SMTP

El Protocolo Simple de Transferencia de Correo (SMTP) es un protocolo para enviar correos electrónicos en una red IP. Puede utilizarse entre un cliente de correo electrónico y un servidor de correo saliente o entre dos servidores SMTP. SMTP se combina a menudo con los protocolos IMAP o POP3, que pueden recuperar y enviar correos electrónicos. En principio, es un protocolo basado en cliente-servidor, aunque SMTP puede utilizarse entre un cliente y un servidor y entre dos servidores SMTP. En este caso, el servidor actúa como cliente.

Default Configuration

Default Configuration

cat /etc/postfix/main.cf | grep -v "#" | sed -r "/^\s*$/d"

smtpd_banner = ESMTP Server 
biff = no
append_dot_mydomain = no
readme_directory = no
compatibility_level = 2
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
myhostname = mail1.inlanefreight.htb
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
smtp_generic_maps = hash:/etc/postfix/generic
mydestination = $myhostname, localhost 
masquerade_domains = $myhostname
mynetworks = 127.0.0.0/8 10.129.0.0/16
mailbox_size_limit = 0
recipient_delimiter = +
smtp_bind_address = 0.0.0.0
inet_protocols = ipv4
smtpd_helo_restrictions = reject_invalid_hostname
home_mailbox = /home/postfix

Dominio	Descripción
AUTH PLAIN	AUTH es una extensión de servicio utilizada para autenticar al cliente.
HELO	El cliente inicia sesión con el nombre de su computadora y así inicia la sesión.
MAIL FROM	El cliente nombra el remitente del correo electrónico.
RCPT TO	El cliente nombra el destinatario del correo electrónico.
DATA	El cliente inicia la transmisión del correo electrónico.
RSET	El cliente aborta la transmisión iniciada pero mantiene la conexión entre el cliente y el servidor.
VRFY	El cliente comprueba si hay un buzón disponible para la transferencia de mensajes.
EXPN	El cliente también verifica si hay un buzón disponible para enviar mensajes con este comando.
NOOP	El cliente solicita una respuesta del servidor para evitar la desconexión por tiempo de espera.
QUIT	El cliente finaliza la sesión.

Telnet-HELO/EHLO

telnet 10.129.14.128 25

Trying 10.129.14.128...
Connected to 10.129.14.128.
Escape character is '^]'.
220 ESMTP Server 


HELO mail1.inlanefreight.htb

250 mail1.inlanefreight.htb


EHLO mail1

250-mail1.inlanefreight.htb
250-PIPELINING
250-SIZE 10240000
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250-SMTPUTF8
250 CHUNKING

Telnet-VRFY

telnet 10.129.14.128 25

Trying 10.129.14.128...
Connected to 10.129.14.128.
Escape character is '^]'.
220 ESMTP Server 

VRFY root

252 2.0.0 root


VRFY cry0l1t3

252 2.0.0 cry0l1t3


VRFY testuser

252 2.0.0 testuser


VRFY aaaaaaaaaaaaaaaaaaaaaaaaaaaa

252 2.0.0 aaaaaaaaaaaaaaaaaaaaaaaaaaaa

Footprinting the Service

sudo nmap 10.129.14.128 -sC -sV -p25

Starting Nmap 7.80 ( https://nmap.org ) at 2021-09-27 17:56 CEST
Nmap scan report for 10.129.14.128
Host is up (0.00025s latency).

PORT   STATE SERVICE VERSION
25/tcp open  smtp    Postfix smtpd
|_smtp-commands: mail1.inlanefreight.htb, PIPELINING, SIZE 10240000, VRFY, ETRN, ENHANCEDSTATUSCODES, 8BITMIME, DSN, SMTPUTF8, CHUNKING, 
MAC Address: 00:00:00:00:00:00 (VMware)

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 14.09 seconds

sudo nmap 10.129.14.128 -p25 --script smtp-open-relay -v

Starting Nmap 7.80 ( https://nmap.org ) at 2021-09-30 02:29 CEST
NSE: Loaded 1 scripts for scanning.
NSE: Script Pre-scanning.
Initiating NSE at 02:29
Completed NSE at 02:29, 0.00s elapsed
Initiating ARP Ping Scan at 02:29
Scanning 10.129.14.128 [1 port]
Completed ARP Ping Scan at 02:29, 0.06s elapsed (1 total hosts)
Initiating Parallel DNS resolution of 1 host. at 02:29
Completed Parallel DNS resolution of 1 host. at 02:29, 0.03s elapsed
Initiating SYN Stealth Scan at 02:29
Scanning 10.129.14.128 [1 port]
Discovered open port 25/tcp on 10.129.14.128
Completed SYN Stealth Scan at 02:29, 0.06s elapsed (1 total ports)
NSE: Script scanning 10.129.14.128.
Initiating NSE at 02:29
Completed NSE at 02:29, 0.07s elapsed
Nmap scan report for 10.129.14.128
Host is up (0.00020s latency).

PORT   STATE SERVICE
25/tcp open  smtp
| smtp-open-relay: Server is an open relay (16/16 tests)
|  MAIL FROM:<> -> RCPT TO:<[e-mail]>
|  MAIL FROM:<[e-mail]> -> RCPT TO:<[e-mail]>
|  MAIL FROM:<antispam@ESMTP> -> RCPT TO:<[e-mail]>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<[e-mail]>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<relaytest%nmap.scanme.org@[10.129.14.128]>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<relaytest%nmap.scanme.org@ESMTP>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<"[e-mail]">
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<"relaytest%nmap.scanme.org">
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<[e-mail]@[10.129.14.128]>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<"[e-mail]"@[10.129.14.128]>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<[e-mail]@ESMTP>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<@[10.129.14.128]:[e-mail]>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<@ESMTP:[e-mail]>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<nmap.scanme.org!relaytest>
|  MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<nmap.scanme.org!relaytest@[10.129.14.128]>
|_ MAIL FROM:<antispam@[10.129.14.128]> -> RCPT TO:<nmap.scanme.org!relaytest@ESMTP>
MAC Address: 00:00:00:00:00:00 (VMware)

NSE: Script Post-scanning.
Initiating NSE at 02:29
Completed NSE at 02:29, 0.00s elapsed
Read data files from: /usr/bin/../share/nmap
Nmap done: 1 IP address (1 host up) scanned in 0.48 seconds
           Raw packets sent: 2 (72B) | Rcvd: 2 (72B)