Sembee Blog of Exchange MVP Simon Butler

One morning you find that there is spam in the queues, your server has been blacklisted etc...

One of the worst experiences for an Exchange administrator is to come in one morning and find that either email is being blocked, the queues are long or the users are getting NDRs saying that the server is blacklisted.

This seems to result in confusion amongst administrators who then go looking for advice only to get conflicting answers on what the problem might be.
I am going to try and clear up some of that confusion which should help Exchange administrators find the source of the problem.

There are two main issues that Exchange administrators seem to see and fail to understand.

  1. There are a large numbers of messages in the queues.
  2. The IP address of the server has been blacklisted.

In both of these occasions many administrators seem to think that a client machine on their network has been compromised and is sending email through the Exchange server.

This is not the case.

To abuse an Exchange server in this way, a BOT writer would need to

  1. get the BOT inside the network
  2. infect the machine
  3. realise that it is on a corporate network where there is an Exchange server
  4. find the Exchange server
  5. send the message.

The above, is not going to happen - at least not at the moment. Too much like hard work. The first two are the most difficult - if the network security has been configured correctly and the users trained to recognise potential suspect emails or web sites.

Then sending the message requires either a MAPI interface or SMTP to be configured on the Exchange server to allow users to relay through the server. While this is default, if you do not have any users who need to relay through the server (Outlook, OWA and Windows Mobile/Blackberry BES users do not need to) then you should disable it.

Then for a successful infection and abuse, the above is also presuming that the user is an administrator and the network admin will not notice the infection!

What the BOT writer is really looking to do is infect clueless home users who are not keeping their machines patched, not using security software and are running as a local admin. Much higher chance of success there involving simpler techniques.

Therefore with the target in mind, the BOT will usually have its own SMTP engine and will be sending out email directly to the internet.


So what has happened?

If you have been blacklisted but the queues are clear, then a client machine has probably been compromised. This is often the case when you have a single IP address on the Internet which is shared among all machines on your LAN.

However to further complicate things - if you are using a smart host - such as your ISPs SMTP server - then your queues could be clear but the server is still being abused. However in that scenario it is likely that your server would not be blacklisted on public lists, but your ISP may have noticed and not be very happy with you. If messages are not being delivered to the smart host then phone your ISP and ask - or they may phone you. Often ISPs will block first and ask questions later.

Finding the Source - Compromised workstation

A quick and dirty method to find the compromised machine is to simply stop Exchange from sending any messages by freezing the outbound traffic, and then block port 25 on the firewall and wait. A compromised machine will quickly show on the logs when it cannot connect. You can then go and find the machine and deal with it.

Having up to date Antivirus is not enough. Once the BOT is on the machine, it is no longer your machine. The only way to ensure that it is clean is to wipe the machine. BOTs are very good at hanging around and they will update themselves regularly.

There is a complication on this as well - if you have been foolish enough to browse from the Exchange server then the server itself may have a BOT and be sending out messages. However those messages would still not show in the queues. If you don't browse from the Exchange server then that shouldn't be the cause of your problems.

Finding the Source - Large Number Of Messages in the Queues

If you have a large number of messages in the queues, then those will be coming from outside your network. That does not mean you are an open relay, there are other ways that the spammer can abuse your server.

The two most common are authenticated relaying and the NDR attack.
I have discussed these in more detail in my spam cleanup article on amset.info - http://www.amset.info/exchange/spam-cleanup.asp .

However in short, authenticated relay is where the spammer has attacked your SMTP port trying to break a password - usually the administrator account. Once broken, the account is used to relay email. Authenticated relaying is enabled by default.

An NDR attack is where messages are sent to your server to non-existent users on purpose. Either as a directory harvest attack (to see what users are valid) or to get your server to bounce the messages to the "sender". The sender is spoofed and is the actual target.
Exchange 2000 is unable to defend itself against these kinds of attack without third party support. Exchange 2003 and higher has features built in to deal with this kind of threat, however if you have Exchange 2003 on Windows 2000 then you should not use them as Windows 2000 is unable to defend itself against a directory harvest.


So what do you do?

When you first notice there is a problem, you need to verify whether it is the result of an attack or compromised machine, or the result of a configuration error or change. Do not presume one or the other.
Once you know which it is then you can look further.

If you are dealing with an ongoing problem then pull the plug on the internet connection. That will stop messages going out and if the spammer is abusing your server will stop the messages from piling up. This will give you some breathing space to clean up and see what is going on.

If your IP address has been blacklisted, then use your ISPs SMTP server to send email through.

Ideally you should have at least two IP addresses so that the Exchange server can have its own address. If a workstation is then abused it does not result in your email IP address getting blacklisted.

Remember, any SMTP server is a target for a spammer. They don't want to use their own resources, they want to use those that belong to someone else so that they can hide.