What Is a “Re-Entrancy Attack” and Why Is It a Common Smart Contract Vulnerability?

A re-entrancy attack is a critical smart contract vulnerability where an external malicious contract repeatedly calls back into the original vulnerable contract before the original contract has finished updating its state (e.g. updating a balance). This allows the attacker to drain funds multiple times.

It is common because developers often overlook the potential for external calls to hijack the execution flow, making it a frequent target for exploitation.