Threat Level: green Handler on Duty: Russell Eubanks

SANS ISC: InfoSec Handlers Diary Blog - Internet Storm Center Diary 2016-09-10 InfoSec Handlers Diary Blog


Sign Up for Free!   Forgot Password?
Log In or Sign Up for Free!

Ongoing IMAP Scan, Anyone Else?

Published: 2016-09-10
Last Updated: 2016-09-10 07:07:37 UTC
by Xavier Mertens (Version: 1)
2 comment(s)

I'm operating a mail server which handles email flows from multiple domains (<20 domains). The server is under a massive IMAPS (port 993) scan for a few days. More details about the ongoing attack:

  • Some logins are valid
  • Some logins seem to be part of a dictionary
  • Some logins are old or unused (like scraped from web pages)
  • Some logins have a format 'user@domain.tld', other just the 'user'

[Update: some IP addresses are also testing SMTP AUTH]

There is a strong password policy in place and no credentials were compromized. This is not a brute-force attack, connection attempts are coming by waves. The only impact until now was a pollution of my logs!

There is an OSSEC active-response[1] with the 'repeated_offender' feature enabled (at 30, 60, 120, 240, 480 minutes) but new IP addresses are always detected (like being part of a bot):

I searched for more information about the offending IP addresses, they do not seem to belong to a known botnet. They are not Tor exit-nodes. Here is the top-10 of active IP addresses:

155.133.82.55
184.71.40.22
50.39.0.12
185.130.6.226
119.29.229.87
80.200.28.68
80.82.64.102
212.118.124.109
139.255.51.42
151.253.48.108

Someone else has already detected the same kind of scan?

[1] http://ossec-docs.readthedocs.io/en/latest/manual/ar/

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

Keywords: botnet imap scan
2 comment(s)
Diary Archives