Vishwa CTF: Web: Redeeem (440)

This is from the 2021 Vishwa CTF

For this challenge, we are given a URL:

Going to the page, we get this:

Entering a coupon code does not appear to do anything. If we click the “BUY” button, we get this:

Inspecting the source, we see a hidden element named “current” with a value of “0”. This appears to be how my current balance is tracked.

I modify that element’s value to “7000” and then click the “BUY” button:

That gives us the flag!