[De1CTF 2019] Write up

过几天有这个比赛  （看V&N大佬秒题） （又是零解的一天）

[De1CTF 2019]SSRF Me

这题BUU上只有1分。。。（我不会）
打开靶机，直接给了源码。美化一下：

#! /usr/bin/env python
#encoding=utf-8
from flask import Flask
from flask import request
import socket
import hashlib
import urllib
import sys
import os
import json
reload(sys)
sys.setdefaultencoding('latin1')
app = Flask(__name__)
secert_key = os.urandom(16)
class Task:
    def __init__(self, action, param, sign, ip):
        self.action = action
        self.param = param
        self.sign = sign
        self.sandbox = md5(ip)
        if(not os.path.exists(self.sandbox)):          #SandBox For Remote_Addr
            os.mkdir(self.sandbox)
    def Exec(self):
        result = {}
        result['code'] = 500
        if (self.checkSign()):
            if "scan" in self.action:
                tmpfile = open("./%s/result.txt" % self.sandbox, 'w')
                resp = scan(self.param)
                if (resp == "Connection Timeout"):
                    result['data'] = resp
                else:
                    print resp
                    tmpfile.write(resp)
                    tmpfile.close()
                result['code'] = 200
            if "read" in self.action:
                f = open("./%s/result.txt" % self.sandbox, 'r')
                result['code'] = 200
                result['data'] = f.read()
            if result['code'] == 500:
                result['data'] = "Action Error"
        else:
            result['code'] = 500
            result['msg'] = "Sign Error"
        return result
    def checkSign(self):
        if (getSign(self.action, self.param) == self.sign):
            return True
        else:
            return False
#generate Sign For Action Scan.
@app.route("/geneSign", methods=['GET', 'POST'])
def geneSign():
    param = urllib.unquote(request.args.get("param", ""))
    action = "scan"
    return getSign(action, param)
@app.route('/De1ta',methods=['GET','POST'])
def challenge():
    action = urllib.unquote(request.cookies.get("action"))
    param = urllib.unquote(request.args.get("param", ""))
    sign = urllib.unquote(request.cookies.get("sign"))
    ip = request.remote_addr
    if(waf(param)):
        return "No Hacker!!!!"
    task = Task(action, param, sign, ip)
    return json.dumps(task.Exec())
@app.route('/')
def index():
    return open("code.txt","r").read()
def scan(param):
    socket.setdefaulttimeout(1)
    try:
        return urllib.urlopen(param).read()[:50]
    except:
        return "Connection Timeout"
def getSign(action, param):
    return hashlib.md5(secert_key + param + action).hexdigest()
def md5(content):
    return hashlib.md5(content).hexdigest()
def waf(param):
    check=param.strip().lower()
    if check.startswith("gopher") or check.startswith("file"):
        return True
    else:
        return False
if __name__ == '__main__':
    app.debug = False
    app.run(host='0.0.0.0')

发现是python写的，对代码进行简单审计发现，首先传入一个param，/geneSign路由会返回的一个sign

通过cookie得到一个sign，到时候回传到exec()中去跟getSign()产生的sign校验，所以这里直接传一个getSign()产生的sign。通过cookie传入一个action参数，看后面的Exec()可以知道action="readscan"。根据提示param=flag.txt，所以这个sign应该是md5('xxxflag.txtreadscan')，但是action在geneSign()写死了为"action"。

• 不妨假设 secert_key 是 xxx ，那么在开始访问 /geneSign?param=flag.txt 的时候，返回的 md5 就是 md5('xxx' + 'flag.txt' + 'scan') ，在 python 里面上述表达式就相当于 md5(xxxflag.txtscan) ，这就很有意思了。

• 直接构造访问 /geneSign?param=flag.txtread ，拿到的 md5 就是 md5('xxx' + 'flag.txtread' + 'scan')

拿到的md5放到cookie中，Cookie: action=readscan，sign=md5
 
在其他师傅的WP中还发现了文件包含：param=local_file:flag.txt

原理：

Python 2.x - 2.7.16 urllib.fopen支持local_file导致LFI(CVE-2019-9948)：https://bugs.python.org/issue35907
参考：https://www.cnblogs.com/chrysanthemum/p/11765046.html