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New Member

Non-SMTP Session Start Question

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.

13 REPLIES
Cisco Employee

Re: Non-SMTP Session Start Question

Hi,

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

signature

http://tools.cisco.com/MySDN/Intelligence/viewSignature.x?signatureId=5748&signatureSubId=0

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

connect.

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

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

Thank you.

Edward

New Member

Re: Non-SMTP Session Start Question

Hi,

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.

Cisco Employee

Re: Non-SMTP Session Start Question

Can you add "produce verbose alert" as an action to 5748-0, then from the cli capture "show event alert | in id=5748" and send that to me offline at wsulym@cisco.com. I might have stumbled across something looking at some other traffic and would like to confirm.

Thanks.

New Member

Re: Non-SMTP Session Start Question

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

Cisco Employee

Re: Non-SMTP Session Start Question

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.

New Member

Re: Non-SMTP Session Start Question

I have the fix-up for smtp disabled

Cisco Employee

Re: Non-SMTP Session Start Question

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.

New Member

Re: Non-SMTP Session Start Question

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

New Member

Re: Non-SMTP Session Start Question

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.

New Member

Re: Non-SMTP Session Start Question

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.

Thanks,

Cisco Employee

Re: Non-SMTP Session Start Question

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

SC

New Member

Re: Non-SMTP Session Start Question

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

New Member

Re: Non-SMTP Session Start Question

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 mail.xxxxx.yyyy.com.

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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