As an abuse administrator, I'm surrounded by spam on a daily basis. When someone sends an abuse-related complaint to our firstname.lastname@example.org contact address, it gets added to our ticket queue, and our Abuse SLayers take time to investigate and follow up with the customers whose servers violate our acceptable use policy. The majority of those abuse-related submissions are reporting spam coming from our network, and in my interaction with customers, I've noticed that spam (and the source of spam) is widely misunderstood.
Most spam tickets we create on customer accounts pinpoint spam sent from a compromised or exploited server. Our direct customer didn't send the phishing email, malware distribution, pharmacy advertisement or pornographic spam, but that activity came from their account. While they're accountable for the abusive behavior coming from their server, in many cases, they don't know that there's a problem until we post an abuse ticket on their account. These servers are targeted and compromised by common techniques and exploits that could have been easily avoided, but they aren't very well known outside the world of abuse.
To protect yourself from a spammer, you need to think like a spammer. You need to understand how someone might try to exploit your environment so that you can prevent them from doing so. As you're looking at ways to secure your server proactively, make sure you target these five exploits in particular:
1. User Auth Login
This is by far the most common exploit to used to send spam. This method involves a person or script using the credentials of a user to send spam through a domain's mail server. The majority of these incidences are caused by malware on a client PC that obtains the login and password for a domain user and uses that information to log on and send mail from the client PC through the server. Often, these spam messages are sent through a botnet command structure.
When an account is compromised, simply changing the password for the compromised user on the server usually won't stop the abuse. We see quite a few accounts that continue to send spam after an initial abuse ticket results in a password change. Most servers that are sending spam with this method are found to only be sending a small amount of spam at any given time to avoid detection. The low volume of spam that is being sent per server is made up for by the fact that there are thousands of servers being used for the same spamming campaigns.
In order to stop the User Auth Login exploit, a customer needs to clean all of the malicious software (malware) from their environments. To prevent future User Auth Login compromises, users should be made aware of the potential dangers of untrusted software, and if they believe their machines are infected, they need to know what to do.
2. Tell-a-friend Exploitation
The User Auth Login technique is the most common method employed by spammers, but the "tell-a-friend" script exploitation isn't far behind when it comes to volume of affected servers. This spamming method find websites that use scripts to invite users to refer friends to a page or product. Spammers will use the 'Your Message' field in one of these scripts to input their own content and links, and they'll push the actual page referral link to the bottom of the message. When these site scripts aren't secure, the spammer will use them to send hundreds or thousands of messages.
To avoid having your website fall victim to this type of spam, be very wary of any widget or script you add. If you need to add Facebook, Twitter and email "share" functionality to your site, make sure you incorporate a tell-a-friend script that does not allow for customizable messages or does not accept input of more than one email address. Also, users won't need the "cc" or "bcc" fields, so you can be sure those are axed as well. If you can't find a good "share" script that you're comfortable with from a security perspective, it might be a good idea to remove that functionality to avoid exploitation.
3. Uploaded Mailers
Spam sent via an uploaded third party mailer can sometimes prove difficult for admins to locate. An uploaded third party mailer could be capable of creating it's own outbound SMTP connection, and that would allow a program to bypass the existing MTA on the server and render any legitimate mail logs useless for investigation. Another challenge is that a php mailer can be uploaded to a location within a user's web content, and that mailer is run by the user 'nobody' (the default Apache user).
We strongly suggest configuring your server to have the mail headers show the script's user (that's not the Apache default user) and the location the script is running from on the server. Many times, these kinds of mailers are maliciously uploaded after a user's FTP password is been compromised, so be sure your FTP login information is secure.
4. Software Exploits
The "software exploits" category casts a huge shadow. Every piece of software on a server — from mail servers, content management systems and control panels to the operating system itself — can be targeted by hackers. They probe servers to find security vulnerabilities and weak coding, and when they find a vulnerability, they take control.
The hacker who found the software vulnerability might not actually take advantage of the exploit immediately. That user may sell access to other entities for their use, and that use often ends up being spam. In addition to having strong firewall rules and access restrictions, you should update and maintain the current stable versions of all software on your servers.
5. WordPress Exploits
WordPress exploits would technically fall under the "Software Exploits" category, but I'm breaking it out into its own category simply due to the volume of spam issues that are the result of exploiting this particular piece of software. The first step to protecting against spam being sent through this source is to make sure you have the latest version of WordPress installed. With that done, be sure to research the latest security plugins for that version and install any that are applicable to your environment.
These five techniques are not the only ones used by spammers to take advantage of your environment, but they are some of the most common. To protect yourself from becoming a source of spam, make your servers a more difficult target to exploit. To stop spam, you need to know spam. Now that you know spam, it's time to stop it. Ask questions, test your environment regularly and watch your logs for any unexplained usage.