PHP: Risks with User Uploaded Text Files

TL;DR

Yes, there are risks! Even simple .txt files sent as strings to PHP can cause problems. This guide shows you how to handle them safely.

Understanding the Risks

While a text file seems harmless, it’s still data that could be misused. Here’s what you need to worry about:

• Path Traversal: A malicious user might try to include directory navigation characters (like ../) in the filename or content to access files outside your intended upload folder.
• Code Injection: Although less likely with plain text, it’s possible a user could craft a file that, when processed by PHP later on, executes unwanted code if not handled correctly.
• Denial of Service (DoS): Very large files can overwhelm your server’s resources.

Solution: Safe Handling of User Uploaded Text Files

Here’s a step-by-step guide to protect yourself:

1. Validate File Extension

1. Check the extension *before* any processing. Don’t rely on client-side validation; it can be easily bypassed.
2. Use PHP’s pathinfo() function to get the file extension and compare it against an allowed list.

2. Sanitize the Filename

1. Remove or replace potentially dangerous characters from the filename. This prevents path traversal attacks.
2. Use preg_replace() to strip out unwanted characters.

3. Secure Storage Location

1. Store uploaded files outside your web root directory if possible. This prevents direct access to the files via a web browser.
2. If you *must* store them within the web root, ensure they are not directly executable (e.g., no .php extensions allowed).

4. Limit File Size

1. Set a maximum file size limit in your PHP configuration (php.ini) and validate it within your script.
2. Use the $_FILES['userfile']['size'] variable to check the file size.

 $max_filesize) {
  die('File too large.');
}
?>

5. Read File Content Safely

1. Don’t directly include or execute the file content. If you need to process it, read it as a string and sanitize it before using it in any operations.
2. Use file_get_contents() to read the file into a variable.
3. If you’re displaying the content on a webpage, use htmlspecialchars() to escape special characters and prevent cross-site scripting (XSS) attacks.

6. Consider a Random Filename

1. Generate a unique, random filename for each uploaded file to further prevent potential attacks and collisions.
2. Use functions like uniqid() or md5(rand()) to create the random name.