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How Spammers Fool Bayesian Filters - 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 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.


About the author:

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 www.ciphertrust.com/products/spam_and_fraud_protection today.

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Winning the War On Spam

For years I didn't worry much about spam.

But lately it's got out of control. Over half of my email is now spam, and it was growing by the week - until I took action.

This article shows you some strategies for winning the war on spam.

------------------------------------------------

How Do They Get Your Address?

------------------------------------------------

In the old days, spammers got their addresses mainly from Newsgroups - if you didn't post to Newsgroups, you were reasonably safe. But they're now using a much more efficient method to build their lists - email harvesters.

Email harvesters are robots that roam the Internet collecting email addresses from web pages. Examples are EmailSiphon, Cherry Picker, Web Weasel, Web Bandit and Email Wolf, to name just a few.

How can you protect yourself from email harvesters?

By 'munging' (mung = 'mash until no good') or cloaking your email address.

There are many ways of munging your address - the easiest technique is to use ASCII code for the punctuation in your email address (instead of symbols).

For the colon after mailto use : and for the @ symbol use @ and for the period use . . With this method, your email address would become:

mailto:yourname@yourdomain.com

but it will display as:

mailto:yourname@yourdomain.com

Your email address will appear exactly as it did before, and it will still be 'clickable', but email harvesters will ignore it and move on.

There are also JavaScript's that you can insert into your web page that will make your email address visible to humans but invisible to harvesting programs. Here's one that works very well: http://pointlessprocess.com/JavaScripts/anti-spam.htm

-----------------------------

How To Fight Spam

-----------------------------

The most important thing is never, ever, reply to spam.

Most spam contains an innocent-looking 'remove me' email address. Do not use it. Here's why:

Spammers typically buy a CD containing a million or so email addresses, but they have no idea how many of those addresses are active. So before beginning their marketing campaign in earnest, they send out a 'test message' to the entire list.

The test message contains an email address for removing yourself. When you reply to that address, it confirms to the spammer that your address is active and therefore worth spamming.

Worse still, the spammer may be distilling from that CD a list of confirmed active addresses that he will then sell to another spammer.

The key to dealing with spam is to report it to a 3rd party: (1) the affiliate program that the spammer is advertising, (2) the spammer's web host, or (3) the ISP the spammer used to connect to the Internet.

When you report spam to a 3rd party, remember to be polite - they didn't send the spam and they're probably just as anti-spam as you are.

(1) Reporting to Affiliate Programs

Many spammers are affiliates advertising someone else's products or services. So look for a website address that contains an affiliate link, something like this: www.affiliateprogramdomain/841526

Then just send an email to the affiliate program (abuse@affiliateprogramdomain.com), informing them that you are receiving spam from one of their affiliates.

Most affiliate programs have zero tolerance for spamming and will remove an affiliate spammer without warning.

Now, affiliate spammers don't want you to see their affiliate link, so many of them send their email as HTML. All you see in the message are the words 'Click Here and Order Now'.

But in your browser just click on 'View Source Code' and search for the letters 'http'. That will take you to the spammer's affiliate link.

(2) Reporting to Web Hosts

If the spam doesn't contain an affiliate link, it's likely that it is coming from the owner of the domain name. In that case you'll have to report it to the spammer's web host or their ISP.

To make a report to the spammer's web host just go to Whois, the directory of registered domain names: http://www.netsol.com/cgi-bin/whois/whois

Type in the spammer's domain (the website address that appears in the spam) together with the extension (.com, .org, .net etc).

The host for that domain will usually be listed as the Technical Contact in the Whois record and there will be an email address for contacting them.

(3) Reporting to ISPs

To report a spammer to his Internet Service Provider, you'll have to look at the spam's 'extended headers'.

Extended headers show the servers that the message passed through in order to get to you. The instructions for viewing extended headers will vary depending on what email client you are using.

=> In Pegasus Mail, open the offending message and then

right-click and choose 'Show raw message data'.

=> In Eudora Light, click on 'Tools' in the top menu

bar, and then 'Options', and then select the

checkbox option that says 'Show all headers (even

the ugly ones)' and click OK.

=> In Outlook Express, open the offending message,

select 'Properties' from the File menu and then

click the 'Details' tab.

Reading and understanding extended headers is quite a detailed subject. Here's an excellent free tutorial on how to decipher extended headers: http://www.doughnut.demon.co.uk/SpamTracking101.html

As an alternative to these reporting techniques, you could use a web-based spam reporting service such as SpamCop (www.spamcop.net). SpamCop deciphers the spam's message headers and traces the mail back to its source.

Wishing you every success in the fight against spam!

------------------------------------------------------------
Michael Southon has been writing for the Internet for over 3
years. He has shown hundreds of webmasters how to use this
simple technique to build a successful online business. Click
here to find out more: http://ezine-writer.com/
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