Quick Overview: File Upload Vulnerabilities

File upload vulnerabilities are the most common vulnerability found in web applications. This blog provides you with a guide to understanding file upload vulnerabilities that include an introduction to vulnerability, how to test, and prevention methods. This blog also recommends using PortSwigger Academy to learn upload vulnerabilities.

Brief Overview of file upload vulnerabilities

File upload vulnerabilities arise in web applications where there is an upload of some files (e.g. photos, resume, mark sheet, videos, etc.) on the application. If there is no validation related to the type of file while uploaded by the web server, there is a high chance of getting file upload vulnerabilities.

How to test file upload vulnerabilities

File upload vulnerabilities may be identified by using the following steps:

  1. Identifying functionality on the web applications where the user is providing external files to the web application. For instance, on the Update Profile webpage, a photo of the user needs to be uploaded.
  2. Upload file which is allowed by the web application. Note which type of files are allowed to upload on web applications.
  3. Now, Try to find a way to upload files not allowed by the web application. There are several test methods available to bypass upload restrictions on web applications.
  4. If found a way, try to execute those files and gain access to back end system.

Risks of identifying file upload vulnerabilities

Upload vulnerabilities are lethal for web applications and may compromise the whole back-end server.

Prevention of file upload vulnerabilities

There is enough literature available to mitigate file upload vulnerabilities. Here, I am listing prevention techniques of mitigations.

  1. Allow upload only extensions that are needed for functionality
  2. Check for file type by using different libraries as the Content-Type header may be spoofed.
  3. Ensure a limit on the size of the file
  4. Authenticate user before uploading on web applications
  5. Web applications must use programs to sanitize the uploaded malicious files
  6. Ensure the filename should be changed after uploaded on the web application
  7. Use a whitelist for file upload rather than a blacklist of file types

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Disclaimer: This tutorial is for educational purpose only. Individual is solely responsible for any illegal act.

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