What is code injection? | Tutorial & examples

Code injection is a type of attack that allows an attacker to inject malicious code into an application through a user input field, which is then executed on the fly. Code injection vulnerabilities are rather rare, but when they do pop up, it is often a case where the developer has attempted to generate code dynamically. Preventing code injection attacks usually comes down to reconsidering the need to dynamically execute code, especially where user input is involved.

In this lesson, you will learn how code injection works and how to protect your applications against it. We will begin by exploiting a code injection vulnerability in a simple application. Then we will analyze the vulnerable code and explore some options for remediation and prevention.

Ready to learn? Buckle your seat belts, put on your hacker's hat, and let's get started!

Let's set the scene. You're a 9-5 developer working for BigCorp and it's 3:30 p.m. on a Friday. The coffee is running hot but your eyes still feel heavy. One more hour and you'll be git pushing yourself all the way home for some corn chips and late-night Call of Duty.

All you need to do is make a string uppercase and greet the customer in onboarding. You've heard something about eval() being bad, but right now you just want to get the job done. You take a deep breath and blow on your piping hot coffee. Let's do this.

Your fingers glide across the keyboard, producing some sub-par code. It's not your best work but it works, and those corn chips are calling you. "That should do the trick", you mumble to yourself. Git add, git commit, git push, git out of here! You turn your phone off for the weekend and go hiking with friends.

Copy and paste the following into the terminal and hit enter:

curl https://bigcorp.com/customerOnboarding?name=Edgar

It's 9 o'clock on Saturday night and Dimitri the attacker wakes up at his desk for another night of hacking. It's a cold night, perfect black hoodie weather. Ooooh, BigCorp has made some changes to their website, what's this? A function that converts a name to uppercase? Dimitri enters his hacker alias by navigating to curl https://bigcorp.com/customerOnboarding?name=Edgar.

Out of habit, Dimitri injects a double quote (") to see if it triggers an error, and sure enough, it does.

Copy and paste the following into the terminal and hit enter:

curl https://bigcorp.com/customerOnboarding?name=Edgar"

Dimitri strokes his beard and raises one eyebrow, why would a double quote cause the application to throw an exception? Could it be because this is a string that is inside an eval() statement? To test it out, he tests a code injection payload, which he hopes will return the Node.js version being used.

Copy and paste the following into the terminal and hit enter:

curl https://bigcorp.com/customerOnboarding?name=";process.version//

Yes, it looks like you have achieved code injection, Dimitri, and you've also discovered that Node.js v16.9.1 is being used on the server. Now that we have code injection, have a think about what other nefarious things we could do. Read arbitrary files, including configs? Read environment variables containing credentials? Execute commands? Delete every file on the server? The possibilities are endless.

Let's jump into the nitty-gritty details of how this code injection works:

As an attacker, we were able to determine that our input was being injected directly into a Node.js eval() function.

The eval function takes a string of JavaScript code and executes it.

We escaped out of the existing string by adding "; at the start of our payload.

Any text after the initial "; will be interpreted as server-side JavaScript code, as an example we accessed the process.version variable using this payload: ";process.version//.

The // at the end of the line is simply to avoid syntax errors from trailing characters.

And now we will have a look at this example in more detail by going through the server-side code.

If a code injection vulnerability exists in an application, the security impact is that an attacker is able to execute arbitrary server-side code.

The ability to execute server-side code can result in a total loss of integrity, availability, and confidentiality within the application. An attacker may also abuse a code injection vulnerability to execute terminal commands on that server and pivot to adjacent systems.

Today, code injection is not a very common vulnerability. There is rarely a situation where it would make sense to inject user input into an eval() function. The use of eval() is typically an indication of bad software design, and there is usually a better alternative.

In general, the proliferation of server-side web frameworks and libraries over the last decade has also decreased the need to custom code many tasks, reducing the likelihood that vulnerabilities will be introduced, and increasing the ease of writing sound, secure code.

Having said this, occasionally code injections can still make their way into popular libraries. One pertinent example is in the LinkedIn DustJS templating engine in 2016. If you are interested, check out the full details of the vulnerability.

Despite the rarity of code injection vulnerabilities, they are included in common checks because of their critical severity.

Perhaps the most straightforward of all prevention techniques is to reconsider the need to evaluate any dynamically generated server-side code. Usually, this dynamic code execution is the result of poor software design rather than necessity, so it's always best to consider other ways to achieve the task.

If the interpreter you are using allows, lock it down by limiting the functionality to the minimum amount required by your application in the server configuration. If you are using PHP, one method is to use the disable_functions directive within the php.ini file. This will instruct the PHP server to ignore any instances of the defined functions. Of course, this will also mean that those functions can not be used at all on that server.

Adding a static application security testing (SAST) tool to your devops pipeline as an additional line of defence is an excellent way to catch vulnerabilities before they make it to production. There are many, but Snyk Code our personal favorite, as it scans in real-time, provides actionable remediation advice, and is available from your favorite IDE.

In Python, avoid the use of eval() or exec() especially when dealing with user input.

For example, instead of the vulnerable example from the interactive example above, which looks like this:

uppercaseName = eval('"' + name + '"' + '.upper()')

We could remove the eval() function, and simplify our code like this:

uppercaseName = name.upper()

Admittedly this is quite a simple example, but the point is that the vulnerability arises because dynamic data is being passed into the eval() function.

Use a security linter. If you are using Python developer, there's a good chance that you're already using linters to enforce the uniformity of your code, why not use one to improve security, too?

That's where flake8-bandit comes in. In order to install it, make sure you have flake8 installed, then run the following command:

pip install flake8-bandit

Then select the new bandit linter in your IDE of choice, or run bandit myapp.py from the terminal.

This plugin contains rules that will test your code for vulnerabilities. It's important to note that linters like these will not catch everything. But they are a good sanity check, and the more layers of checks we have the less likely you are to introduce vulnerabilities.

To learn more about code injection, check out some other great content:

• Have a look at the OWASP code injection page that goes over some other examples of code injection.

• We all love cheatsheets, to check this one here, where you can read about other types of injection and how to prevent them.

• If you loved the lesson, don't forget to check out our article about command injection.