WEB｜[De1CTF 2019]SSRF Me

Posted 2023-05-06 scarecr0w7

页面代码为python代码，题目提示为SSRF，并且flag is in ./flag.txt

格式化代码

#! /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)

代码审计

代码中有三个路由
@app.route(\'/\')，读取本地code.txt文件
@app.route("/geneSign", methods=[\'GET\', \'POST\'])，根据param、action和secert_key参数调用getSign()方法计算出一个值sign
@app.route(\'/De1ta\', methods=[\'GET\', \'POST\'])，获取action、param、sign和ip参数调用Task.Exec方法，扫描本地并返回结果，可以利用这里读取flag.txt文件然后输出

@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())

访问/De1ta路径，传入cookies中action和sign，get方法中传入param，调用waf()函数

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

waf()函数将param参数值转换为小写，判断是否以gopher或file字符串开头，这里需要返回Ture，调用task.Exec()

def Exec(self):
        result = 
        result[\'code\'] = 500
        if (self.checkSign()):
            ......

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

task.Exec()中判断传入的Sign是否与getSign()计算出的Sign值相同

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

相同则，判断scan是否在action中，在则scan文件param，结果不为Connection Timeout则输出结果
所以这里需要解决的就是传入Sign值与计算的Sign值相同，并且param不以gopher或file字符串开头，action含有scan字符串。因为这里给出了Sign计算路径，且action判断用的是in，所以很好绕过，只要action含有scan字符串就行

if "scan" in self.action:

思路

访问/geneSign，param参数为flag.txtread，获取Sign
获取Sign：65f37c68d5eb8a857159d661f1817a15

Sign = secert_key + flag.txtread + scan

访问/De1ta，param参数为flag.txt，添加Cookie为action=readscan;sign=65f37c68d5eb8a857159d661f1817a15，sign为刚刚计算出的值

Sign = secert_key + flag.txt + readscan

flag0eea8996-266f-4d36-970d-2e5406aba2e7

方法二：文件包含

使用urllib.urlopen(param) 包含文件，可以直接填写文件路径./flag.txt，也可以使用伪协议file:///app/flag.txt ，但是waf()禁止以file开头，可以使用local_file，使用local_file时需要注意，路径必需为绝对路径，可以使用/proc/self/cwd/代表当前路径

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

获取Sign

# file:///app/flag.txt
# local_file:flag.txt
# local-file:///proc/self/cwd/flag.txt

/geneSign?param=local_file:flag.txtread

627888ff16803e4bbc91c5f8113c8f9e

获取flag

/De1ta?param=local_file:flag.txt

cookie: action=readscan;sign=627888ff16803e4bbc91c5f8113c8f9e

[De1CTF 2019]SSRF Me

看了好久的wp才做出来的一道题目，感觉自己太菜了。

给出源码（加了几条注释）

#! /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):       #构造方法self代表对象,其他是对象的属性
        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"))   #提取cookie信息中的，名为action得对应值
    param = urllib.unquote(request.args.get("param", ""))    #提取get方法传入的，参数名叫param对应得值
    sign = urllib.unquote(request.cookies.get("sign"))       #70-72 将url编码解码
    ip = request.remote_addr
    if(waf(param)):
        return "No Hacker!!!!"
    task = Task(action, param, sign, ip)
    return json.dumps(task.Exec())                           #Python 对象编码成 JSON 字符串
@app.route(‘/‘)
def index():
    return open("code.txt","r").read()


def scan(param):
    socket.setdefaulttimeout(1)
    try:
        return urllib.urlopen(param).read()[:50]            #读取网络文件参数可以是url
    except:
        return "Connection Timeout"



def getSign(action, param):
    return hashlib.md5(secert_key + param + action).hexdigest()  #hashlib.md5()#获取一个md5加密算法对象，hexdigest()是获得加密吼的16进制字符串


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‘)

字符串拼接

访问 /geneSign?param=flag.txt 得到

 

 看到 getSign 处的凭拼接

def getSign(action, param):
    return hashlib.md5(secert_key + param + action).hexdigest()  #hashlib.md5()#获取一个md5加密算法对象，hexdigest()是获得加密吼的16进制字符串

如果 secert_key 为 x，则访问 /geneSign?param=flag.txt 时，返回的md5为 md5(‘xxx‘ + ‘flag.txt‘ + ‘scan‘) ，相当于 md5(xxxflag.txtscan)

于是构造 /geneSign?param=flag.txtread 得到 等价于 md5(‘readflag.txtreadscan‘)的md5

 

 

 bp 抓包后 构造 Cookie 得到

 

 

 

 参考了：https://xz.aliyun.com/t/5927；https://joychou.org/web/hash-length-extension-attack.html

大佬们的文章。