Brute Force Dictionary Attack on Dovecot

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In a brute force dictionary attack, a remote host attempts to connect using common phrases for username and password, connecting and trying combinations as fast as possible, until either entry is achieved by a match, or the overload system experiences an overflow resulting in breach.

The most common brute force dictionary attack against Dovecot is continuous connections to port 25, the port responsible for listening to incoming mail, and connections from clients attempting to send mail.

messages 

Feb  6 22:21:24 mailserver01 saslauthd[1150]: do_auth         : auth failure: [user=summer] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]
Feb  6 22:21:24 mailserver01 saslauthd[1149]: do_auth         : auth failure: [user=summer] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]

secure 

Feb  6 22:21:22 mailserver01 saslauthd[1150]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost= 
Feb  6 22:21:22 mailserver01 saslauthd[1150]: pam_succeed_if(smtp:auth): error retrieving information about user summer
Feb  6 22:21:22 mailserver01 saslauthd[1149]: pam_unix(smtp:auth): check pass; user unknown
Feb  6 22:21:22 mailserver01 saslauthd[1149]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost= 
Feb  6 22:21:22 mailserver01 saslauthd[1149]: pam_succeed_if(smtp:auth): error retrieving information about user summer

maillog 

Feb  6 22:21:24 mailserver01 postfix/smtpd[16812]: warning: s15449723.onlinehome-server.com[74.208.17.177]: SASL LOGIN authentication failed: authentication failure
Feb  6 22:21:24 mailserver01 postfix/smtpd[16814]: warning: s15449723.onlinehome-server.com[74.208.17.177]: SASL LOGIN authentication failed: authentication failure

It is a shortcoming of postfix/dovecot/sasl that the IP address of the attacker is not stored in the messages log or the secure log. Also, the only way to match entries from either of those two logs against the maillog is by the timestamp. This is problematic on a server with a great deal of mail activity.

It would be nice if the IP address was in the messages or secure log, or if there was a key value that could be used to match exactly with the correct entries in maillog.

mechanisms in play

  • postfix - The mail transfer agent (MTA)
  • dovecot - The IMAP and POP3 service, how you retrieve your mail
  • pam (pam.d) - Linux Pluggable Authentication Modules (PAM) is what authenticates you when you try to login to get your mail from dovecot. PAM handles authentication.
  • saslauthd (SASL) - This is what authenticates you when you try to send an email. Rather than managing a relay list, SASL requires authentication before permitting the sending of email.
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