How Spammers Fool Whitelists - And How to Stop Them

by: Paul Judge, CTO, CipherTrust, Inc.
Effectively stopping spam over the long-term requires much more than blocking individual IP addresses and creating rules based on keywords that spammers typically use. The increasing sophistication of spam tools coupled with the increasing number of spammers in the wild has created a hyper-evolution in the variety and volume of spam. The old ways of blocking the bad guys just don’t work anymore. Examining spam and spam-blocking technology can illuminate how this evolution is taking place and what can be done to combat spam and reclaim e-mail as the efficient, effective communication tool it was intended to be. One method used to combat spam is whitelisting. Whitelists are databases of trusted email sources. The list may contain specific email addresses, IP addresses or trusted domains. Emails received from a whitelisted source are allowed to pass through the system to the user’s email box. The list is built when users and email administrators manually add trusted sources to the whitelist. Once built, the catch-rate for spam can be close to 100%, however, whitelists produce an inordinate number of false positives. It is virtually impossible to produce an exhaustive list of all possible legitimate email senders because legitimate email can come from any number of sources. To get around this difficulty, some organizations have instituted a challenge-response methodology. When an unknown sender sends an email to a user’s account, the system automatically sends a challenge back to the sender. Some challenge-response systems require the sender to read and decipher an image containing letters and numbers. The image is designed to be unreadable by a machine, but easily recognizable by a human. Spammers would not spend the time required to go through a large number of challenge-response emails, so they drop the address and move on to those users who don’t use such a system. Whitelists are only partially successful and impractical for many users. For example, problems can arise when users register for online newsletters, order products online or register for online services. If the user does not remember to add the new email source to their whitelist, or if the domain or IP address is entered incorrectly, the communication will fail. Additionally, whitelists impose barriers to legitimate email communication and are viewed by some as just plain rude. Whitelists are not widely used by email users and administrators as a primary tool to fight spam because of the high number of false positives, and the difficulties in creating a comprehensive list of email sources. Because whitelists are not widely used, spammers typically do not develop countermeasures. As with other spam fighting techniques, whitelists are most effective when used in conjunction with other anti-spam tools. The Solution When used individually, each anti-spam technique has been systematically overcome by spammers. Grandiose plans to rid the world of spam, such as charging a penny for each e-mail received or forcing servers to solve mathematical problems before delivering e-mail, have been proposed with few results. These schemes are not realistic and would require a large percentage of the population to adopt the same anti-spam method in order to be effective. You can learn more about the fight against spam by visiting our website at www.ciphertrust.com and downloading our whitepapers.