File upload vulnerabilities are when a web server allows users to upload files to its filesystem without sufficiently validating things like their name, type, contents, or size. Failing to properly enforce restrictions on these could mean that even a basic image upload function can be used to upload arbitrary and potentially dangerous files instead. This could even include server-side script files that enable remote code execution.

Exploiting unrestricted file uploads to deploy a web shell

The worst possible scenario is when a website allows you to upload server-side scripts, such as PHP, Java, or Python files, and is also configured to execute them as code. This makes it trivial to create your own web shell on the server.

<aside> 💡 Web shell

A web shell is a malicious script that enables an attacker to execute arbitrary commands on a remote web server simply by sending HTTP requests to the right endpoint.

</aside>

Flawed file type validation

When submitting HTML forms, the browser typically sends the provided data in a POST request with the content type application/x-www-form-url-encoded. This is fine for sending simple text like your name or address. However, it isn't suitable for sending large amounts of binary data, such as an entire image file or a PDF document. In this case, the content type multipart/form-data is preferred with a POST request structure like this one:

POST /images HTTP/1.1
    Host: normal-website.com
    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...]

    ---------------------------012345678901234567890123456
    Content-Disposition: form-data; name="description"

    This is an interesting description of my image.

    ---------------------------012345678901234567890123456
    Content-Disposition: form-data; name="username"

    wiener
    ---------------------------012345678901234567890123456--

As you can see, the message body is split into separate parts for each of the form's inputs. Each part contains a Content-Disposition header, which provides some basic information about the input field it relates to. These individual parts may also contain their own Content-Type header, which tells the server the MIME type of the data that was submitted using this input.

One way that websites may attempt to validate file uploads is to check that this input-specific Content-Type header matches an expected MIME type. If the server is only expecting image files, for example, it may only allow types like image/jpeg and image/png. Problems can arise when the value of this header is implicitly trusted by the server. If no further validation is performed to check whether the contents of the file actually match the supposed MIME type, this defense can be easily bypassed using tools like Burp Repeater.