CAPTCHA used to be an easy and useful way for Web administrators to authenticate users. Now it's an easy and useful way for malware authors and spammers to do their dirty work.

CAPTCHA -- Completely Automated Public Turing Test to Tell Computers and Humans Apart -- was a good idea in its day. You presented users with an obfuscated string of characters and then had them decode and type the string in to get an e-mail account, a social networking account or comment access on an online forum. Not much fuss (though users justifiably complained that the difference between 1 (one) and l (the lower-case letter l) can be hard to see in many fonts) and certainly no muss from a Web administrator's point of view.

So it was that CAPTCHA went from relatively obscure security measure perfected in 2000 by researchers at Carnegie Mellon University to deployment by most of the major Web e-mail sites and many other Web sites by 2007. Sites such as Yahoo Mail, Google's Gmail and Microsoft's Hotmail all used -- and, for that matter, continue to use -- CAPTCHA to make sure that only human beings, not bots, could get accounts or make postings.

Those days are long gone.

By January 2008, Yahoo Mail's CAPTCHA had been cracked . Gmail was ripped open in April. Hotmail's top got popped during the same month.

And then things got bad.

There are now programs available online (no, we will not tell you where) that automate CAPTCHA attacks. You don't need to have any cracking skills. All you need is a desire to spread spam, make anonymous online attacks against your enemies, propagate malware or, in general, be an online jerk.

It's not just free e-mail sites that can be made to suffer, though.

John Nagle, founder of SiteTruth , a site that tries to identify bogus businesses and their Web sites, wrote in late May on Techdirt that while spam on the popular online classified ad service Craigslist "has been a minor nuisance for years ... this year, the spammers started winning and are taking over."

Craigslist tried "to stop spamming by checking for duplicate submissions," Nagle explained. "They check for excessive posts from a single IP address. They require users to register with a valid e-mail address. They added a CAPTCHA to stop automated posting tools. And users can flag postings they recognize as spam."

For more enterprise computing news, visit Computerworld. Story copyright Computerworld, Inc.