How Spammers Fool Bayesian Filters - And How to Stop Them

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 Bayesian Filtering. Named after Thomas Bayes, an English mathematician, Bayesian Logic is used in decision making and inferential statistics. Bayesian Filers maintain a database of known spam and ham, or legitimate email. Once the database is large enough, the system ranks the words according to the probability they will appear in a spam message.

Words more likely to appear in spam are given a high score (between 51 and 100), and words likely to appear in legitimate email are given a low score (between 1 and 50). For example, the words "free" and "sex" generally have values between 95 and 98, whereas the words "emphasis" or "disadvantage" may have a score between 1 and 4. Commonly used words such as "the" and "that", and words new to the Bayesian filters are given a neutral score between 40 and 50 and would not be used in the system's algorithm.

When the system receives an email, it breaks the message down into tokens, or words with values assigned to them. The system utilizes the tokens with scores on the high and low end of the range and develops a score for the email as a whole. If the email has more spam tokens than ham tokens, the email will have a high spam score. The email administrator determines a threshold score the system uses to allow email to pass through to users.

Bayesian filters are effective at filtering spam and minimizing false positives. Because they adapt and learn based on user feedback, Bayesian Filers produce better results as they are used within an organization over time. They are not, however, foolproof. Spammers have learned which words Bayesian Filters consider spammy and have developed ways to insert non-spammy words into emails to lower the message's overall spam score. By adding in paragraphs of text from novels or news stories, spammers can dilute the effects of high-ranking words. Text insertion has also caused normally legitimate words that are found in novels or news stories to have an inflated spam score. This may potentially render Bayesian filters less effective over time.

Another approach spammers use to fool Bayesian filters is to create less spammy emails. For example, a spammer may send an email containing only the phrase, "Here's the link?". This approach can neutralize the spam score and entice users to click on a link to a Web site containing the spammer's message. To block this type of spam, the filter would have to be designed to follow the link and scan the content of the Web site users are asked to visit. This type of filtering is not currently employed by Bayesian filters because it would be prohibitively expensive in terms of server resources and could potentially be used as a method of launching denial of service attacks against commercial servers.

As with all single-method spam filtering methodologies, Bayesian filters are effective against certain techniques spammers use to fool spam filters, but are not a magic bullet to solving the spam problem. Bayesian filters are most effective when combined with other methods of spam detection.

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.

Dr. Paul Judge is a noted scholar and entrepreneur. He is Chief Technology Officer at CipherTrust, the industry's largest provider of enterprise email security. The company's flagship product, IronMail provides a best of breed enterprise anti spam solution designed to stop spam, phishing attacks and other email-based threats. Learn more by visiting http://www.ciphertrust.com/prod ucts/spam_and_fraud_protection today.

---

Stop Spam: How To Escape The Spam Hell-Hole

If you're anything like me, you're pretty sick of it,... Read More

Where Did The Word Spam Come From?

We've all become familiar with the term spam. It's become... Read More

I Must Be The Luckiest Person Alive! Spam

I must be the luckiest person alive! My inbox is... Read More

Wiki Reek-y Havoc

The Vandals are coming! And this time they're after your... Read More

The Business Of Sending Spam

You all tear your hair out in frustration every time... Read More

All About Spam

Spam is annoying. Period. Why people would want to send... Read More

Fight Spam and Fortify Your Web Site with RSS

RSS is the answer to the Spam epidemic of the... Read More

The Cybermagic of Whitelists

Before we start getting deep into the meat of this... Read More

Six Tips to Get Rid of Spam Email

1. Ignore Spam EmailDo not open an unsolicited email. Spammers... Read More

Spammer in the Slammer: Jeremy Jaynes Sentenced to Nine Years

Will other spammers take heed? Don't count on it.Jeremy Jaynes... Read More

Demand for Spam? It exists

Do you like spam? No, I'm not kidding. Everybody knows... Read More

Do You Know Whats in Your Trash?

A hearty welcome to all the spam fighting filters and... Read More

Internet Theft and Fraud

My friends in the web hosting business have recently informed... Read More

Spam eMails Are Not Just Annoying - They Are A Main Distributor Of Viruses

Why is someone from India, Africa, or elsewhere writing you... Read More

Are You A Spam Zombie?

Over the past few years you've all become familiar with... Read More

How to Fight Back BEFORE Youre Falsely Accused of Spam

A friend of mine received a chilling email message from... Read More

Having a Bad e MALE Day? Email, Spam, Spam and More Spam

You just sat at your desk, opened your email account... Read More

How Spammers Fool Whitelists - And How to Stop Them

Effectively stopping spam over the long-term requires much more than... Read More

BUSTED: Anti Spam Forces Bankrupt Super-Spammer Scott Richter

Microsoft scores one for the good guysScott Richter, the self-proclaimed... Read More

Eight Quick Tips For Stopping SPAM

If you are buried in SPAM then you're not alone.... Read More

Managing Spam in 2005

In 1998, nearly 10% of all email traffic on the... Read More

Why Your ISP Takes Bribes From Spammers

The lifeblood of the spammer is email. They need to... Read More

How Spammers Fool Spam Blacklists - And How to Stop Them

Effectively stopping spam over the long-term requires much more than... Read More

Learn Simple Strategies That Will Stop Spammers From Bombarding Your Inbox!

Spam filter is the solution that most people resort when... Read More

The Trouble With Spam Is....

Each day we all face the same challenge. Spam. It... Read More

Block Spam with An Easy Behavioral Change

E-mails now have a connection back to their servers. I... Read More

Avoiding the Spam Trap: Get Your Message Delivered!

Your message is not being delivered.If you send emails to... Read More

ANTI-S*P^A#M: Protecting Your Web Sites Email Address(es)

Did you know that there are software programs that view... Read More

Is Your Website Blacklisted?

A blacklist, as the name implies, is a list of... Read More

How to Protect Your Email ID from Spammers?

You must be one of them experiencing a lot of... Read More

A War on SPAM: Attacking The Evil

As most small, medium and large businesses in this country... Read More

Stuffing the Spammers!

I'm really, truly fed up with spam. Every day when... Read More

Dealing With SPAM - An E-mail Address Strategy

With SPAM being such a problem it might seem the... Read More