SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP, that let the user save messages in a server mailbox and download them periodically from the server.
In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail. On Unix-based systems, sendmail is the most widely-used SMTP server for e-mail. A commercial package, Sendmail, includes a POP3 server. Microsoft Exchange includes an SMTP server and can also be set up to include POP3 support. From here.
Default port: 25,465(ssl),587(ssl)
PORT STATE SERVICE REASON VERSION25/tcp open smtp syn-ack Microsoft ESMTP 6.0.3790.3959
If you have the opportunity to make the victim send you a email (via contact form of the web page for example), do it because you could learn about the internal topology of the victim seeing the headers of the mail.
You can also get an email from a SMTP server trying to send to that server an email to a non-existent address (because the server will send to the attacker a NDN mail). But, be sure that you send the email from an allowed address (check the SPF policy) and that you can receive NDN messages.
You should also try to send different contents because you can find more interesting information on the headers like:
X-Virus-Scanned: by av.domain.com
You should send the EICAR test file.
Detecting the AV may allow you to exploit known vulnerabilities.
nc -vn <IP> 25
openssl s_client -starttls smtp -crlf -connect smtp.mailgun.org:587
dig +short mx google.com
nmap -p25 --script smtp-commands 10.10.10.10
Check if you sniff some password from the packets to port 25
Authentication is not always needed
$ telnet 10.0.10.1 25Trying 10.0.10.1...Connected to 10.0.10.1.Escape character is '^]'.220 myhost ESMTP Sendmail 8.9.3HELO x250 myhost Hello [10.0.0.99], pleased to meet youMAIL FROM:email@example.com 2.1.0 firstname.lastname@example.org... Sender okRCPT TO:test550 5.1.1 test... User unknownRCPT TO:admin550 5.1.1 admin... User unknownRCPT TO:ed250 2.1.5 ed... Recipient ok
$ telnet 10.0.0.1 25Trying 10.0.0.1...Connected to 10.0.0.1.Escape character is '^]'.220 myhost ESMTP Sendmail 8.9.3HELO501 HELO requires domain addressHELO x250 myhost Hello [10.0.0.99], pleased to meet youVRFY root250 Super-User <root@myhost>VRFY blah550 blah... User unknown
$ telnet 10.0.10.1 25Trying 10.0.10.1...Connected to 10.0.10.1.Escape character is '^]'.220 myhost ESMTP Sendmail 8.9.3HELO501 HELO requires domain addressHELO xEXPN test550 5.1.1 test... User unknownEXPN root250 2.1.5 <ed.williams@myhost>EXPN sshd250 2.1.5 sshd privsep <sshd@mail2>
Metasploit: auxiliary/scanner/smtp/smtp_enumsmtp-user-enumnmap –script smtp-enum-users.nse <IP>
root@kali:~# sendEmail -t email@example.com -f firstname.lastname@example.org -s 192.168.8.131 -u Important Upgrade Instructions -a /tmp/BestComputers-UpgradeInstructions.pdfReading message body from STDIN because the '-m' option was not used.If you are manually typing in a message:- First line must be received within 60 seconds.- End manual input with a CTRL-D on its own line.IT Dept,We are sending this important file to all our customers. It contains very important instructions for upgrading and securing your software. Please read and let us know if you have any problems.Sincerely,
Most of this section was extracted from the book Network Security Assessment 3rd Edition.
SMTP messages are easily spoofed, and so organizations use SPF, DKIM, and DMARC features to prevent parties from sending unauthorised email.
A complete guide of these countermeasures can be found in https://seanthegeek.net/459/demystifying-dmarc/
Sender Policy Framework (SPF) provides a mechanism that allows MTAs to check of ahost sending an email is authorized. Then, the organisations can define a list of authorised mail servers and the MTAs can query for this lists to check if the email was spoofed or not.
