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In Internet usage, an email bomb is a form of net abuse consisting of sending huge volumes of email to an address in an attempt to overflow the mailbox or overwhelm the server where the email address is hosted in a denial-of-service attack.
There are three methods of perpetrating an email bomb: mass mailing, list linking and zip bombing.
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, also known as "email cluster bomb", means signing a particular email address up to several email list subscriptions. The victim then has to unsubscribe from these unwanted services manually. The attack can be carried out automatically with simple scripts: this is easy, almost impossible to trace back to the perpetrator, and potentially very destructive. A massive attack of this kind targeting .gov email addresses was observed in August 2016.
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. However, even the confirmation emails contribute to the attack. A better defense would prevent Web sites from being exploited without abandoning subscription forms. After a subscription form is filled out, the Web site would dynamically create a mailto link to itself. A legitimate user would then send a message to validate the request without receiving any email from the Web site. While the sender's email could be spoofed, the sender's SMTP IP address cannot. The list manager can therefore verify that the email in the form request matches the originating SMTP server in the validation message.
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).
- Jakobsson, Markus; Menczer, Filippo (December 2003). "Untraceable Email Cluster Bombs". ;login:. USENIX. 28 (6). Retrieved 23 August 2016.
- Jakobsson, Markus; Menczer, Filippo (2010-01-01). Huang, Scott C.-H.; MacCallum, David; Du, Ding-Zhu, eds. Web Forms and Untraceable DDoS Attacks. Springer US. pp. 77–95. arXiv: . doi:10.1007/978-0-387-73821-5_4. ISBN 9780387738208.
- "Massive Email Bombs Target .Gov Addresses — Krebs on Security". krebsonsecurity.com. Retrieved 2016-08-23.