Yesterday, I read an interesting blog post about exploiting XXE (XML eXternal Entity) flaws to send e-mails [1]. In short: It is possible to trick the application to connect to an FTP server, but since mail servers tend to be forgiving enough, they will just accept e-mail if you use the FTP client to connect to port 25 on a mail server. The mail server will of course initially see the "USER" and "PASS" commands, but it will ignore them.

Initially, I considered this a lesser issue. A similar attack has been used in the past via HTTP proxies. HTTP proxies can also connect to port 25, and relay mail connections that way. But from my experience, mail servers tend to ignore them. For example:

220 mail.dshield.org ESMTP
GET
221 2.7.0 Error: I can break rules, too. Goodbye.
Connection closed by foreign host.

However, (and thanks to Alexander, the author of the blog for pointing this out), it looks like the list of blocked command is limited to HTTP verbs:

smtpd_forbidden_commands (default: CONNECT, GET, POST)

List of commands that cause the Postfix SMTP server to immediately terminate the session with a 221 code. This can be used to disconnect clients that obviously attempt to abuse the system. In addition to the commands listed in this parameter, commands that follow the "Label:" format of message headers will also cause a disconnect.

This feature is available in Postfix 2.2 and later.

Only CONNECT, GET, and POST will be blocked by default. To extend the list, use the following line in your main.cf file for postfix:

smtpd_forbidden_commands = CONNECT,GET,POST,USER,PASS

I don't think either USER or PASS is ever used legitimately in SMTP. Instead, SMTP uses "AUTH" to log in a user. To test, just connect to the mail server via telnet or netcat:

$ nc localhost 25
220 mail.dshield.org ESMTP
USER
221 2.7.0 Error: I can break rules, too. Goodbye.

 [1] https://shiftordie.de/blog/2017/02/18/smtp-over-xxe/

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Johannes B. Ullrich, Ph.D.
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