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Web Dev Two

File Uploads

PHP makes it easy to upload files to our web server by way of an HTML form.

The upload files are stored in temporary directory with file details stored in a $_FILES super-global.

Table of Contents

  1. Introduction
  2. Objectives
  3. File Upload Forms
  4. File Upload Meta-Data
  5. Saving an Uploaded File
  6. Security Implications
  7. Testing For Images
  8. Filtering Out Non-Image Uploads

Introduction

Introduction

PHP makes it easy to upload files to our web server by way of an HTML form.

The upload files are stored in temporary directory with file details stored in a $_FILES super-global.

Objectives

Upon completion of this module, you should be able to:

  • Create HTML forms configured to allow for file uploads.
  • Retrieve upload details from the $_FILES super-global.
  • Move uploaded file from temporary storage to a known file-system location.
  • Perform security related tests on uploaded files.

File Upload Forms 

<!DOCTYPE html>
<html>
    <head><title>File Upload Form</title></head>
<body>
    <form method="post" enctype="multipart/form-data">
        <label for="image">Image Filename:</label>
        <input type="file" name="image" id="image">
        <input type="submit" name="submit" value="Upload Image">
    </form>
</body>
</html>

This form will allow users to browse for a single local file and upload it when the form is submitted.

Note that the form element has an extra attribute named enctype that has been set to multipart/form-data. This attribute is required in order for the upload dialog to function properly. Also required is a method attribute with a value of post.

Resources

File Upload Meta-Data

Details about each file input in our HTML form will be accessibly by name in the $_FILES super-global.

The name attribute of our file input in the previously shown form was image, so we look in $_FILES['image'].

<?php if (isset($_FILES['image']) && $_FILES['image']['error'] > 0): ?>

    <p>Error Number: <?= $_FILES['image']['error'] ?></p>

<?php elseif (isset($_FILES['image'])): ?>

    <p>Client-Side Filename: <?= $_FILES['image']['name'] ?></p>
    <p>Apparent Mime Type:   <?= $_FILES['image']['type'] ?></p>
    <p>Size in Bytes:        <?= $_FILES['image']['size'] ?></p>
    <p>Temporary Path:       <?= $_FILES['image']['tmp_name'] ?></p>

<?php endif ?>

Tips & Gotchas:

  1. The file metadata is not accessed through the $_POST super-global, but instead through the $_FILES super-global.

  2. The uploaded file will be deleted from the temporary path automatically after this script finishes executing.

  3. $_FILES["image"]["error"] is an error number:

  • 0: No error. File upload successful.
  • 1: Maximum file-size, as specified in php.ini, exceeded.
  • 4: No file uploaded with POST. Not actually an error for forms where file uploads are optional.

Resources

Saving an Uploaded File

In order to save an uploaded file your PHP script must move it to a known location on your file-system.

<?php
    function file_upload_path($original_filename, $upload_subfolder_name = 'uploads') {
       $current_folder = dirname(__FILE__);
       $path_segments = [$current_folder, $upload_subfolder_name, basename($original_filename)];
       return join(DIRECTORY_SEPARATOR, $path_segments);
    }

    $image_upload_detected = isset($_FILES['image']) && ($_FILES['image']['error'] === 0);

    if ($image_upload_detected) {
        $image_filename       = $_FILES['image']['name'];
        $temporary_image_path = $_FILES['image']['tmp_name'];
        $new_image_path       = file_upload_path($image_filename);

        move_uploaded_file($temporary_image_path, $new_image_path);
    }
?>

Resources

Security Implications

Vulnerabilities Make Me Scream

By allowing file uploads we are giving our users write-access to our file-system. This can be a very dangerous move.

Imagine the following scenario:

  1. Attacker creates a malicious PHP script named p0wn.php.
  2. Attacker uploads this script using our PHP upload form, causing the file to be moved to known location.
    For example, the file might be moved to a /uploads/ folder.
  3. Attacker accesses the uploaded script at the known location by URL:
    http://host.ca/uploads/p0wn.php

An example p0wn.php file:

<?php system($_GET['command']) ?>

This script would allow a hacker to execute any command on your system under the same security permissions as the Apache user. Here is our hacker deleting everything in the current directory:

http://host.ca/uploads/p0wn.php?command=rm%20-rf%20.

(Remember that %20 in a URL means a space.)

Testing For Images

File uploads can be checked for “image-ness” by looking at the temporary file path, the potential new path, and the file mime-type.

<?php
    function file_is_an_image($temporary_path, $new_path) {
        $allowed_mime_types      = ['image/gif', 'image/jpeg', 'image/png'];
        $allowed_file_extensions = ['gif', 'jpg', 'jpeg', 'png'];

        $actual_file_extension   = pathinfo($new_path, PATHINFO_EXTENSION);
        $actual_mime_type        = getimagesize($temporary_path)['mime'];

        $file_extension_is_valid = in_array($actual_file_extension, $allowed_file_extensions);
        $mime_type_is_valid      = in_array($actual_mime_type, $allowed_mime_types);

        return $file_extension_is_valid && $mime_type_is_valid;
    }
?>

Resources

Filtering Out Non-Image Uploads

If you are only accepting image uploads, then only move the uploaded file when it’s an image:

<?php
    $image_upload_detected = isset($_FILES['image']) && ($_FILES['image']['error'] === 0);

    if ($image_upload_detected) {
        $image_filename       = $_FILES['image']['name'];
        $temporary_image_path = $_FILES['image']['tmp_name'];
        $new_image_path       = file_upload_path($image_filename);

        if (file_is_an_image($temporary_image_path, $new_image_path)) {
            move_uploaded_file($temporary_image_path, $new_image_path);
        }
    }
?>