Horizontal Privilege Escalation on Quora which can compromise all users on Quora

Hey, I am SpyD3r(@TarunkantG) and in this blog, I will be discussing the bug I have found in Quora which can compromise all users on Quora due to Horizontal Privilege Escalation.

I worked more than 18 hrs a day and did it for a week to get familiar with all Quora functionality/workings, then finally I got two bugs 1 medium and 1 critical, here I will be discussing critical one only because I didn’t get the bounty for Medium one yet ;). It was really fun working with Quora because it has a lot of functionalities and this was my first bug (which comes out to be critical). I started finding the bug because I need to finance my site(https://spyclub.tech) because subscription gonna end in March. Eventually, now I can take a subscription for 10 years, so let’s start.

As I was testing each functionality, I was also figuring out which only thing can make a proper impact in the response like only m-b cookie is responsible for login, we can enumerate uid to get user details like Name, Profile pic etc.

For enumerating uid, I figured out that we can do it from kwargs.

json=%7B%22args%22%3A%5B%5D%2C%22kwargs%22%3A%7B%22uid%22%3A123456%7D ...

Changing the uid will let you know other user details. (BUT ONLY HERE?)
After digging more I also got to know that in the some of the places putting the uid on args, will also get you the same (COOL). Now let’s go forward.

When I was checking the functionality of sending the request for an Question, I saw this the request:

js_init=%7B%22asked_uid%22%3A365110562%2C%22viewer%22%3A730730786

Here, asked_uid is the uid of the person you asked and viewer is your uid, I tried changing viewer uid, but, obviously it didn’t worked(it’s not that easy IDOR). So I started checking here, which specific thing is responsible for telling that it’s me (Maybe I can overwrite that). My gut feeling was saying there can be a bug, because if we can enumerate uid to get the details, then why the same we can do here.
So I left this part here and went to find the solution for that, by checking other things. I observed that in the one place uid parameter is given at kwargs, then I remembered back that, we could have actually do it, yeah, then I went back and tried adding one more parameter(uid) in kwargs, but it didn’t worked, okay, now I tried with args and guess!! HURRAY!! The uid got overwrite.

json=%7B%22args%22%3A%5B<PUT_VICTIM_UID>%5D%2C%22kwargs%22%3A%7B ......

I was happy on that moment because this can be a bug because I can request for any Question to anyone on the Quora, also from account of most followed person, celeb etc.
I was doing this testing in my working days, so I had classes, so on the way to my college, one idea clicked that was, while giving Answer or Ask Question if I can overwrite uid. But the problem was I had to wait for 3hrs (because of my class). As the class ended, I ran for the lab, takeout the laptop and tried it and guess what!!! IT WORKED!!! I got the critical bug… Now, I can put answers to any questions, ask any questions from a different account.

Instantly, I reported this bug through HackerOne to Quora and they fixed it in less than 4hrs and awarded me the bounty next day.

Thanks for reading it, I hope you get fun reading.