What is a Reentrancy Attack in Smart Contracts and How to Prevent It?

Blockchain Hacks
Published in
6 min readFeb 21, 2024
What is a Reentrancy Attack in Smart Contracts and How to Prevent It?
Photo by AstraKode created with Canva

Smart contracts revolutionize the way we conduct financial transactions, but they still face security vulnerabilities. One such vulnerability known as reentrancy attack can jeopardize the security of smart contracts, causing significant financial loss. Understanding reentrancy attacks in Solidity is crucial for safeguarding smart contracts against this threat. We’ll examine reentrancy attacks, their methods, and prevention strategies to bolster smart contract security.

What is a Reentrancy Attack?

A reentrancy attack in the context of Ethereum smart contracts, particularly those written in Solidity, is a critical vulnerability where an attacker manipulates the contract’s withdrawal process. This exploit allows the attacker to repeatedly withdraw funds, akin to a thief continually drawing money from an account even after it should be empty. A reentrancy attack begins with a contract flaw, using an early external call to trigger a fund-draining loop.As famously demonstrated in the DAO hack, understanding and preventing reentrancy attacks are essential for secure smart contract development and operation.

Explaining How Reentrancy Attack Work with Illustrations

A reentrancy attack happens when the contract allows a function to be called again before the previous call finishes. This can lead to unexpected behavior and security vulnerabilities. To illustrate, imagine a function that transfers funds from one account to another. A malicious user could exploit a vulnerability in the function to repeatedly call it and drain the account before the transaction is complete. This demonstrates how reentrancy attacks can disrupt the expected flow of execution and compromise the security of a system.

  1. Initiate: The malicious contract’s fallback function activates before the vulnerable contract can update its balance.
  2. Invoke: The malicious contract calls a withdrawal function on the vulnerable contract.
  3. Interrupt: Before the vulnerable contract updates its balance, the malicious contract’s fallback function is triggered.
  4. Repeat: The fallback function calls the withdrawal function again, exploiting the fact the balance hasn’t updated.
  5. Drain: This loop continues, recursively calling the withdrawal and draining the contract’s funds.
  6. Complete: The reentrancy attack ceases once the contract’s funds run out or the vulnerability is detected and addressed.

Types of Reentrancy Attacks

There are several types of reentrancy attacks, including:

  1. Classic Reentrancy Attack: This type of attack is a common security risk in smart contract development. It involves exploiting unsynchronized states during external contract calls, allowing repeated execution of actions that should only happen once, like unauthorized state alterations or excessive fund withdrawals.
  2. Cross-Function Reentrancy: This attack occurs when a vulnerable function within a smart contract shares its state with another external function that the attacker manipulates. The attacker exploits the asynchronous nature of smart contracts, persistently calling back into multiple susceptible functions.
  3. Cross-Contract Reentrancy: Similar to cross-function reentrancy, this type involves two contracts sharing the same state. The vulnerability arises if a contract fails to update its state to mirror the immediate changes from transactions before initiating any cross-contract calls, both low-level and high-level.
  4. Mono-Function Reentrancy: This happens when a single function within a smart contract is subject to repeated recursive invocations before the completion of previous invocations. It is a specific form of attack focusing on a singular function within the contract.

Examining A Few Contracts That Were Exploited with Reentrancy in the Past

The DAO Attack (2016): This event is a landmark in the history of Ethereum and smart contracts. “The DAO” was an innovative idea, aiming to revolutionize how decentralized organizations operate. Tragically, it became the victim of a reentrancy attack, leading to a staggering loss of about $150 million in Ether. This attack not only drained the funds but also shook the trust in decentralized systems and was a wake-up call for the need for enhanced security in smart contracts.

Lendf.Me Hacks (April 2020): Both these platforms suffered significant financial losses due to a reentrancy attack, amounting to $25 million. These attacks showcased the vulnerabilities present even in well-known decentralized finance (DeFi) platforms and emphasized the necessity for continuous security monitoring and upgrades in the DeFi ecosystem.

