Jai Sharma

Aug 1, 2021

3 min read

~/BugBounty/IDOR/”How I was able to exfiltrate any user’s credit coupons”

Hi guys, I will explain how a simple Verb Tampering led to credit-stealing IDOR via this article.

#whoami: JAI SHARMA | Part Time Open Source Security Researcher

To begin with, I will walk you through the target subdomains.

#target: BugCrowd Private Program

#In-scope: a.redacted.com, b.redacted.com

Subdomain: a.redacted.com, allows authenticated users to create discount Coupons for their individual business stores.

I looked for any loophole while creating the Coupons from a.redacted.com. However, got no luck.

After spending 2–3 hours on the same domain, I decided to move onto the next in-scope subdomain: b.redacted.com. I was doing a basic website crawling without prior user authentication.

Next, I tried to log in to the application with valid user credentials. While crawling the target, I found an interesting preflight API call:

Noticed anything? Yes, the user email id was sent via the OPTIONS method, resulting in a plain 204 response. Basically, it was triggered to ensure if the oncoming requests are safe or not.

The next thing I tried, to tamper with the OPTIONS verb, I changed it to GET, I got some strange response, which includes everything user: user@web.com has created in a.redacted.com — Credit Coupons, Credit History, Expired Coupons, etc.

At this point, I wanted to see, if by just manipulating the email id, if I can fetch data(from a.redacted.com) of other users.

I was able to pull that off. However, this requires an attacker to know the victim’s email and external_id. This seems doable. However, it was still incomplete.

I observed that the external_id value is an incremental value, which is unique for each unique user.

I found the same API call to be missing any rate limit. Well Okay! What else?

Surprisingly, the same external_id is shared once a password reset request was initiated.

Now all I have to do is, create another test account obtain the victim's email id and external ID(easy enumeration/exposed via password reset functionality). Using the same vulnerable request with tampered OPTIONS to GET Method.

And I can fetch the Coupons created in a.redacted.com by the victim, not only the currently active credit coupons, but also, expired coupons, current balance, and other transaction details.

Initially, it was triaged as a P3, later changed to P2 after seeing the potential impact.


Submitted on: 02 Jun 2021

VRT: Broken Access Control (BAC) > Insecure Direct Object References (IDOR)

Priority: P2

Triaged on: 07 Jun 2021

Rewarded on: 21 Jun 2021

PO Feedback:

If you make till here, thank you so much for giving it a read, do share your thoughts on this.

Thanks, lordjerry0x01, Abhinav_one & Abhind5 for the proofreading.

Connect with me on Twitter: https://twitter.com/ja1sharma