A directory traversal attack aims to access files and directories that are stored outside the intended folder. By manipulating files with "dot-dot-slash (../)" sequences and its variations, or by using absolute file paths, it may be possible to access arbitrary files and directories stored on the filesystem; including application source code, configuration, and other critical system files.
In this lesson, you will learn how directory traversal works and how to mitigate it in your application. You will first use a directory traversal attack to hack a vulnerable web server. We will then explain directory traversal by showing you the backend code of that vulnerable server. Finally, we will teach you how to prevent directory traversal from affecting your code.
Ready to learn? Buckle your seat belts, put on your hacker's hat, and let's get started!
The directory traversal vulnerability wears many faces. Some people also call it path traversal, path manipulation, dot-dot-slash, directory climbing, or the backtracking vulnerability. All of these are actually the same vulnerability.
To increase revenue and survive until the next funding round, a company called startup.io decided to create a side product. Since the market for image hosting platforms has recently become a bit saturated, the firm made a call to build an app for managing to-do lists instead.
Sadly, their to-do app is vulnerable to directory traversal attack. Let's use a terminal window and
curl to exploit the vulnerability. Our goal is to view the
/etc/passwd stored on the backend server.
The application is hosted on
https://todoapp.startup.io. First, let's try to curl a page we should have access to by running the following in the terminal:
We see the
about.html page returned, which is to be expected. Notice that this HTML page is being served from the public directory.
Essentially, the attack is accomplished by adding characters such as
../ into a URL that serves content from a directory structure. The content is usually served from a base directory, such as
/public. An attacker can supply filenames that contain
../ or a URL encoded equivalent
%2e%2e%2f. These URLs allow the attacker to break out of the base directory and view files stored in other folders on the filesystem.
A directory traversal attack which shows a hacker sending malicious payload to a server and accessing files which shouldn't be publicly accessible
To illustrate this, let's jump into the code. Below you will find the a function, which constructs a filesystem path from the URL. All files and directories returned by the function are served statically by the web server.
In PHP, the
include() function has a sibling:
require()! These functions are identical except for how they handle errors (require will stop the script, include will only flag that an error occurred). Both, however, are vulnerable to directory traversal!
We can use the “realpath” function to dereference the requested (relative) path, and return the absolute path of the file.
By defining a base path (or an array of allowable paths) in our program, we can compare them to the output of realpath and decide whether a request should be permitted or not. If the prefix paths do not match up, then our user could be attempting to access files they’re not supposed to!
include() will only be called if a given file is prefixed with “/todoapp/public/images”, and otherwise does nothing.
As presented in this lesson, directory traversal is a read-only vulnerability: it allows the attacker to read sensitive files. However, there is a more dangerous cousin in the directory traversal family tree. That cousin is called Zip Slip, and it allows the attacker to execute commands by overwriting files on a remote server. Sounds scary? It is! Check out more about Zip Slip on Snyk's Zip Slip research page.
To learn more about directory traversal, check out some other great content produced by Snyk:
Read our white paper on Zip Slip, a directory traversal vulnerability that results in remote code execution.
If the white paper got you worried, learn how to mitigate Zip Slip in your code-base with our cheat sheet.
Have a look at our YouTube video which further explains directory traversal and digs into a real-world example of directory traversal affecting a well-known open-source library.
You’ve learned what directory traversal is and how to protect your systems from it. We hope you will apply your new knowledge wisely and make your code much safer. Feel free to rate how valuable this lesson was for you and provide feedback to make it even better! Also, make sure to check out our lessons on other common vulnerabilities.