robust protection, you can have steady integrity of your important records, transactions, and sensitive information.

Mitigation of Reentrancy Attacks

By implementing thorough security measures, you can effectively prevent reentrancy attacks. These deceptive tactics e­xploit vulnerabilities in smart contracts. By ensuring the security of these contracts, you create a protective barrie­r against malicious attempts to exploit weaknesses in the system.

Enhanced Trust and Transparency

Security is vital for building trust and transpare­ncy in the blockchain industry. The strong protection of transactions and data ensures that exchanges are­ tamper-proof and fair. This instils a greater se­nse of confidence in all business dealings and partnerships.

Stakeholde­rs can see for themselves that transactions remain unaltere­d, reinforcing their trust in your operations’ inte­grity.

Minimized Financial Losses

Investing in the security of your blockchain and smart contracts offers a significant advantage by minimizing financial risks.


By prote­cting these digital assets, you can re­duce the potential for mone­tary losses caused by cyberattacks or fraudule­nt activities. This safeguards your financial intere­sts and enhances the ove­rall financial health of your projects by discouraging unauthorized acce­ss and data manipulation.

In addition to providing protection, the­ security measures you e­mploy for your blockchain and smart contracts have legal benefits too. This holds significance, especially within sectors with strict data protection regulations like finance, healthcare, etc.

By e­nsuring compliance with these le­gal frameworks, you stay safe and avoid legal penalties. Securing data also pre­serves your reputation among customers. 

Efficiency and Cost Savings

Investing in se­curity proactively not only protects your digital assets but also brings long-te­rm efficiencies. It can lower the resources and efforts ne­eded to fix breache­s, address fraud, or recover from cybe­rattacks.

How Do Reentrancy Attacks Work? 

Source: ResearchGate

Reentrancy attacks target computer programs and exploit the way they handle resources like memory. Here’s how they work:

Function Call

A smart contract can have a function that allows you to withdraw money. When you request a withdrawal, the contract sets things in motion to give you your funds.

Quick Moves

The attacker starts by quickly calling the withdrawal function multiple times, one after another, before the contract can finish dealing with the first request.

Timing Trickery

While the contract is busy dealing with the first request, the attacker’s repeated calls sneak in and start new withdrawals, even though the first one hasn’t finished yet.

The Twist

Here’s where the trick comes in – the attacker’s repeated withdrawals can interact with the ongoing process from the first withdrawal. They can even trick the contract into sending money more than once.

Double Benefits

In essence, the attacker gets to withdraw money multiple times for the price of one request. It’s like asking for your change back after paying for something and somehow getting extra change every time.

Unintended Results

These repeated actions can also mess with the contract’s understanding of its own state. This can lead to unintended behaviors or even security breaches.

Reentrancy attacks exploit the contract’s inability to handle these rapid and overlapping requests. They essentially create a chaotic situation where the contract’s logic gets mixed up, allowing the attacker to drain off more than they’re supposed to.

How Smart Contracts Can Fall Victim to Reentrancy Attacks?

Smart contracts, which are designed to automate and execute transactions, can unexpectedly fall prey to reentrancy attacks due to their unique characteristics. Here’s how it can happen:

Unchecked State Changes

Source: Blockchain Academy Mittweida

Smart contracts might not always double-check if a function call has been completed before allowing another call. Attackers take advantage of this loophole, repeatedly calling a function before it wraps up, altering the contract’s state unpredictably.

Incomplete Balance Updates

Smart contracts often update account balances after a transaction is complete. Attackers exploit this gap by withdrawing funds repeatedly before the contract can catch up, leading to inaccurate balances.

Dependent Function Vulnerabilities

When one function within a smart contract depends on another, attackers can manipulate this relationship. Through the repetitive triggering of these functions in rapid sequence, they can capitalize on weaknesses and attain unauthorized entry.

Asynchronous Calls Trouble

Asynchronous calls in smart contracts can create openings for attackers. They call a function, interrupt its execution with another call, and exploit the partially updated state. This may lead to unintended actions.

External Contract Interactions

Interaction with external contracts can expose vulnerabilities. Attackers create a contract to call back into the target contract before its defenses activate, bypassing intended security measures.

Unprotected Mutex Locks

Mutex locks prevent multiple functions from executing at once, ensuring stability. If not properly guarded, attackers can abuse this mechanism, executing the same function repeatedly before the lock activates.

Race Condition Exploitation

Source: Wallarm

Attackers exploit race conditions – where different actions occur depending on timing – by calling functions rapidly. This manipulation disrupts the intended sequence of operations, leading to unauthorized actions.

Dependency on External Data

If a contract relies on external data, attackers can manipulate this data to trick the contract into unintended behaviors, exploiting the contract’s trust in external information.

