Adithyan's Blog

How I Won 90 Days OSCP Lab Voucher for Free

I don't participate in giveaways as I and luck broke up a few years ago. But I do participate rarely in CTFs especially when the reward is an OSCP voucher.
On to the point, It was a pretty straight-forward CTF. I was given a web application and thank god it doesn't have any crypto, steg, etc. It was purely based on web attacks. But there was a catch. We have to solve it in 10 mins as only the first person to solve will get the voucher.
So, I visited the target URL
The target site is built with Python and Django Framework. Like everyone, it’s time to flood the server with gobuster.
Haha Gobuster go brrrr….
As soon I saw the flag endpoint, I visited it and got nothing, just an API endpoint. So I visited other endpoints such as login and report. The report endpoint was not accessible at that time.
So I moved on to the login endpoint and tried inspecting the cookies.
Login endpoint’s biscuits
At first, it seemed like a JWT token. Then, I examined the page source.
Looking at John, it struck me with two possibilities. One is, it might be referring to John for Bruteforce or the other might be the username for the login page. I tried both lol and to my surprise, both worked. First, I tried decoding the cookies by brute-forcing it with Flask-Unsign as the source also mentioned “Stop eating all the cookies”.
flask-unsign -u -c "eyJ1c2VybmFtZSI6IkFub255bW91c19Vc2VyIn0.X2h0pQ.BH7pliC3PH_YFeLJDEc2i_Uc7I4" --wordlist /home/kali/Desktop/rockyou.txt --no-literal-eval --threads 8
Bruteforce completed in 1 min 43 seconds
Flask-unsign gave me the secret key rogger555. Now using the same secret key, I re-encoded the message with {‘username’: ‘John’}
flask-unsign --sign --cookie "{'username': 'John'}" --secret 'rogger555' --legacy
Flask-unsign’s resigned key output
Copied the cookie and pasted it on the login page. It redirected me to the report page which was inaccessible earlier.
Report endpoint after pasting John’s cookie
Hmm, so it asks for a URL as input. So SSRF struck my find at first as a wise man once said,
I didn't have time to set up Burp collaborator, I ran ngrok ASAP.
Ngrok HTTP listening on 4444
I gave the https URL in the input box of the report endpoint and looked for the incoming request in the Ngrok dashboard.
Ngrok request inspection dashboard
Taking a look at the cookie received revealed it was different from what I sent. So, I copied this cookie and pasted it into the report endpoint.
Report endpoint asking me to GTFO
Still, it didn't show me anything. I was stuck here for about a minute and I visited other endpoints and refreshed them. Then finally…
API Endpoint revealing the flag
And that’s how I was able to win the OSCP challenge with 90 days of lab access. I thank the Secarmy, Offensive Security, the organizers, and everyone who supported me.