When a blockchain platform like Ethereum is being used to process transactions, it is important to consider the possibility of reentrancy attacks. A reentrancy attack is when a malicious actor is able to call a function multiple times in quick succession, before the first call has had a chance to complete.

This can allow the attacker to siphon off funds, or otherwise tamper with the normal functioning of the blockchain.

Reentrancy attacks are made possible by the fact that many Ethereum smart contracts are written in a language called Solidity, which allows for functions to be called recursively. In other words, a function can call itself, or call another function, and so on.

This can be useful in some cases, but it also opens up the possibility for an attacker to abuse this feature.

There have been a few high-profile cases of reentrancy attacks in the past, most notably the one that resulted in the theft of over $50 million worth of ether from The DAO in 2016. Thankfully, there are now some best practices that developers can follow to avoid falling victim to such attacks.

In short, a reentrancy attack on Ethereum is when a malicious actor is able to exploit the recursive nature of Solidity functions in order to tamper with transactions or otherwise disrupt the normal functioning of the blockchain. These attacks can be prevented by following best practices when writing smart contracts.