Yesterday, I received a few interesting emails in my honeypot. I set up catch-all email addresses for domains that are well known by spammers. I'm capturing emails and extracting MIME attachments for further analysis. Today, my honeypot received three ICS files. iCalendar[1] is a file format used to exchange meeting information between users, mainly via email or a file sharing system. Such files use the extension ".ics".

The emails were sent as a Yahoo calendar event:

They were sent from Yahoo SMTP servers (with a nice typo in the From: field)

Oct 18 11:27:07 marge postfix/cleanup[9842]: 444817C2519: warning: header From: "OFICE FILE" \
  from n9-vm2.bullet.mail.bf1.yahoo.com[66.196.80.169]; from=xxxx to=xxxx proto=ESMTP helo=xxxx

The ICS file attached to the mail had a valid format but with some interesting characteristics. First, it was a cancellation request:

Then, many recipients (approximately 50) were added as "required":

ATTENDEE;ROLE=REQ_PARTICIPANT;RSVP=TRUE:mailto:kiotoambiental@ig.com.br
ATTENDEE;ROLE=REQ_PARTICIPANT;RSVP=TRUE:mailto:kirk.pearson@leg.wa.gov
ATTENDEE;ROLE=REQ_PARTICIPANT;RSVP=TRUE:mailto:kissimmeesdb@yahoo.com
ATTENDEE;ROLE=REQ_PARTICIPANT;RSVP=TRUE:mailto:kitanamileene@bol.com.br
ATTENDEE;ROLE=REQ_PARTICIPANT;RSVP=TRUE:mailto:kitty.hotel@hotmail.com
ATTENDEE;ROLE=REQ_PARTICIPANT;RSVP=TRUE:mailto:kittythebulldog-1@yahoo.com

Once the file opened in Outlook, it looked like this:

You can see that all the participants are listed. Depending on the way the user will cancel or reply to the mail, a notification could be sent to all the attendees, propagating the spam. Note that the mail was sent approximately 30 minutes (11:27 GMT+2) before the scheduled time in the meeting request (12:00 - 13:00 GMT+2).

The message in itself does not contain malicious content (an ICS file contains only text) but your mail server could be used to spread the message to other attendees and affect its reputation in anti-spam lists. The meeting details could also contain a link to a malicious website.

Did you also see such emails or do you have more information? Feel free to share!

[1]  http://icalendar.org/

Xavier Mertens (@xme)
ISC Handler - Freelance Security Consultant
PGP Key