Website File Upload Security

TL;DR

Generally, a website shouldn’t directly see the full file path on your computer. However, they *do* receive the filename and file contents when you upload. Good websites will rename files server-side and store them in secure locations to prevent malicious access or information leaks.

Understanding What Happens When You Upload

When you click ‘Upload’, several things happen:

• Your browser prepares the file: It packages the file data (contents) along with some metadata, like the filename and file type.
• Data is sent to the website’s server: This happens over a secure connection (HTTPS is important!).
• The server receives the file: The website’s code then handles what to do with it.

Can They See My File Path?

No, not usually. Your browser prevents websites from directly accessing your local filesystem for security reasons. However…

What Information *Do* Websites Get?

1. Filename: The website will almost always receive the original filename you used on your computer (e.g., ‘report_2024.docx’).
2. File Contents: They get the actual data inside the file – the text, images, etc.
3. File Type (MIME type): Your browser tells the website what kind of file it is (e.g., ‘application/vnd.openxmlformats-officedocument.wordprocessingml.document’ for a Word document). This isn’t always reliable and can sometimes be faked, which is why websites need further checks.

How Websites Should Protect Your Uploads

A secure website will take these steps:

1. Server-Side Renaming: The most important step! They should immediately rename the file on their server to a random, unique name. This prevents attackers from predicting filenames and accessing files they shouldn’t.

   // Example PHP code (simplified)

   $originalFilename = $_FILES['file']['name'];
   $extension = pathinfo($originalFilename, PATHINFO_EXTENSION);
   $newFilename = uniqid() . '.' . $extension;
   move_uploaded_file($_FILES['file']['tmp_name'], 'uploads/' . $newFilename);

2. Secure Storage Location: Files should be stored outside the website’s public web directory. This prevents direct access via a URL.

   For example, instead of storing files in /var/www/html/uploads/ they might use /var/secure_storage/files/.

3. File Type Validation: They should verify the file type on the server-side (not just rely on what your browser says).

   // Example PHP code (simplified)

   $allowedTypes = ['pdf', 'doc', 'docx'];
   $fileExtension = strtolower(pathinfo($_FILES['file']['name'], PATHINFO_EXTENSION));
   if (!in_array($fileExtension, $allowedTypes)) {
     die('Invalid file type');
   }

4. Content Scanning: Some websites scan uploaded files for viruses or malicious code.

What Can *You* Do?

1. Only Upload to Trusted Websites: Be careful where you upload sensitive documents.
2. Check the Website’s Privacy Policy: See how they handle uploaded files.
3. Use Strong Passwords: Protect your account on the website.
4. Be Wary of Suspicious Links: Don’t click links in emails that ask you to upload documents to unknown websites.

In Summary

While a website can’t see the full path to your file, they receive the filename and contents. Reputable websites take steps to protect this information by renaming files and storing them securely. Always be cautious about where you upload sensitive data.