๐Ÿ›ก Deep Dive into Path Traversal with   ‘Send the Alien Back home’ย ๐Ÿ›ก

Hey Ninjas! ๐Ÿฅทย 

Welcome to our thrilling writeup for the "Send the Alien Back Home" CTF challenge! This series of challenges, hosted in our labs, is designed to test your cybersecurity mettle and sharpen your skills in identifying and exploiting path traversal vulnerabilities. ๐Ÿš€

If you are eager to dive into the action, access the challenges on our labs and website here: Access Send the Alien Back Home Challenges. In this writeup, we will navigate through each challenge, unraveling the secrets behind successful exploitation and the critical defense strategies to protect your systems. So gear up, get ready, and let's embark on this cybersecurity adventure together! ๐ŸŒŸ


  • Make sure your computer is connected to SNA Labs [Essential lab]


If you're unfamiliar with connecting to Challenge labs, we recommend you check out our Connecting to CTF Labs Guide for essential tips and guidelines. Always approach these challenges with a responsible and ethical mindset.

Before starting the writeup, let's clarify what path traversal vulnerability is:

Exploring Path Traversal:

Path traversal is a critical security vulnerability in web applications, where attackers can gain unauthorized access to files by exploiting inadequately sanitized user inputs. This issue arises when applications fail to properly validate or sanitize file paths provided by users, leading to potential access to sensitive areas of the server's file system.

In our 'Send the Alien Back Home' challenge series, you'll get hands-on experience with this concept, where each level intensifies in complexity. Through a series of engaging challenges, youโ€™ll learn all about path traversal vulnerabilities and how to protect against them.

๐ŸŒŸ Letโ€™s embark on this educational adventure together!

Challenge 1: The Home Intel :

After initiating the mission, let's first review the brief mentioned above. Pay close attention to the code within the brief as it holds key insights for our challenge.

$file_path = $_GET['file'];

function fetch_file($file_path)
        $output = file_get_contents($file_path);
        print $output;


In our challenge, we're exploring a scenario where the file parameter in a GET request is used to access files on the server. This technique can potentially expose sensitive files, depending on how the server handles these requests.

Also, take a look at the code snippet: file_get_contents($file_path);. This function is crucial, as it's used to retrieve the contents of the specified file. ๐Ÿง

Given this information, let's put our detective hats on ๐Ÿ•ต๏ธโ€โ™‚๏ธ and examine the website. Remember, the brief mentioned the location of the flag. With this key piece of information, we can strategize our approach to testing the website and ultimately capture that elusive flag. ๐Ÿšฉ Let's dive in and see what we can uncover!

Home page:


Get the file mentioned in the mission brief just by entering it in the url: ?file=/documents/zoraxians_home_planet.

๐ŸŽ‰ Hooray! We've got our first flag by passing the value of the file location of our flag /documents/zoraxians_home_planet. It turned out to be easier than expected to snag that flag, right? ๐Ÿ˜Š

๐Ÿ’ก Remember, this flag is uniquely generated for every user. Copying this flag won't work for you. The real goal here is to practice and tackle the challenge on your own. So, get ready to dive in and learn!

Are you excited for the next challenge? Let's keep up the momentum and move forward! ๐Ÿš€

Challenge 2: The Secret Project ๐Ÿ—‚๏ธ

Just like in our first mission, let's kick off this challenge by delving into the mission brief. Within it, we've uncovered an interesting PHP code snippet:

$directory_path = "uploads/";
$file_path = $directory_path . $_GET['file'];

function fetch_file($file_path)
    if (file_exists($file_path)) {
        $output = file_get_contents($file_path);
        echo $output;
    } else {
        echo $file_path . " " . "not found";

Check out this key variable in the code: $directory_path = "uploads/";. It's quite similar to what we encountered in the first challenge, involving the file parameter in a GET request and using the file_get_contents function. Plus, we've got a clue about the location of our flag! ๐Ÿ•ต๏ธ Let's take this information and test it out on the website.


Voila! ๐ŸŽ‰ We've successfully captured the second flag, located in the 'uploads' directory. By cleverly navigating to file=../../../../media/usb/project_51, we uncovered it with ease. Remember, each challenge progressively becomes more intricate.๐Ÿš€

Great job on the puzzles so far!, But guess what? There's a whole bunch of new challenges coming up, and this time, you're on your own!. Thereโ€™s nothing like the feeling of solving tough puzzles on your own.

Defense Measures:

๐Ÿ”’ Defending against such advanced path traversal attempts is a complex task. It requires a comprehensive approach, encompassing thorough input validation, constant security protocol updates, and a deep understanding of the underlying server and application architecture. Regular security audits and penetration testing are crucial in identifying potential bypass techniques and fortifying defenses against them.


For those keen on understanding advanced path traversal defense strategies, OWASPโ€™s Advanced Path Traversal Guide is an excellent resource. It delves into sophisticated attack vectors and provides guidance on creating robust security frameworks to counter them.


Our journey through 'Send the Alien Back Home' has equipped us with a deeper understanding of path traversal vulnerabilities. Each challenge, varying in difficulty, provided unique insights and strategies. Thanks for joining us on this adventure, and stay tuned for more CTF challenges!

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