Malicious Hackers and Spam, Part 1

My consulting company recently received a call from a client company that was having problems with backup failures and poor server performance when sending and receiving email. When we arrived at the client site, we found the problem was more serious than a failed tape drive and slow server. I logged on to the server and noticed it was running extremely slow. The server showed a lot of drive activity and high CPU usage. I pressed Ctrl+Alt+Delete to open Windows Task Manager and sorted the processes by CPU usage. I noted that store.exe was taking up most of the CPU cycles. Microsoft Exchange 2000 Server and Windows 2000 Server were running on this machine. Could the problem be a corrupted Exchange Store? Large email volume? The organization wasn't a heavy email user and had only 15 users connected to the server.

I started the Exchange System Manager (ESM), which took awhile to load and looked at the configuration. I opened Administrative Groups, Admin_Group_Name, Servers, Server_Name, Protocols, Smtp, Default SMTP Virtual Server, Current Sessions and noticed six connections had been connected to the SMTP virtual server for more than 5 minutes. This finding was the first clue that something was very wrong on the server. Typically, an Exchange session lasts for a few seconds at most, unless the connection is sending or receiving an email message with a large attachment. I looked at the queues on the default SMTP virtual server and noticed that more than 50 queues were in various states of sending email or waiting for a retry. Obviously, someone was using the company's mail server as a relay. But how? As you know, Exchange 2000 isn't an open relay by default. The server was current with the latest versions (Win2K Service Pack 4—SP4—and Exchange SP3) and had the latest critical security updates. I reviewed the relay settings and used the open relay test from Open Relay DataBase (— to ensure that the relay was closed.

Whenever I tried to clear a connection to the default SMTP virtual server, the connection would reappear, usually with a different domain name but from the same IP scheme. I traced the IP addresses to a block allocated by an ISP in China. After concluding that the server was not an open relay, I decided that someone was probably authenticating to the server and sending spam. The backup was failing because it was trying to back up all the mail the spammer was attempting to send.

I opened the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in and, with the client’s help, removed all the invalid users. I noticed that some users in the Administrators Group didn’t belong in the group, so I removed the unauthorized users. I first suspected that a former employee had “sold” the password to a spammer so that the spammer could use the mail server to relay messages (more about this later). I thought some malicious activity might be occurring on the server, so I checked the following run subkeys in the registry to determine whether any hacking programs were loaded:
· HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
· HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
· HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersionPolicies\Explorer\Run

The subkeys turned out to be clean. I also ran a virus scan on the server, and the server was clean. From the surface, the server looked free from any hacking tools. In Part 2, I’ll discuss how I closed the hole and how to prevent this type of problem from recurring.

Tip Some ISPs now check for a domain's DNS record in an attempt to reduce the amount of spam received by their mail servers. If you’re making a change to or establishing new mail service for a domain, make sure that the ISP adds a DNS entry for the domain. A common symptom of not having the correct DNS records is the inability to send mail to certain domain names. Most DNS providers will automatically do add a DNS entry for the domain when establishing a mail exchange (MX) record for the domain and when entering a reverse Pointer (PTR) record, in case the mail server performs a reverse lookup for the domain. By ensuring that you have an MX record and reverse record for your domain, you should be able to reduce the sending problems you have with specific domains.

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