Webhacking.kr old-20

Challenge

Submit the captcha before the two-second deadline.

在两秒时限内提交验证码。

https://webhacking.kr/challenge/code-4/

Analysis

GET 响应同时给出本次页面实例的 captcha 和 st Cookie。服务端用 st 判断时限，并要求 POST 的 captcha 与本次页面生成的值相同；因此 GET 和 POST 必须使用同一个 requests.Session。不能预先保存验证码，也不能把下一次 GET 得到的 st 与旧验证码混用。

Solution

下面的脚本完整完成一次 GET、解析 captcha、复用 Cookie 并构造 POST。它只打印响应，不包含账号或固定 session 值：

import re

import requests

URL = "https://webhacking.kr/challenge/code-4/"
CAPTCHA_PATTERN = re.compile(
    r"<input[^>]+name=[\"']?captcha_[\"']?[^>]+value=[\"']([^\"']+)[\"']",
    re.IGNORECASE,
)

def extract_captcha(html):
    match = CAPTCHA_PATTERN.search(html)
    if not match:
        raise RuntimeError("captcha input was not found")
    return match.group(1)

def solve_once():
    with requests.Session() as session:
        response = session.get(URL, timeout=10)
        response.raise_for_status()

        captcha = extract_captcha(response.text)
        if "st" not in session.cookies:
            raise RuntimeError("st cookie was not set by the GET response")

        post_response = session.post(
            URL,
            data={"id": "demo", "cmt": "hello", "captcha": captcha},
            timeout=10,
        )
        post_response.raise_for_status()
        return post_response

if __name__ == "__main__":
    result = solve_once()
    print(result.status_code)
    print(result.text)