buuctf [De1CTF 2019]SSRF Me

[De1CTF 2019]SSRF Me

打开题目发现一堆不知名乱码

不过仔细观察发现应该是python源码

#! /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',port=80)

代码审计：
Flask框架下 有三个路由
/geneSign:用于测试页面返回的是md5加密后的secert_key + param + action

#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)

/De1ta:获取一系列的数据,waf(param)过滤了gopher和file,生成Task对象,调用Exec()命令执行函数去读取文件

@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())

/:get返回源代码
就是一打开题目看到的那一堆源码

@app.route('/')
def index():
    return open("code.txt","r").read()

waf：禁止了flie和gopher协议
if判断

 def checkSign(self):
        if (getSign(self.action, self.param) == self.sign):
            return True
        else:
            return False

传入的sign要和getSign()生成的sign要一致
可以访问/geneSign路由get上传?param=flag.txtread
访问/geneSign得到的就是当action=readscan param=flag.txt时 页面返回的md5加密后的值
param=flag.txt， action=readscan和param=flag.txtread， action=scan
最后的哈希值应该是一样的
因为action在生成的时候已经固定为scan了 所以只需要param=readscan
解题：
构造sign值
访问url/geneSign，构造param=flag.txtread，通过action获得对应哈希值
访问url/De1ta?param=flag.txt
添加cookie:sign=哈希值，action=readscan绕过checkSign检查cookie中的getSign(self.action, self.param) == self.sign)

就达到利用exce()读取flag.txt