Advent of Cyber 2022| [Day15]| TryHackMe write-up

Amol Rangari
3 min readDec 16, 2022

Learning Objectives

  • Input validation of file upload funtionality
  • Unrestricted file upload vulnerabilities
  • Phishing through file uploads
  • How to properly secure file upload functionality

The Unrestricted in Unrestricted File Uploads

The ability to upload files to a server has become integral to how we interact with web applications. Just think of file uploads like a profile picture for a social media website, a report being uploaded to cloud storage, or saving a project on GitHub; the applications for file upload features are limitless.

Unfortunately, when poorly handled, file uploads can also open up severe vulnerabilities in the server. This can lead to anything from relatively minor nuisance problems; all the way up to full Remote Code Execution (RCE) if an attacker manages to upload and execute a shell. With unrestricted upload access to a server (and the ability to retrieve data at will), an attacker could deface or otherwise alter existing content — up to and including injecting malicious webpages, which lead to further vulnerabilities such as Cross-Site Scripting (XSS) or Cross-Site Request Forgery (CSRF). By uploading arbitrary files, an attacker could potentially use the server to host and/or serve illegal content or to leak sensitive information. Realistically speaking, an attacker with the ability to upload a file of their choice to your server — with no restrictions — is very dangerous indeed.

Unrestricted File Uploads usually have two main exploitation paths:

  • If the attacker can retrieve the uploaded file, it could lead to code execution if the attacker uploads a file such as a web shell.
  • If the file is viewed by a user, think of a CV application, then an attacker could embed malware in the uploaded file that would execute on the user’s workstation once they view the file.

There has been quite a lot of focus on RCE through web shells in previous rooms, so in this task, we will focus on the latter exploitation path.

Santa is Looking for a Sidekick

Santa is looking to hire new staff for his security team and has hired a freelance developer to create a web application where potential candidates can upload their CVs. Elf McSkidy is aware that third-party risks can be serious and has tasked you, Exploit McRed, with testing this application before it goes live. Since the festivities are right around the corner, we will have to focus on the core feature of the website, namely the ability to upload a CV.

Elf McSkidy has provided you with the latest version of the application. Start the machine attached to this task to load the website. Once loaded (roughly 2 minutes), you can navigate to http://MACHINE_IP/ to view the website, using either the AttackBox of your TryHackMe VPN connection:

Follow me on Medium and stay tuned for more write-ups.

https://linktr.ee/alexhack

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Amol Rangari

I am Cyber Security Expert, Security Researcher and bug hunter