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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 thisa 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 dont 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.
STI|Twitter|LinkedIn

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Enlarge (credit: Defense Advanced Research Projects Agency)

Researchers have uncovered an advanced malware-based operation that siphoned more than 600 gigabytes from about 70 targets in a broad range of industries, including critical infrastructure, news media, and scientific research.

The operation uses malware to capture audio recordings of conversations, screen shots, documents, and passwords, according to a blog post published last week by security firm CyberX. Targets are initially infected using malicious Microsoft Word documents sent in phishing e-mails. Once compromised, infected machines upload the pilfered audio and data to Dropbox, where it's retrieved by the attackers. The researchers have dubbed the campaign Operation BugDrop because of its use of PC microphones to bug targets and send the audio and other data to Dropbox.

"Operation BugDrop is a well-organized operation that employs sophisticated malware and appears to be backed by an organization with substantial resources," the CyberX researchers wrote. "In particular, the operation requires a massive back-end infrastructure to store, decrypt, and analyze several GB per day of unstructured data that is being captured from its targets. A large team of human analysts is also required to manually sort through captured data and process it manually and/or with Big Data-like analytics."

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