[De1CTF 2019]SSRF Me

[De1CTF 2019]SSRF Me

整理代码

#!/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
import imp

imp.reload(sys)

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.request.unquote(request.args.get("param", ""))
    action = "scan"
    return getSign(action, param)

@app.route('/De1ta',methods=['GET','POST'])
def challenge():
    action = urllib.request.unquote(request.cookies.get("action")) # Get action from cookie
    param = urllib.request.unquote(request.args.get("param", "")) # Get param from url query
    sign = urllib.request.unquote(request.cookies.get("sign")) # Get sign from cookie
    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.request.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): # param should not start with gopher or file
    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)

三个路由：

/geneSign

传递参数param和设置好的action=scan传入getSign生成签名

/De1ta

从cookie中获取action和sign，再获取参数param以及ip然后传入Task类中，以json形式返回Task->Exec()

/

首页，读取code.txt中的源码返回

waf()过滤了param中gopher和file关键字

def waf(param): # param should not start with gopher or file
    check=param.strip().lower()
    if check.startswith("gopher") or check.startswith("file"):
        return True
    else:
        return False

secert_key未知，param和action可控

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

scan方法用的是urllib.urlopen()，这里有两种方法可以直接写文件名读取本地文件

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

Exec()主要是action中如果有read和scan关键字及操作，如果action中有scan，利用scan()方法读取内容并放入result.txt，如果有read即读取result.txt

    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

根据hint，确定我们要读取的是flag.txt

利用scan将flag.txt的内容写入到result.txt中，然后read读取result.txt

接下来满足验证签名即可

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()

拼接参数生成签名时param和action可控，并且action是拼接在最后。这样就可以利用哈希长度扩展攻击，在action后加上read关键字，并且预测出签名。

首先访问下/geneSign路由，传参param=flag.txtread，拿到这个签名

hashlib.md5(secert_key + 'flag.txt' + 'scan').hexdigest()

45e041f7cedbaa834de7d143f345a2b2

这里为了找到key，令/geneSign?param=flag.txt，action是固定值，得出的md5是拼接字符，反编译后得到key值，而输入flag.txtread的原因是因为action是scan，而在getSign里几个字符是.号会连接起来，所以就变成了key.flag.txtreadscan，这样sign就出现了我们想要的md5值，避免出现过多操作导致失误

接着在/De1ta路由传入param，并在cookie设置action和sign即可得到flag，注意action里既要有read也要有scan，因为要先将flag写入result后再读出

总体思路就是利用getSign拼接顺序的漏洞绕过Task.checkSign和geneSign共同构成的对可用action的限制

令param='flag.txtread'，通过geneSign拼接而成的md5(secret_key + 'flag.txtreadscan')同指定param='flag.txt' action='readscan'相同