You probably have sent email. Or have you?
If you use a major mail provider like GMail, Yahoo, or Hotmail, they do all the work of taking the information you want to send and who you want to send it to, and then communicate that to an “SMTP” mail server on your behalf in a special protocol. SMTP is easy to remember as it stands for “Send Mail To People”.
At #! you get a free email account and a “Mutt” email client for interacting with email on the terminal, but here we will dive a bit deeper and use telnet to interact directly with our SMTP mail server to send an email in raw form like your mail client would normally do for you.
The hard way
- Install a local smtpd server
apt install opensmtpd
- Start local mail server
sudo systemctl start opensmtpd
Connect to your local mail server
> telnet localhost 25 Trying ::1... Connected to localhost. Escape character is '^]'. 220 sf1.hashbang.sh ESMTP Postfix (Debian/GNU)
Announce yourself to the mail sever
The server will respond with a list of protocals it supports:
250-localhost 250-PIPELINING 250-SIZE 52428800 250-VRFY 250-ETRN 250-STARTTLS 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN
Tell the server who you want to send mail as
MAIL FROM: <firstname.lastname@example.org>
Note: you can put literally any email you want here. This is like a return address on an envelope. Most email servers will blindly let you impersonate anyone. We call this email “spoofing”.
Tell the server who you want to receive the mail
RCPT TO: <email@example.com>
Tell the server the body of the message you want to send
Note: end with a period on a line by itself
You need to type everything below in order for the message to be accepted.
“DATA” announces that you want to send a message, followed by From, to and subject followed by the body of your message.
DATA From: John Doe <firstname.lastname@example.org> To: Your Name <email@example.com> Subject: some subject My really awesome message here
Just a single period on a line by itself lets the server know you are done.
Go check your inbox!
The Easy Way
A bash one liner using the “sendmail” command:
echo "Subject: yeah cool\n\n my super rad message" | \ sendmail \ -F "John Doe <firstname.lastname@example.org>" \ -f email@example.com \ firstname.lastname@example.org
Depending on who you spoofed, it may or may not be in spam. In practice many mail providers will refuse mail from home IP addresses. You will generally have more success from an office or university. You also will only be able to spoof domains that don't have special settings on their domain. If spoofing actually works for a real domain like this, it is a sign they have a significant security hole. Point it out to them and collect your first bug bounty as a security researcher! Please don't abuse this, and be mindful of the laws in your local area ;)