A smart attack on SMTP evading fail2ban

Question

I have an ongoing brute-force attack on my SMTP server in a way that evades typical fail2ban settings:

attacks take place from several IPs in the same subnet
a single IP attacks in intervals longer than an hour, apparently to evade being blocked by fail2ban that typically uses findtime and bantime shorter than an hour

This is the log:

2017-09-05 01:11:19 LOGIN authenticator failed for (User) [91.200.12.165]:57519 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=soft)
2017-09-05 01:11:36 LOGIN authenticator failed for (User) [91.200.12.164]:51973 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=sandy)
2017-09-05 01:15:22 LOGIN authenticator failed for (User) [91.200.12.121]:51545 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=brown)
2017-09-05 01:28:57 LOGIN authenticator failed for (User) [91.200.12.105]:64938 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=family)
2017-09-05 01:48:32 LOGIN authenticator failed for (User) [91.200.12.165]:64730 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=soi)
2017-09-05 01:48:47 LOGIN authenticator failed for (User) [91.200.12.164]:59184 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=sanjeev)
2017-09-05 01:50:13 LOGIN authenticator failed for (User) [91.200.12.166]:64999 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=until)
2017-09-05 01:54:05 LOGIN authenticator failed for (User) [91.200.12.121]:58756 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=bryan)
2017-09-05 02:04:34 LOGIN authenticator failed for (User) [91.200.12.105]:55772 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=famous)
2017-09-05 02:25:36 LOGIN authenticator failed for (User) [91.200.12.165]:55563 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=soldiers)
2017-09-05 02:25:48 LOGIN authenticator failed for (User) [91.200.12.164]:50017 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=sanmiguel)
2017-09-05 02:30:33 LOGIN authenticator failed for (User) [91.200.12.166]:55835 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=update)
2017-09-05 02:32:56 LOGIN authenticator failed for (User) [91.200.12.121]:49589 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=bsd)
2017-09-05 02:40:11 LOGIN authenticator failed for (User) [91.200.12.105]:62983 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=far)
2017-09-05 03:02:29 LOGIN authenticator failed for (User) [91.200.12.165]:62775 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=solution)
2017-09-05 03:02:40 LOGIN authenticator failed for (User) [91.200.12.164]:57228 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=santamaria)
2017-09-05 03:10:50 LOGIN authenticator failed for (User) [91.200.12.166]:63046 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=upgrade)
2017-09-05 03:11:37 LOGIN authenticator failed for (User) [91.200.12.121]:56803 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=bubba)
2017-09-05 03:15:41 LOGIN authenticator failed for (User) [91.200.12.105]:53820 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=farias)
2017-09-05 03:39:36 LOGIN authenticator failed for (User) [91.200.12.165]:53612 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=solve)
2017-09-05 03:39:48 LOGIN authenticator failed for (User) [91.200.12.164]:64441 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=santos)
2017-09-05 03:50:50 LOGIN authenticator failed for (User) [91.200.12.121]:64015 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=buchhaltung)
2017-09-05 03:51:30 LOGIN authenticator failed for (User) [91.200.12.166]:53880 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=upload)
2017-09-05 03:51:34 LOGIN authenticator failed for (User) [91.200.12.105]:61032 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=farm)
2017-09-05 04:17:31 LOGIN authenticator failed for (User) [91.200.12.165]:60825 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=somebody)
2017-09-05 04:17:34 LOGIN authenticator failed for (User) [91.200.12.164]:55274 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=sara)
2017-09-05 04:27:43 LOGIN authenticator failed for (User) [91.200.12.105]:51865 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=farmer)
2017-09-05 04:30:06 LOGIN authenticator failed for (User) [91.200.12.121]:54848 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=buddy)
2017-09-05 04:32:02 LOGIN authenticator failed for (User) [91.200.12.166]:61091 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=upon)
2017-09-05 04:55:24 LOGIN authenticator failed for (User) [91.200.12.164]:62485 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=sarah)
2017-09-05 04:55:24 LOGIN authenticator failed for (User) [91.200.12.165]:51657 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=someone)
2017-09-05 05:03:42 LOGIN authenticator failed for (User) [91.200.12.105]:59078 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=farmers)
2017-09-05 05:09:10 LOGIN authenticator failed for (User) [91.200.12.121]:62060 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=build)
2017-09-05 05:12:27 LOGIN authenticator failed for (User) [91.200.12.166]:51923 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=ups)
2017-09-05 05:33:26 LOGIN authenticator failed for (User) [91.200.12.164]:53321 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=sat)
2017-09-05 05:33:26 LOGIN authenticator failed for (User) [91.200.12.165]:58869 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=somethin)
2017-09-05 05:40:00 LOGIN authenticator failed for (User) [91.200.12.105]:49912 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=fast)
2017-09-05 05:48:34 LOGIN authenticator failed for (User) [91.200.12.121]:52896 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=building)
2017-09-05 05:53:00 LOGIN authenticator failed for (User) [91.200.12.166]:59136 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=uranus)
2017-09-05 06:11:20 LOGIN authenticator failed for (User) [91.200.12.165]:49705 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=son)
2017-09-05 06:11:21 LOGIN authenticator failed for (User) [91.200.12.164]:60535 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=saturn)
2017-09-05 06:16:06 LOGIN authenticator failed for (User) [91.200.12.105]:57124 I=[192.168.1.224]:25: 535 Incorrect authentication data (set_id=father

Is there a way to make some sort of selective fail2ban setting that would catch such attacks in a way other than just increasing findtime and bantime to several hours?

sebix · Answer 1 · 2017-10-20T08:18:09.290

0

Is there a way to make some sort of selective fail2ban setting that would catch such attacks in a way other than just increasing findtime and bantime to several hours?

You can create an additional rule with a regular expression only matching these clients, with higher findtime. Using an extra rule does not influence your other rules which work fine.

edited Oct 20 '17 at 08:18

answered Oct 19 '17 at 08:28

sebix

4,175
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Considering that they try to brute-force guess password (via SMTP LOGIN), that might be harmful, yes. – LetMeSOThat4U Oct 19 '17 at 13:05

score 0 · Answer 2 · answered Oct 20 '17 at 08:51

Be careful that you don't end up creating a rule that may get triggered by normal login failures. This depends on how many users you might have using the same ISP and, therefore, connecting from the same subnet, occasionally providing wrong credentials.

Fail2Ban is good for automating prevention of typical brute force attacks. This kind of malicious activities from a wider subnet are not so usual and are quite OK, probably even better, to be handled manually:

If you don't need authenticated SMTP from 91.200.12.0/22, block it on your firewall.

This might be easier if you had separate submission for authenticated SMTP on port 587 and never allow authentication on port 25; that would prevent giving any information about SMTP logins on 25 and still allows possible legitimate incoming mail from 91.200.12.0/22.

If you think this network is hijacked, the whois database has contact information. By contacting the owner of this IP block you may help them to solve the actual problem!

inetnum:        91.200.12.0 - 91.200.15.255

remarks:        **********************************Attention***************
remarks:        The pool is used other Department!
remarks:        In case of questions related to SPAM, HACKING, SECURITY
remarks:        Please contact directly abuse@vhoster.net
remarks:        tel: +38 (044) 379-28-50; +7 (499) 404-16-45
remarks:        **********************************************************

A smart attack on SMTP evading fail2ban

2 Answers2