Recently we began seeing lots of these log entries on our off-site mx
smtp host. I have googled this but I am not clear from what I have
read if this is something we can stop altogether or should even worry
about.
Comments?
Logwatch. . .
--------------------- sendmail Begin ------------------------
SMTP SESSION, MESSAGE, OR RECIPIENT ERRORS
------------------------------------------
WARNING!!!! Possible Attack:
Attempt from 104.Red-83-50-106.dynamicIP.rima-tde.net
[83.50.106.104] with:
command=HELO/EHLO, count=3: 1 Time(s)
--
*** E-Mail is NOT a SECURE channel ***
James B. Byrne mailto:ByrneJB at Harte-Lyne.ca
Harte & Lyne Limited http://www.harte-lyne.ca
9 Brockley Drive vox: +1 905 561 1241
Hamilton, Ontario fax: +1 905 561 0757
Canada L8E 3C3
James B. Byrne wrote:> Recently we began seeing lots of these log entries on our off-site mx > smtp host. I have googled this but I am not clear from what I have > read if this is something we can stop altogether or should even worry > about. > > Comments? >I'm not real good with smtp, but it looks as though someone from Spain is trying to directly connect to your smtp server. Unless you know that they're legitimately using your system, I'd block that IP now. fail2ban's your friend.... mark> Logwatch. . . > > --------------------- sendmail Begin ------------------------ > > SMTP SESSION, MESSAGE, OR RECIPIENT ERRORS > ------------------------------------------ > > WARNING!!!! Possible Attack: > Attempt from 104.Red-83-50-106.dynamicIP.rima-tde.net > [83.50.106.104] with: > command=HELO/EHLO, count=3: 1 Time(s) > > > -- > *** E-Mail is NOT a SECURE channel *** > James B. Byrne mailto:ByrneJB at Harte-Lyne.ca > Harte & Lyne Limited http://www.harte-lyne.ca > 9 Brockley Drive vox: +1 905 561 1241 > Hamilton, Ontario fax: +1 905 561 0757 > Canada L8E 3C3 > > _______________________________________________ > CentOS mailing list > CentOS at centos.org > http://lists.centos.org/mailman/listinfo/centos >
On Thu, 20 Sep 2012, James B. Byrne wrote:> Recently we began seeing lots of these log entries on our off-site > mx smtp host. I have googled this but I am not clear from what I > have read if this is something we can stop altogether or should even > worry about. > > WARNING!!!! Possible Attack: > Attempt from 104.Red-83-50-106.dynamicIP.rima-tde.net > [83.50.106.104] with: > command=HELO/EHLO, count=3: 1 Time(s)My understanding is that this is indicative of a (almost certainly malicious) SMTP client trying different HELO or EHLO identities within the same session. Sendmail is hard-coded to reject the connection after three HELO/EHLO commands. So you've got a dynamic address (83.50.106.104) trying to identify itself as three different hostnames -- and finally Sendmail gets angry and slams the door. If you've configured a blacklist service like spamhaus, you're likely to see the 'possible SMTP attack' warning shortly after Sendmail has already rejected mail from the remote host, e.g., Aug 19 11:45:01 myserv sendmail[16804]: ruleset=check_relay, arg1=ill90.internetdsl.tpnet.pl, arg2=127.0.0.4, relay=ill90.internetdsl.tpnet.pl [79.190.37.90], reject=550 5.7.1 mail rejected - see http://www.spamhaus.org/ Aug 19 11:45:02 myserv sendmail[16804]: q7JIj1pM016804: ill90.internetdsl.tpnet.pl [79.190.37.90]: possible SMTP attack: command=HELO/EHLO, count=3 -- Paul Heinlein heinlein at madboa.com 45?38' N, 122?6' W
On Thu, September 20, 2012 11:10, m.roth at 5-cent.us wrote:> I'm not real good with smtp, but it looks as though someone from > Spain is trying to directly connect to your smtp server. Unless > you know that they're legitimately using your system, I'd block > that IP now. >The list of sources is far too long to include in a message to the list. Suffice to say that each IP address is automatically blocked for varying lengths of time following any failed attempt. What I am trying to discover is what in particular, if anything, caused this traffic to suddenly start hitting our external server and whether or not we should be concerned about a specific vulnerability. This host is our last remaining Sendmail server. All the rest have been switched to Postfix. None of the other MX hosts are reporting this and so the questions arise: Is this an attack? Is it specifically directed at the Sendmail server or is it just a co-incidence? -- *** E-Mail is NOT a SECURE channel *** James B. Byrne mailto:ByrneJB at Harte-Lyne.ca Harte & Lyne Limited http://www.harte-lyne.ca 9 Brockley Drive vox: +1 905 561 1241 Hamilton, Ontario fax: +1 905 561 0757 Canada L8E 3C3