Defending Against File Upload Vulnerabilities

Essential Strategies for File Upload Vulnerability Protection

4 min readOct 14, 2023

File upload vulnerabilities represent a critical threat to web servers. These vulnerabilities occur when a web server allows users to upload files without adequately verifying essential attributes such as file name, type, contents, or size. When these validations are not properly enforced, it creates an avenue for potentially harmful files, including scripts that can execute code remotely.

In simple terms, servers typically don’t thoroughly check what you’re uploading. For instance, if you intend to upload an image, an attacker might exploit a PHP vulnerability to upload a PHP payload and retrieve valuable data.

Understanding the Origins of File Upload Vulnerabilities

Developers commonly strive to establish robust validation mechanisms. However, there are several common pitfalls:

  1. Inadequate Blacklisting: Developers may attempt to blacklist dangerous file types, but they might overlook parsing discrepancies when checking file extensions. Additionally, obscure yet potentially harmful file types can be inadvertently omitted from the blacklist.
  2. Manipulation of File Properties: Some websites try to verify file types by inspecting properties that can be manipulated by attackers using tools like Burp Proxy or Repeater.
  3. Inconsistent Validation: Even when robust validation measures are in place, they may not be consistently applied across the entire network of hosts and directories, resulting in exploitable discrepancies.

This is honest review of a server after these type of vulnerabilty :-

Exploiting Unrestricted File Uploads: Creating Web Shells

The most severe scenario arises when a website permits the upload and execution of server-side scripts such as PHP, Java, or Python files. This enables the creation of a web shell, which is a malicious script that allows an attacker to execute commands on the remote web server through HTTP requests.

Upon successful upload of a web shell, the attacker gains complete control over the server. This includes the ability to read and write arbitrary files, extract sensitive data, and employ the server for attacks on both internal and external infrastructure.

Example PHP Web Shell:

<?php echo file_get_contents('/path/to/target/file'); ?>

The above PHP script, when uploaded, enables an attacker to read arbitrary files from the server’s filesystem.

A more versatile web shell might look like:

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

This script facilitates the passing of an arbitrary system command via a query parameter.

Flaws in File Type Validation: Acquiring Web Shells

While it’s unusual to find a website completely devoid of protections against file upload attacks, existing defenses may still have vulnerabilities.

Flawed File Type Validation

When submitting forms, the browser sends data in a POST request. For binary data (e.g., image or PDF files), the content type multipart/form-data is used.

We can visualize this in a sample request:

POST /images HTTP/1.1
Content-Length: 12345
Content-Type: multipart/form-data; boundary=---------------------------012345678901234567890123456---------------------------012345678901234567890123456
Content-Disposition: form-data; name="image"; filename="example.jpg"
Content-Type: image/jpeg
[...binary content of example.jpg...]
Content-Disposition: form-data; name="description"This is an interesting description of my image.
Content-Disposition: form-data; name="username"anonymous---------------------------012345678901234567890123456--

This request contains separate parts for each form input. Each part provides information about the input field it corresponds to, including its Content-Type header.

Pitfalls of Implicit Trust in Content-Type Headers

Websites may attempt to validate uploads by checking if the input-specific Content-Type header matches an expected MIME type. However, if the server implicitly trusts this header and doesn't validate the actual contents of the file, this defense can be easily bypassed using tools like Burp Repeater.

Protecting Against File Upload Vulnerabilities

To fortify against file upload vulnerabilities, consider implementing the following measures:

  1. Whitelisting: Rather than blacklisting dangerous file types, consider allowing only specific, safe formats.
  2. Server-Side Validation: Conduct server-side validation to cross-verify the file type with its actual content.
  3. File Sandboxing: Store uploaded files in a separate, restricted directory with limited permissions.
  4. Server-Side Validation: Conduct server-side validation to cross-verify the file type with its actual content.
  5. Regular Security Audits: Continuously monitor and audit your application for potential vulnerabilities.
  6. Educate Developers: Provide training on secure coding practices and common pitfalls related to file uploads.

By adhering to these practices, you can significantly reduce the risk of file upload vulnerabilities and enhance the security of your web application.

If you want to do a practical on your own, go to PortSwigger and search for this topic. You can find a lab on this topic. Let me provide you with the link; it’s free, so just go ahead and start. But before that, let me give you a simple tip: you’ll need Burp Suite.

Keep the bytes all Bytebusters

BBX signing off..




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