What Is a Reentrancy Attack? A Beginner’s Guide to Understanding the DeFi Exploit That Cost GMX $42 Million

If you have been following crypto news in July 2025, you probably saw headlines about GMX losing $42 million to a hack. The term “reentrancy attack” was mentioned in nearly every report, but most coverage assumed readers already understood what that means. With Bitcoin trading near $119,000 and more newcomers entering the crypto space than ever before, understanding the basics of how DeFi exploits work is essential for anyone holding or using digital assets. This guide breaks down reentrancy attacks in plain language, using the GMX incident as a real-world example.

The Basics

A reentrancy attack is a type of smart contract exploit where an attacker tricks a contract into giving away more funds than it should. To understand how it works, think of a vending machine. Normally, you insert a dollar, press a button, and the machine gives you one snack while recording that your dollar has been spent. But imagine if you found a way to press the button multiple times before the machine registered your payment — you could get several snacks for the price of one. In smart contract terms, this happens when a contract sends funds to another address before it updates its internal records. The receiving address can be a malicious smart contract that immediately calls back into the original contract, requesting another withdrawal before the first transaction has fully completed. Because the internal balance has not been updated yet, the original contract thinks no funds have been sent and processes the second withdrawal as well. This cycle can repeat multiple times within a single transaction, draining far more funds than the attacker originally deposited.

Why It Matters

Reentrancy attacks are not theoretical — they are one of the most common and costly exploits in DeFi history. The most famous example is the 2016 DAO hack on Ethereum, where an attacker used reentrancy to steal approximately $60 million worth of ETH, ultimately leading to the hard fork that created Ethereum and Ethereum Classic. The GMX exploit in July 2025 demonstrates that even nine years later, this same vulnerability pattern continues to affect major protocols. For everyday crypto users, understanding reentrancy matters because it directly affects the safety of funds deposited in DeFi protocols. When you provide liquidity to a protocol like GMX, your funds are held in smart contracts. If those contracts contain reentrancy vulnerabilities, your funds are at risk regardless of how established or reputable the protocol appears to be. The GMX V1 GLP pool had been operating for years and had undergone security audits, yet the vulnerability persisted undetected.

Getting Started Guide

If you want to protect yourself from reentrancy-related losses, here are the practical steps to take. First, understand which protocols you are using and whether they run on V1 or V2 versions. When protocols upgrade to V2, it is often because they have identified and fixed vulnerabilities in V1. Migrating to the newer version as soon as it is available and audited is generally the safer choice. Second, diversify your DeFi positions. Never put all your funds into a single protocol. If one protocol is exploited, you want your exposure limited to a fraction of your total portfolio. A good rule of thumb is to limit any single protocol to no more than 10-20% of your total DeFi allocation. Third, monitor your positions actively. Set up alerts for abnormal activity on protocols where you have funds deployed. Tools like DeFi Llama can track total value locked in protocols, and sudden drops may indicate an ongoing exploit. Fourth, check whether protocols you use have undergone recent security audits from reputable firms. While audits do not guarantee safety — as the GMX case shows — protocols that have not been audited at all carry significantly higher risk. Fifth, understand the concept of time-locked withdrawals. Some protocols implement delays on large withdrawals, giving the community time to detect and respond to potential exploits before funds are fully drained. Protocols with time locks offer an additional layer of protection.

Common Pitfalls

New DeFi users often make several mistakes that increase their vulnerability to exploits. The first is yield chasing — depositing funds into the protocol offering the highest returns without understanding the risks. Higher yields almost always come with higher risk, which may include unaudited contracts or experimental features. The second mistake is ignoring version numbers. When a protocol launches a V2, users who remain on V1 are often taking on unnecessary risk. V1 contracts are typically maintained for backwards compatibility but do not receive the same security attention as the current version. The third pitfall is failing to revoke token approvals. When you interact with a DeFi protocol, you typically grant it permission to spend your tokens. Even after you withdraw your funds, this permission may remain active. If the protocol is later exploited, attackers can potentially use these approvals to access your remaining tokens. Tools like Revoke.cash allow you to review and remove unnecessary approvals. The fourth mistake is relying solely on protocol reputation or total value locked as indicators of safety. The GMX exploit shows that even protocols with billions in TVL and established track records can harbor critical vulnerabilities.

Next Steps

Understanding reentrancy attacks is just the beginning of your DeFi security education. To deepen your knowledge, explore resources on other common smart contract vulnerabilities including flash loan attacks, oracle manipulation, and front-running. The Solidity documentation provides detailed explanations of each vulnerability type along with recommended mitigation patterns. For practical experience, platforms like Damn Vulnerable DeFi offer intentionally vulnerable smart contracts that you can practice exploiting in a safe environment. This hands-on approach builds intuition for how attacks work and what defensive patterns look like. Consider following security researchers and firms on social media — accounts like CertiK, Trail of Bits, and OpenZeppelin regularly publish educational content about emerging threats. Finally, stay informed about protocol upgrades and security incidents. The crypto security landscape evolves rapidly, and staying current is your best defense against becoming the next victim of a preventable exploit.

Disclaimer: This article is for educational purposes only and does not constitute financial or investment advice. Always conduct your own research before using any DeFi protocol.