cpolo@DESKTOP-RVEHVKS:~$ dig txt google.com | grep spfgoogle.com. 235 IN TXT "v=spf1 include:_spf.google.com ~all"cpolo@DESKTOP-RVEHVKS:~$ dig txt _spf.google.com | grep spf; <<>> DiG 9.11.3-1ubuntu1.7-Ubuntu <<>> txt _spf.google.com;_spf.google.com. IN TXT_spf.google.com. 235 IN TXT "v=spf1 include:_netblocks.google.com include:_netblocks2.google.com include:_netblocks3.google.com ~all"cpolo@DESKTOP-RVEHVKS:~$ dig txt _netblocks.google.com | grep spf_netblocks.google.com. 1606 IN TXT "v=spf1 ip4:22.214.171.124/24 ip4:126.96.36.199/19 ip4:188.8.131.52/20 ip4:184.108.40.206/20 ip4:220.127.116.11/18 ip4:18.104.22.168/16 ip4:22.214.171.124/21 ip4:126.96.36.199/16 ip4:188.8.131.52/17 ip4:184.108.40.206/19 ip4:220.127.116.11/19 ~all"cpolo@DESKTOP-RVEHVKS:~$ dig txt _netblocks2.google.com | grep spf_netblocks2.google.com. 1908 IN TXT "v=spf1 ip6:2001:4860:4000::/36 ip6:2404:6800:4000::/36 ip6:2607:f8b0:4000::/36 ip6:2800:3f0:4000::/36 ip6:2a00:1450:4000::/36 ip6:2c0f:fb50:4000::/36 ~all"cpolo@DESKTOP-RVEHVKS:~$ dig txt _netblocks3.google.com | grep spf_netblocks3.google.com. 1903 IN TXT "v=spf1 ip4:18.104.22.168/19 ip4:22.214.171.124/20 ip4:126.96.36.199/19 ip4:188.8.131.52/20 ip4:184.108.40.206/19 ip4:220.127.116.11/21 ip4:18.104.22.168/20 ip4:22.214.171.124/19 ip4:126.96.36.199/16 ip4:188.8.131.52/22 ~all"
To check the SPF of a domain you can use online tools like: https://www.kitterman.com/spf/validate.html If you get No valid SPF record you probably can send an email using that domain.
Use checkdmarc to check the SPF record: https://pypi.org/project/checkdmarc/
DomainKeys Identified Mail (DKIM) is a mechanism by which outbound email is signed and validated by foreign MTAs upon retrieving a domain’s public key via DNS. The DKIM public key is held within a TXT record for a domain; however, you must know both the selector and domain name to retrieve it.
Then, to ask for the key you need the domain name and the selector of the mail from the mail header
DKIM-Signature for example:
dig 20120113._domainkey.gmail.com TXT | grep p=20120113._domainkey.gmail.com. 280 IN TXT "k=rsa\; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA1Kd87/UeJjenpabgbFwh+eBCsSTrqmwIYYvywlbhbqoo2DymndFkbjOVIPIldNs/m40KF+yzMn1skyoxcTUGCQs8g3
Domain-based Message Authentication, Reporting & Conformance (DMARC) is a method of mail authentication that expands upon SPF and DKIM. Policies instruct mail servers how to process email for a given domain and report upon actions performed.
To obtain the DMARC record, you need to query the subdomain _dmarc
root@kali:~# dig _dmarc.yahoo.com txt | grep DMARC_dmarc.yahoo.com. 1785 IN TXT "v=DMARC1\; p=reject\; sp=none\; pct=100\;rua=mailto:email@example.com, mailto:firstname.lastname@example.org\;"root@kali:~# dig _dmarc.google.com txt | grep DMARC_dmarc.google.com. 600 IN TXT "v=DMARC1\; p=quarantine\; rua=mailto:email@example.com"root@kali:~# dig _dmarc.paypal.com txt | grep DMARC_dmarc.paypal.com. 300 IN TXT "v=DMARC1\; p=reject\; rua=mailto:firstname.lastname@example.org\;ruf=mailto:email@example.com,mailto:firstname.lastname@example.org"
PayPal and Yahoo instruct mail servers to reject messages that contain invalid DKIM signatures or do not originate from their networks. Notifications are then sent to the respective email addresses within each organization. Google is configured in a similar way, although it instructs mail servers to quarantine messages and not outright reject them.
Use checkdmarc to check the DMARC record: https://pypi.org/project/checkdmarc/
You can attack some characteristics of mail clients to make the user think that the mail is coming from any address, more info: https://www.mailsploit.com/index