SMTP (Simple Mail Transfer Protocol)
Default Port: 25
SMTP (Simple Mail Transfer Protocol) is a communication protocol for electronic mail transmission. The vulnerabilities in SMTP protocols can allow malicious attackers to spoof the origin of the messages, perform phishing attacks, and even execute code remotely in the system.
Connect
Connect Using Telnet
We can use Telnet to connect to the remote server. Here is a command using Telnet:
telnet example.com 25
Enumeration
Identifying a SMTP Server
You can use Nmap to check if there's an Telnet server on a target host like this:
nmap -p25,465,587 -sV -Pn target.com
Enumerate Users
Nmap has a script for SMTP user enumeration
nmap -p25 --script smtp-enum-users.nse target.com
DNS Mail Exchange (MX) Record Enumeration
We can use the dig tool to find out the mail servers (MX servers) of a domain. This tool sends a DNS query and returns the list of MX servers.
dig +short mx example.com
Information Disclosure with NTLM Auth
Some SMTP servers with NTLM Authentication enabled can disclose sensitive information, like Windows Server version and internal IP, if Anonymous Logon is allowed.
nmap -p25 --script smtp-ntlm-info --script-args smtp-ntlm-info.fingerprint=on target.com
Attack Vectors
Open Relay Exploit
SMTP Open Relay occurs when the SMTP server is configured to accept and transfer messages on the network that were neither for nor from local users.
Here is a simple example of how to test for open relay:
telnet target.com 25
MAIL FROM:<test@example.com>
RCPT TO:<test2@anotherexample.com>
DATA
Subject: Test open relay
Test message
.
QUIT
Post-Exploitation
Common SMTP Commands
| Command | Description | Usage |
|---|---|---|
| HELO | Identifies the client to the server. | HELO example.com |
| EHLO | Extended HELLO. | EHLO example.com |
| MAIL FROM: | Specifies the sender's email address. | MAIL FROM: <sender@example.com> |
| RCPT TO: | Specifies the recipient's email address. | RCPT TO: <recipient@example.com> |
| DATA | Indicates the start of the message body. | DATA |
| RSET | Resets the session. | RSET |
| NOOP | No operation; used for testing. | NOOP |
| QUIT | Terminates the session. | QUIT |