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How do I protect against authenticated users being used as spam relay

Posted on 2009-05-20
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Last Modified: 2013-11-16
I have recently joined a company that has Exchange 2003 SP1 on a 2003 SP2 server. Our Firewall is a Watchguard Edge X55e using common packet filter policies. A Sendio I.C.E. box scans for spam & viruses before routing SMTP traffic to the mail server. OWA is enabled, so OWA traffic is NATed through the firewall directly to the mail server. We are not an open relay.

We recently got spam blacklisted, I think because a user account had been compromised & been used to relay spam. In the past users had been allowed very weak passwords & hardly ever changed them. A more rigorous regime is now in place & all passwords have been changed. We have been delisted & so far we're ok.

My question is, what else should I do to prevent against this happening again?

Any suggestions would be greatly appreciated as I am going on holiday on Saturday & would like the peace of mind of knowing that our system is secure whilst I'm away!
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Question by:alec1836
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by:karstieman
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You've done the best thing by hardening the password security policy of your company.
I don't know if your firewall allows to check for 'if more than x mails are sent from the same receipient then block for xxx-period'. but that would be a great option to stop relaying immediately. There's a lot of software that can do this, but it has extra costs of course.
In my opinion you've done the best by hardening your password security policity ( require x characters. ,maybe even require a number and a special character like - or @ or something like that.
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by:Akhater
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If I understood correctly you do not have SMTP opened from outside directly to your Exchange server so you are left with two options


1) The spam is coming from inside the company (not necessary from exchange) a good practice would be to allow only the exchange server to send outgoing SMTP, and/or NAT the exchange to a different public IP than the one used by your clients

2) the spam was sent using a legitimate username and pass on your domain, the solution for that is definitely to use a stronger password policy
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by:alec1836
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Karstieman,

Thanks for your block for xxx-period suggestion, I'll look into that. It seems my firewall can check for spam over POP3, so I'll test that.

The password policy is now 8 characters to include upper & lower case, numbers & a special character. Come the next password change though I think I'll enforce a minimum of 10 characters & try to deter them from using "words" to make dictionary attacks harder.
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by:alec1836
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Akhater,

Thanks for your reply. SMTP is routed from Exchange to the Sendio I.C.E box & the firewall only allows outbound SMTP from this. So hopefully that covers any internal threat. I'm scanning all of our Windows PCs for viruses, malware & adware. I'm assuming our Macs are clean. I'm not sure what I can use to check the Win servers, so far all I've used is the MS Malicious Software Removal tool.

I'm not quite sure what you meant by "NAT the exchange to a different public IP than the one used by your clients"?
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by:Rajith Enchiparambil
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Disable authenticated relay in exchange 2003. Yuo don't need it unless you have POP/IMAP clients.

http://www.amset.info/exchange/smtp-relaysecure.asp
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by:alec1836
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Hi Rajith,

Don't I need that enabled for OWA?
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by:alec1836
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In fact, don't I need authenticated relay enabled for my users to email externally?
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by:Akhater
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what client are they using ?

If oulook configured as exchange client or OWA then the answer is no

If you are using POP3 or IMAP then yes
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by:alec1836
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At present several of my users only access their email from a web browser, both from within our network & remotely on the internet. Also, I have users using the Mail app on a Mac & Entorage on a Mac.
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by:Mestha
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The usual target account is Administrator, so you should ensure that the Administrator account cannot be used. Ideally by turning off authenticated relaying. If you have MAC users, then you cannot do that, so restrict down to only those that need it.

Simon.
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by:Akhater
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Entourage can be configured as an Exchange client so if it is the case you do not need relay, that being said most of the exchange servers I am administering have this checkbox enabled and have never faced any spam issue.

1) Is there any public IP you can telnet to from outside your company and access your exchange server ? or incoming emails are first being delivered to your anti spam ?

2) How is your exchange server sending emails, is it configured to use DNS or to send to your spam filter first ?

If I understood correctly all incoming email are routed through your spam filter so your exchange box is NOT directly accessible from outside i.e. no external user can use it to relay.

3) check your exchange queues are they suspicious ?

4) When you are sending out emails the IP address you are using (the one that is being blacklisted) is it the same one your clients are using to browse the internet ? if so then the issue is probably that you have an infected client, why not just restrict outgoing SMTP to the IPs of your exchange and/or spam filter ?
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by:alec1836
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Hi Simon,

I've created an access group called POP3 Relay & allowed only that group to relay. The administrator account is not in the group. However, nor is my account (I'm an administrator as well) & I can still send via Outlook & OWA - is that the correct behaviour?
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by:Akhater
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Yes I told you previously that the "Allow relaying for computers that has authenticated irrespectively of the list above" checkbox is not for MAPI/OWA clients it is for POP3/IMAP clients
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by:alec1836
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OK, sorry I'm a bit of a noobie.

So, in my POP3 Relay group I should only have users that are NOT using Outlook or OWA?

All Outlook/OWA clients can relay regardless of those settings because they are MAPI?
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by:Akhater
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it is not for users NOT using outlook or owa but rather any user using IMAP or POP3 (maybe user is using IMAP and OWA for example)
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by:Rajith Enchiparambil
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""In fact, don't I need authenticated relay enabled for my users to email externally?""

No, unless you have pop/imap clients.
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by:alec1836
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OK guys, I've removed all the users that I know to only use Outlook/OWA from my POP3 Relay group.

Akhater, to go back to your 4 questions:

1) No, I can only telnet the external IP of my firewall. This passes all SMTP to my Spam box.
2) All SMTP-OUT is via the Spam box.
3) My queues seem to be ok now, but the problem was last week. I just want to prevent it happeneing again.
4) Yes, the IP that was blacklisted was the external IP of our firewall. The same IP that users will display when browsing the net. The firewall is already configured to only allow outbound SMTP from the Spam box.
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Akhater earned 500 total points
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Since you answered that your exchange box is not reachable from outside then the spam can't be from outside

since only your Spam box can send SMTP traffic externally I think it is there that you should be checking
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Author Closing Comment

by:alec1836
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Sorry for the late response, I've been away. Thank you all for your assistance.
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