代码审计

整体代码

#! /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')

逐块审计

'/De1ta’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())

get传一个参数，cookie传两个参数

param经过一个waf函数过滤

实体化对象task

waf函数

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

这个过滤的内容：param参数中不能有gopher和file

exec()

def Exec(self):
        result = {}
        result['code'] = 500
        if (self.checkSign())://if1
            if "scan" in self.action://if2
                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://if3
                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

if1 需要满足self.checkSign()，先暂时忽略

我们要保证action的值有read和scan，才能进入两个if2、if3实现相应功能，所以action=readscan

if2：打开一个文本文档并写入resp的内容,resp =scan(self.param)

scan函数

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

它的作用是，打开一个 URL，并读取前50个字符的内容。题目说flag在flag.txt中，所以param=flag.txt

因为resp的值改变，所以并没有将flagf的值传入result[‘data’]，而是写入了./%s/result.txt，这个文件

if (resp == "Connection Timeout"):
    result['data'] = resp
else:
    print resp
    tmpfile.write(resp)
    tmpfile.close()

if3成功将flag的内容读入result[‘data’]中

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"

最后return json.dumps(task.Exec())，即可得到flag

self.checkSign()

然后我们回过头来看第一个if 条件self.checkSign()

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

getSign()

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

将三个参数连在一起连在一起，然后转换为md5值，secert_key是一个固定值，param=flag.txt，action=readscan

然后我们需要通过函数构造sign的md5值，需要使用"/geneSign"路由

在构造md5的过程中出现了问题，action的值固定了，但getSign()可被利用一下

getSign(self.action, self.param)的md5：secert_key+flag.txt+filescan=secert_keyflag.txtfilescan（固定值不变）

sign的md5:scert_key+flag.txt+scan=secert_keyflag.txtscan（我们构造的）

二者不同，但是param的值可控制，我们令param=flag.txtfile，即可得到md5:secert_keyflag.txtfilescan

解题过程

/geneSign目录下get传值，param=flag.txtread，得到md5值

/De1ta目录下，get传值：param=flag.txt

cookie传值：Cookie: action=readscan;sign=634054ed897aa798d902c856c7dc79a6

最终得到flag