E-Mail Bombing

I. Description

Email bombing is characterized by abusers repeatedly sending an email message to a particular address at a specific victim site. In many instances, the messages will be large and constructed from meaningless data in an effort to consume additional system and network resources. Multiple accounts at the target site may be abused, increasing the denial of service impact.

Email spamming is a variant of bombing; it refers to sending email to hundreds or thousands of users (or to lists that expand to that many users). Email spamming can be made worse if recipients reply to the email, causing all the original addressees to receive the reply. It may also occur innocently, as a result of sending a message to mailing lists and not realizing that the list explodes to thousands of users, or as a result of a responder message (such as vacation(1)) that is setup incorrectly.

Email bombing/spamming may be combined with email spoofing (which alters the identity of the account sending the email), making it more difficult to determine who actually sent the email.

II. Technical Issues

• If you provide email services to your user community, your users are vulnerable to email bombing and spamming.
• Email spamming is almost impossible to prevent because a user with a valid email address can spam any other valid email address, newsgroup, or bulletin-board service.
• When large amounts of email are directed to or through a single site, the site may suffer a denial of service through loss of network connectivity, system crashes, or failure of a service because of
• overloading network connections
• using all available system resources
• filling the disk as a result of multiple postings and resulting syslog entries

III. What You Can Do

1. Detection
2. If your system suddenly becomes sluggish (email is slow or doesn't appear to be sent or received), the reason may be that your mailer is trying to process a large number of messages.
3. Reaction
1. Identify the source of the email bomb/spam and configure your router (or have your Network Service Provider configure the router) to prevent incoming packets from that address.

Review email headers to determine the true origin of the email. Review the information related to the email bomb/spam following relevant policies and procedures of your organization.

2. Follow up with the site(s) you identified in your review to alert them to the activity. Contact them to alert them to the activity.
3. Ensure you are up to date with the most current version of your email delivery software (sendmail, for example) and increase logging capabilities as necessary to detect or alert you to such activity.
3. Prevention
4. Unfortunately, at this time, there is no way to prevent email bombing or spamming (other than disconnecting from the Internet), and it is impossible to predict the origin of the next attack. It is trivial to obtain access to large mailing lists or information resources that contain large volumes of email addresses that will provide destination email addresses for the spam.
1. Develop in-house tools to help you recognize and respond to the email bombing/spamming and so minimize the impact of such activity. The tools should increase the logging capabilities as well as check for and alert you to incoming/outgoing messages that originate from the same user or same site in a very short span of time. Once you identify the activity, you can use other in-house tools to discard the messages from the offending users or sites.
2. If your site uses a small number of email servers, you may want to configure your firewall to ensure that SMTP connections from outside your firewall can be made only to your central email hubs and to none of your other systems. Although this will not prevent an attack, it minimizes the number of machines available to an intruder for an SMTP-based attack (whether that attack is a email spam or an attempt to break into a host). It also means that should you wish to control incoming SMTP in a particular way (through filtering or another means), you have only a small number of systems--the main email hub and any backup email hubs--to configure.
3. Consider configuring your mail handling system(s) to deliver email into filesystems that have per-user quotas enabled. Doing this can minimize the impact of an email bombing attack by limiting the damage to only the targeted accounts and not the entire system.
4. Educate your users to call you about email bombing and spamming.
5. Do not propagate the problem by forwarding (or replying to) spammed email.

There are three methods of perpetrating an email bomb; Mass mailing, list linking and zip bombing

Mass mailing

Mass mailing consists of sending numerous duplicate mails to the same email address. These types of mail bombs are simple to design but their extreme simplicity means they can be easily detected by spam filters. Email-bombing using mass mailing is also commonly performed as a DDoS attack by employing the use of "zombie" botnets; hierarchical networks of computers compromised by malware and under the attacker's control. Similar to their use in spamming, the attacker instructs the botnet to send out millions or even billions of emails, but unlike normal botnet spamming, the emails are all addressed to only one or a few addresses the attacker wishes to flood. This form of email bombing is similar in purpose to other DDoS flooding attacks. As the targets are frequently the dedicated hosts handling website and email accounts of a business, this type of attack can be just as devastating to both services of the host.

This type of attack is more difficult to defend against than a simple mass-mailing bomb because of the multiple source addresses and the possibility of each zombie computer sending a different message or employing stealth techniques to defeat spam filters.

List linking

List linking means signing a particular email address up to several email list subscriptions. The victim then has to unsubscribe from these unwanted services manually. In order to prevent this type of bombing, most email subscription services send a confirmation email to a person's inbox when that email is used to register for a subscription. This method of prevention is easily circumvented: if the perpetrator registers a new email account and sets it to automatically forward all mail to the victim, he or she can reply to the confirmation emails, and the list linking can proceed.

Zip bombing

A ZIP bomb is a variant of mail-bombing. After most commercial mail servers began checking mail with anti-virus software and filtering certain malicious file types, EXE, RAR, Zip,7-Zip, mail server software was then configured to unpack archives and check their contents as well. A new idea to combat this solution was composing a "bomb" consisting of an enormous text file, containing, for example, only the letter z repeating millions of times. Such a file compresses into a relatively small archive, but its unpacking (especially by early versions of mail servers) would use a greater amount of processing, which could result in a DoS (Denial of Service).