Tree Viewer

Overview
Background
Enumeration
Exploitation
Conclusion

Overview

360 solves / 50 points
Difficulty: Intro
Overall difficulty for me (From 1-10 stars): ★☆☆☆☆☆☆☆☆☆

Background

Author: Eteck#3426

Here, you can check the content of any directories present on the server.

Find a way to abuse this functionality, and read the content of /home/flag.txt

Enumeration

Home page:

In here, we can view the source code, and an input box, which allows us to check a directory.

Let’s look at the source code:

When the source GET parameter is provided, it’ll highlight the index file.

<?php
$parsed = isset($_POST['input']) ? $_POST['input'] : "/home/";

preg_match_all('/[;|]/m', $parsed, $illegals, PREG_SET_ORDER, 0);
if($illegals){
    echo "Illegals chars found";
    $parsed = "/home/";
}

if(isset($_GET['source'])){
    highlight_file(__FILE__);
}
?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Tree Viewer</title>
</head>
<body>
    <a href="/?source">Source code</a>
    <hr/>
    <form action="/" method="post">
        <label for="input">Directory to check</label>
    <input type="text" placeholder="Directory to see" id="input" name="input" value="<?= $parsed ?>">
    </form>

    <h3>Content of <?= $parsed ?>: <?= shell_exec('ls '.$parsed); ?></h3>
    
</body>
</html>

Let’s break it down!

When input POST parameter is provided, it’ll check the input contains ; OR | character via regular expression (regex). If no input parameter is provided or it contains ; OR |, default value will be /home/.

Finally, it’ll parse our input to a shell_exec() function, which will execute shell command!

Nice, we found a sink (Dangerous function)!

Let’s look at the shell_exec() function:

<?= shell_exec('ls '.$parsed); ?>

This function will execute ls <path>!

That being said, although it has a regex filter, it’s still vulnerable to OS command injection!

Exploitation

To bypass it, I’ll use the new line character \n (%0a in URL encoding)!

%0aid

Also, I’ll be using Burp Suite’s Repeater to send the payload:

Boom! We have Remote Code Execution (RCE)!

Let’s read the flag!

%0acat /home/flag.txt

Flag: PWNME{U53R_1NpU75_1n_5h3lL_3x3c_77}

Conclusion

What we’ve learned:

Exploiting OS Command Injection & Bypassing Filters

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