BurgerSwap Hack (May 2021): The $7.2 million loss due to a reentrancy exploit involving a fake token contract highlighted the sophisticated methods attackers use. It demonstrated that even seemingly secure smart contracts could be vulnerable to well-orchestrated exploits, stressing the importance of rigorous testing and validation of smart contracts, especially those dealing with token exchanges.

SURGEBNB Hack (August 2021): This attack, resulting in a $5 million loss, was particularly notable for its use of reentrancy in combination with price manipulation. It exemplified the multifaceted nature of smart contract attacks, where exploiters may use a mix of vulnerabilities to maximize their impact.

CREAM FINANCE Hack (August 2021): The $25 million loss in this incident showed how a single reentrancy vulnerability could be exploited to perform unauthorized actions like repeated borrowing, underlining the critical need for robust mechanisms like reentrancy guards in smart contract design.

How to Prevent Reentrancy Vulnerabilities in Solidity

In Solidity smart contracts, guarding against reentrancy vulnerabilities is crucial to prevent security breaches. Leveraging the inherent event-driven architecture of smart contracts, developers can employ events to create a tamper-proof record of all pivotal activities and state changes, ensuring a secure and verifiable transaction history on the blockchain ledger. This historical data is invaluable for tracking and substantiating the contract’s activity over time, serving as a crucial tool in detecting anomalies and reinforcing the robustness of security practices. It should work in tandem with the strategic state updates and meticulous management of external calls to form a holistic security framework.

To effectively mitigate these risks, focus on the following key prevention mechanisms:

  1. Reentrancy Guards/ Mutex : Utilize reentrancy guards or mutually exclusive flag, which are mechanisms that prevent a function from being called again before its first invocation is completed. This is crucial in stopping attackers from exploiting vulnerabilities during the execution of sensitive functions.
  2. State Updates Before Transfers: Ensure that the contract’s state is updated before any Ether transfers occur. This practice is vital to prevent inconsistencies in the contract’s state even if an external call is hijacked.
  3. Formal verification: Formal verification strengthens smart contract security by employing mathematical proofs to confirm that contract execution aligns with specified logic. In Solidity, developers can utilize tools like Mythril alongside the K framework to uncover and address reentrancy risks, among other vulnerabilities, ensuring reliable and secure contract operations. This method is essential for crafting smart contracts that are both resilient and trustworthy.
  4. Minimize External Calls in Critical Functions: Critical functions, especially those that alter the contract’s state or handle financial transactions, should minimize external calls. If external calls are necessary, they should be placed after all critical state changes to reduce risk.
  5. Pull Over Push Payment Methods: Implement a pull payment strategy, which allows users to withdraw funds themselves, rather than the contract pushing out payments automatically. This approach significantly lessens the risk of reentrancy during fund transfers.
  6. Consider Gas Limits and Loops: While gas limits play a crucial role in Ethereum by preventing infinite loops and managing computation costs, they aren’t a direct solution for thwarting reentrancy attacks. Focus on secure contract design over gas limits to prevent resource abuse and enhance security. This includes updating the contract’s state before making external calls and employing mechanisms like reentrancy guards. Robust practices protect smart contracts from reentrancy attacks, ensuring their security.
  7. Regular Contract Audits: Regularly conducting thorough audits and testing of smart contracts is essential. These audits can identify and rectify potential vulnerabilities, reducing the risk of exploitation.


Keeping up with the latest Solidity versions is crucial for enhancing smart contract security. Recent versions introduce improved syntax and compiler checks to mitigate vulnerabilities like reentrancy attacks, offering developers stronger safeguards against exploits. Developers must use reentrancy guards, update states before transfers, limit external calls, adopt pull payments, and conduct audits to lower vulnerability risks. Being informed about smart contract security developments is essential as the blockchain landscape evolves. Proactive security measures safeguard smart contracts on Ethereum, maintaining their integrity.



Blockchain Hacks

AstraKode helps you revolutionize the way you leverage enterprise blockchian technology through low-code. Sign-up to AKB for free: https://bit.ly/44nBbUm