[De1CTF 2019]SSRF Me

[De1CTF 2019]SSRF Me

题目直接给了源代码和提示

#! /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')

打开题目发现里面的东西就是题目中给的源代码

直接审计代码，发现了三个路由，先看使用最广的/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())

审计代码得知，我们需要传入三个值分别是action、param、sign，而param用get方法传值，action、sign用cookie传值，get传入的param需要经过waf函数，其次还要传入Task类对象，并执行Exec函数，先看一下waf函数

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

waf函数过滤了以gopher和file开有的协议，所以我们不能通过协议来读取文件，再看一下Exec函数

    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

它首先会通过一个checkSign方法检测登录，if通过以后要求传入的action必须要有关键词scan、read，才能够获取flag，看一下checkSign函数

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

我们传入的action和param参数经过getSign函数之后与sign相等才会进行下一步，继续看一下getSign函数

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

发现getSign函数是secret_key、param、sction三者拼接后的md5，但是发现/geneSign路由，返回了getSign，可以生成我们需要的md5

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

构造payload：/geneSign?param=flag.txt

给了md5，但是这里只有scan功能，还需要read功能，/geneSign?param=flag.txt，返回的md5就是md5(secret_key+flag.txt+scan)，即(secret_keyflag.txtscan)，但如果我们param传入的值是flag.txtread（payload：/geneSign?param=flag.txtread），返回的值就是md5(secret_key+flag.txtread+scan)，即(secret_keyflag.txtreadscan)。

这样的话就满足了action需要有scan与read关键词的要求，给出的md5也就是sign的值，action传入的值直接为readscan，剩下的param只能等于flag.txt，准备完毕可以执行了。