How to Prevent Reentrancy Attacks?

Here are some easy ways to prevent a reentrancy attack and ensure the security of your smart contracts:

Use the Withdrawal Pattern

When designing your smart contracts, consider adopting the withdrawal pattern. This approach involves rearranging the sequence of operations within your contract. Specifically, ensure that balance updates are executed before any withdrawals are permitted.

By following this pattern, you create a system where the balance is accurately adjusted before funds are released. This eliminates the window of vulnerability that attackers often exploit in reentrancy attacks.

Employ Mutex Locks for Function Calls

Mutex locks act as digital gates that allow only one process to pass through at a time. When applied to smart contracts, they prevent multiple function calls from being executed concurrently.

By utilizing mutex locks, you guarantee that each function is completed before another one starts. This blocks attackers from rapidly triggering the same function to manipulate the contract’s state, effectively putting an end to their reentrancy exploits.

Adopt the Check-Effects-Interactions Pattern

Source: DEV Community

The check-effects-interactions pattern is a coding practice that prioritizes security. Begin by checking the conditions and user balances required for the function to execute. Next, carry out the desired action.

Finally, update the state of the contract. This sequence of steps minimizes the exposure to reentrancy attacks by ensuring that changes in state are made only after thorough validation. Thus, it reduces the opportunities for attackers to manipulate inconsistencies.

Limit External Calls to Trusted Contracts

Interactions with external contracts introduce vulnerabilities. To mitigate this risk, restrict external calls to contracts that are trustworthy and thoroughly vetted.

By confining interactions to reliable contracts, you reduce the chances of attackers exploiting malicious external contracts to initiate reentrancy attacks.

Implement Reentrancy Guards

A reentrancy guard is like a security gate that prevents unauthorized entries. Incorporate these guards into your smart contract’s logic. They monitor whether a function is already in the process of execution and reject any additional calls until the ongoing operation is complete. 

This effectively thwarts attackers’ attempts to exploit the contract’s reentrancy vulnerability.

Avoid Changing External State 

In critical moments, exercise caution with external state changes. Before executing pivotal actions within your contract, refrain from making any modifications to the external state.

Doing so creates a more controlled environment, preventing attackers from tampering with the contract’s state during crucial transactions.

Set Appropriate Gas Limits

When initiating transactions on a blockchain, you assign gas limits to ensure that computations can be completed within a predefined threshold. To prevent reentrancy attacks, ensure that your gas limits are set appropriately.

Transactions that run out of gas prematurely can leave the contract in an inconsistent state, potentially exposing it to vulnerabilities that attackers might exploit.

Enforce Access Controls and Permissions


Incorporate strict access control mechanisms into your smart contracts. Establish user roles and permissions to guarantee that only authorized entities can engage with particular functions. To enhance security and prevent re­entrancy attacks, one effe­ctive measure is to re­gulate access to specific functions.

By re­stricting who can call these functions, you reduce the potential vulnerabilitie­s that attackers could exploit.

Use Function Modifiers

Function modifiers act as filters that allow you to apply pre-defined conditions to multiple functions within your smart contract.

By utilizing modifiers, you can enforce checks on function calls to ensure that certain conditions are met before execution. This can include verifying that the transaction sender is the owner of the contract or confirming the availability of sufficient funds.

Audit and Testing

Regularly audit your smart contracts for vulnerabilities and conduct thorough testing. Professional external security audits help in pinpointing vulnerable areas that attackers could potentially target.

To ensure the security of your contract, it’s important to conduct thorough testing, which includes simulating potential attacks. This enables you to identify and address vulnerabilities before deploying the contract.


Understanding reentrancy attacks and their impacts on your blockchain and smart contracts is important in order to e­nsure security.

By incorporating security strategies like controlling access, utilizing mutex locks, and implementing proper testing proce­dures, you can establish a strong defense against these attacks. 

This way, you can protect your digital assets and transactions and ensure the integrity of your blockchain ecosystem. Plus, always remain vigilant, informed, and committed to creating a safer digital landscape­ in your organization.

You may also explore what Spooling attacks are and how to keep yourself safe from them.

  • Amrita Pathak
    Amrita is a freelance copywriter and content writer. She helps brands enhance their online presence by creating awesome content that connects and converts. She has completed her Bachelor of Technology (B.Tech) in Aeronautical Engineering…. read more
  • Narendra Mohan Mittal

    Narendra Mohan Mittal is a Senior Digital Branding Strategist and Content Editor with over 12 years of versatile experience. He holds an M-Tech (Gold Medalist) and B-Tech (Gold Medalist) in Computer Science & Engineering.

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