Developers often build applications that need to interact with the server’s underlying operating system (OS). Maybe it’s a tool to check network latency, manage files, or run a background script. It seems straightforward, but what if a user could trick your application into running their commands instead of the ones you intended?

This is the basis of a critical vulnerability known as Command Injection. It’s a classic attack, but one that continues to appear in applications today. Let’s break down what it is, how an attacker thinks about exploiting it, and most importantly, how to stop it cold.

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What is Command Injection?

Command Injection is a security flaw that allows an attacker to execute arbitrary operating system commands on the server running an application. The vulnerability occurs when an application takes user-supplied data and includes it directly in a command that is passed to the system’s shell for execution.

Think of your application as a helpful assistant and the server’s OS as a powerful tool. You ask the assistant to use the tool in a very specific way. But a clever attacker whispers extra instructions into your request, and the assistant, not knowing any better, passes them right along to the tool. The tool executes everything…