What Is chroot in Linux and How Does It Work?
Are you looking to run applications or services in an isolated space on Linux? Or maybe you want to test and debug an application without it interfering with the rest of your system? If so, you’ll want to learn about chroot, a powerful mechanism built into Linux that allows you to create a separate environment within your system.
Let’s explore chroot in detail, learn about its benefits, use cases, how to set it up on a Linux system, and if needed, how to escape it.
What Is chroot in Linux?
chroot or change-root is arguably one of the easiest and ancient forms of containerization software that allow a user to safely sandbox applications and services. Sandboxing, in computing terms, is the process of isolating a program in a confined space with pre-defined resources.
If you’re familiar withDocker and how it works, you can think of chroot as a heavily stripped-down version of it. chroot works by changing the root directory of a program, restricting access and visibility, and thus providing an added layer of isolation and security.
Essentially, you create a separate directory, copy over all the program’s dependencies into the new directory, and then run the chroot command. This allows the program to function properly whilst having no access to the base file system.
chrooting a program is a great way of testing its reliability in a safe space without altering the actual system’s files. Moreover, you could also reduce the security risk caused by a compromised package as in a chrooted environment, the compromised package will not be able to access and modify sensitive system files.
The program will only be able to access and view files imported over to the chrooted directory, otherwise known as a “chroot jail”. This keeps the program and its sub-processes from interfering with the base system.
What Is a chroot Jail?
A chroot jail is an isolated environment where chroot programs reside and are executed. The term chroot jail is derived from the concept that the process and its sub-processes inside the chrooted environment have no access or visibility to the base file system and are trapped within the bounds of chroot with pre-determined resources.
Now that the concept of chroot is clear to you, let’s get hands-on and learn how you can create a chroot jail and spawn processes in it.
How to Create a chroot Jail and Execute Programs in It
A chroot jail is essentially a directory that contains all the required resources, files, binaries, and other dependencies a program would need to function properly.
However, unlike in a regular Linux environment, the chroot jail’s environment is highly restricted and the program can’t access external or additional files and system resources.
For instance, to run the Bash shell in a chroot jail, you would need to copy over the Bash binary and all its dependencies to the chroot directory.
Here are the steps to create a chroot jail and spawn a Bash shell:
That’s all the steps to spawn a chroot jail and run a program inside it.
How to Break Out of a chroot Jail
While chroot jails are good for testing unstable software, they’re also useful when handling SSH connections as chrooting connected users is one of the manyways to secure your SSH server.
On the flip side, if you’re a penetration tester and have logged into your target’s SSH server, finding it to be a chrooted environment can be frustrating and feel like a dead end.
However, there are quite a lot of ways to escape out of a misconfigured chroot jail, some requiring C programming skills while others can be automated with tooling. A few easy ways of chroot escape are:
Do note that to perform chroot escape using any of these methods, you will need to have escalated privileges in the system. Learn more about these escape methodologies by heading over tochw00t’s GitHub repository, a chroot escape automation tool.
Now You Know All About chroot in Linux
Now that you know what chroot is, how it works, and how to implement it, you should be able to sandbox applications effortlessly from the terminal. Keep in mind that, as you know, there are multiple ways to break out of a chrooted environment.
This goes to show that chroot was in no way designed to be implemented as a security feature. So you should use chroot with a lot of caution as an improper configuration may end up backfiring and compromise the safety of your system.
It’s best to stick to dedicated sandboxing solutions like containerization software or virtual machines. Both of them are centered around sandboxing and security, so unless there’s a new zero-day in town, you wouldn’t need to worry about the security and integrity of your system.
If you’re unsure which to choose between containerization and virtualization software, you should try out both and see for yourself which one is better.
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