So last night I played a CTF. Of course, it was free and with no prize. I know you are not here to listen my bla bla bla about my CTF journey, but wait brooo, I’ll share.
Actually, there is a web challenge named FLAG SHOP 2.0. This challenge was HARD level for those who don’t know about this vulnerability, but let me tell you how it looks like.
Challenge Overview
In this challenge, the application allows users to:
- Grab Voucher
- Check Voucher
- Buy Flag
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When you enter a name such as shadow_monarch and click on Buy Flag, the application tells you that you must collect 5 vouchers with the same name.
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Normal Behavior
First, enter the name shadow_monarch and click on Grab Vou{{Screenshot}}cher. You will receive one voucher code.
If you try grabbing another voucher using the same name, you get an error because the backend restricts this action to a single voucher per user.