How Spammers Fool Rule-based and Signature-Based Spam Filters

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.

Heuristics (Rule-based Filtering)

One method used to combat spam is Rule-based, or Heuristic Filtering. Rule-based filters scan email content for predetermined words or phrases that may indicate a message is spam. For example, if an email administrator includes the word "sex" on a company's rule-based list, any email containing this word will be filtered.

The major drawback of this approach is the difficulty in identifying keywords that are consistently indicative of spam. While spammers may frequently use the words "sex" and 'Viagra" in spam emails, these words are also used in legitimate business correspondence, particularly in the healthcare industry. Additionally, spammers have learned to obfuscate suspect words by using spellings such as "S*E*X", or "VI a a GRR A".

It is impossible to develop dictionaries that identify every possible misspelling of "spammy" keywords. Additionally, because filtering for certain keywords produces large numbers of false positives, many organizations have found they cannot afford to rely solely on rule-based filters to identify spam.

Signature-Based Spam Filters

Another method used to combat spam is Signature-based Filtering. Signature-based filters examine the contents of known spam, usually derived from honey pots, or dummy email addresses set up specifically to collect spam. Once a honey pot receives a spam message, the content is examined and given a unique identifier. The unique identifier is obtained by assigning a value to each character in the email. Once all characters have been assigned a value, the values are totaled, creating the spam's signature. The signature is added to a signature database and sent as a regular update to the email service's subscribers. The signature is compared to every email coming in to the network and all matching messages are discarded as spam.

The benefit of signature-based filters is that they rarely produce false-positives, or legitimate email incorrectly identified as spam. The drawback of signature-based filters is that they are very easy to defeat. Because they are backward-looking, they only deal with spam that has already been sent. By the time the honey pot receives a spam message, the system assigns a signature, and the update is sent and installed on the subscribers' network, the spammer has already sent millions of emails. A slight modification of the email message will render the existing signature useless.

Furthermore, spammers can easily evade signature-based filters by using special email software that adds random strings of content to the subject line and body of the email. Because the variable content alters the signature of each email sent by the spammer, signature-based spam filters are unable to match the email to known pieces of spam.

Developers of signature-based spam filters have learned to identify the tell-tale signs of automated random character generation. But as is often the case, spammers remain a step ahead and have developed more sophisticated methods for inserting random content. As a result, most spam continues to fool signature-based filters.

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.

Earthlink and the devil's spam filter
Macworld, CA - Aug 20, 2008
by Christopher Breen, Macworld.com Earthlink’s spamBlocker is the tool of the devil if it’s not configured properly. Let’s suppose you’ve sent me a message ...

Duplicate email arrives once an hour for up to 96 hours
TechRepublic, KY - Aug 28, 2008
3) Watchguard Spamblocker - if it was the spam filter causing timeouts we'd see one or two duplicates...not NINETY-SIX. Having elimintated everything, I am ...

6 Simple Ways To Put A Computer Technician Out Of Business
Corsavoo.com, France - Aug 20, 2008
Spam Filter: There are so many free email programs out there that do amazing jobs at blocking spam that this is nearly obsolete, but just to name my ...

Olympics and Intel Developer Forum
Macworld, CA - Aug 27, 2008
In the early part of the show, I don my cranky-pants to rail against Earthlink’s spamBlocker technology, which includes a Challenge and Response “feature. ...

International Nepal Solidarity Network
COCAP press release
International Nepal Solidarity Network, Nepal - Aug 20, 2008
We use a spam blocker that filters comments with certain words. If you cannot post a comment, please send it to the webmaster. Also, comments with more than ...

Steer Spotlight: BSHS Center Zack Saldivar
Big Spring Herald (subscription), TX - 2 hours ago
He's a good blocker and a great teammate. A lot of what we'll do offensively this year will be based on his abilities.” For Saldivar, the transition has ...

On the prowl
Scotts Valley/San Lorenzo Valley Press-Banner, CA - Aug 28, 2008
Senior Will Lawton will start at tight end at a position the Cougars have traditionally utilized as a blocker and a deep threat on play-action passes. ...

U-Verse for Business Has Wi-Fi Perks
Wi-Fi Planet, CT - Aug 28, 2008
... installation (eventually it will cost $95), unlimited e-mail storage and security applications for anti-spyware, anti-virus, pop-up blocker and spam. ...

OpEdNews
To the Editors at OpEdNews:
OpEdNews, PA - Aug 1, 2008
Multiple apologies aside, am I being picky, or does having to jump a spam-blocker seem a bit much for a regular contributor’s access to an editor? ...

RICHARD OSBORNE: E-mail, voice mail, cell phones make life easy ...
The Morning Journal, OH - Aug 10, 2008
Even with a "spam blocker" (now there's a term we can live without), for example, I would guesstimate that more than 75 percent of the e-mail I receive is ...

Beware Of Spam Withdrawals

Q: I am so sick of all the spam that is sent to my business... Read More

Bayesian Spam Filters Explained

In a word Bayesian spam filters are "intelligent". Bayesian spam filters are intelligent in so... Read More

Spam - How to Report it

I'm sure you find spam just as frustrating and annoying as I do. So I've... Read More

Ignoring These Tips Could Result in an Inbox Full of Spam

Although there still seem to be some differences among the US Government, the lawmakers, the... Read More

Phishing for an Identity

Phishing is rapidly becoming on the largest threats to your personal, financial, and emotional wellbeing.... Read More

Three Faces of SPAM

Like everybody who will ever read this, I get spam in my e-mail. Mine seems... Read More

How Spammers Fool Whitelists - And How to Stop Them

Effectively stopping spam over the long-term requires much more than blocking individual IP addresses and... Read More

Anti Trackback and Comment Spam Methods

What is spam ? Spam is text or... Read More

Demand for Spam? It exists

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

How You Can Avoid The New Dangers Of Spam

Until recently, spam has been an annoyance, a definite load on your email system and... Read More