Non-SMTP Session Start Question

Unanswered Question
Feb 19th, 2007

I'm getting hundreds of triggers on signature 5748 Non-SMTP Session Start. When I put a block host on this signature I stop getting e-mail. Should this be considered normal traffic.

I have this problem too.
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edwakim Mon, 02/19/2007 - 12:39


Regarding signature 5748 firing SMTP session initiation with something other than HELO or EHLO. See below for MySDN link on this


I'm assuming subsig 0. Is this true?

This is likely a type of reconnaissance attack to see if you are running

an smtp service at this IP address and what type and version number of

smtp software you're running (i.e., Sendmail, Postfix, Microsoft

Exchange, etc.) as they'll get the smtp banner after their initial


When you see the signature alert, who's the attacker?

You can turn on 'produce verbose alert' to see more information.

Thank you.


rrutledge Mon, 02/19/2007 - 14:01


Today the signature was triggered 2698 times, from 349 hosts (90% public addesses). I am also seeing this triggered by local addresses, but I suppose the public one's are what I should be concerned with. As I stated before I did try and block hosts on this signature, but I am considering adding and exception for local address, and only block public.

bitterman Tue, 02/20/2007 - 06:25

PIX smtp fixup causes this. If you have a pix, disable the sig or disable fixup.

wsulym Tue, 02/20/2007 - 06:36

It shouldn't. The signature looks for either HELO EHLO or XXXX at the beginning of the stream - if it's not one of those, the signature will fire. The pix uses XXXX in smtp fixup.

wsulym Wed, 02/21/2007 - 07:06

I took this offline with rrutledge. Just so that there's some closure to this thread, in the end, what happened was that 'produce-alert' was set on the subsignatures, and that was what was seen flooding the event store (specifically subsigs -1 & -2). The subsigs will fire on normal traffic and should not have produce alert set.

hannatest Thu, 01/17/2008 - 13:12

Not in my case. I have the Sig 5748/3 set to "None", but Sig 5748/0 still fires on the "XXXX" command.

hannatest Fri, 01/18/2008 - 08:12

The IPS version is 6.0(3)E1.The triggered packets were captured.They are the 0x58 0x58 0x58 0x58.

Any known bug on this signature?Thanks.

mai2mai2m Mon, 02/11/2008 - 07:50

Still firing on xxxx in our case. We are running IPS-4260 with the signature S291.0 of 2007-06-18. The smtp payload of the triggering packet starts with xxxx.


scothrel Mon, 02/11/2008 - 10:30

Are they lower case 'x' or uppercase 'X' ? The signature only accepts uppercase as a valid start.


hannatest Mon, 02/11/2008 - 10:38

In my case,they are the uppercase 'X's.The start bytes are:0x58 0x58 0x58 0x58.

bnidacoc Mon, 03/09/2009 - 14:06

I have this sig firing very frequently. This sig constitutes about 80-90% of all of my alerts. Often the alert is firing on data as "RSET.."

The source IPs are scattered, some have even had domain names associated with them, like

Over the course of 72 hours I have 2331 Sig 5748/0 events.

I am sure that one grouped source attack IP which consists of 27 events (including summaries) in 10 minutes is most likely a malicious activity.

However, about 95% of unique attacker IPs consist of only 1-3 attempts (alerts) with rarely a summary among them.

I was on the latest sig a few weeks ago.

We have so much email activity; it would be difficult to analyze packet captures for RSETs coming in immediately after the TCP handshake.

Is this sig really correct?


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