According to Nagle, waxing sarcastic, "Several commercial products are now available to overcome those little obstacles to bulk posting. A tool called CL Auto Posting Tool is one such product. It not only posts to Craigslist automatically, it has built-in strategies to overcome each Craigslist anti-spam mechanism." It's not the only one. There are, he added, "other desktop software products [such as] AdBomber and Ad Master. For spammers preferring a service-oriented approach, there's ItsYourPost." The result? "The defenses of Craigslist have been overrun. Some categories on Craigslist have become over 90 per cent spam. The personals sections were the first to go, then the services categories, and more recently, the job postings."

Of course, you don't have to pay anything. There are now free CAPTCHA crackers available online.

Craigslist is fighting back. The organization is now using phone verification for some ads. Crackers, in return, are working on a way to break Craigslist's phone defenses. With combat costs mounting, it's hard to see how Craigslist, which has always been a free service, can continue to survive with its no-visible-means-of-revenue model.

It's not, as the Craigslist situation shows, that malicious e-mail is the only problem coming from broken CAPTCHA security. Paul Wood, senior analyst at MessageLabs, a UK-based e-mail security company, says, "MessageLabs have already begun to see examples of spammers exploiting other techniques once they have bypassed the CAPTCHA of Google and Hotmail -- for example, using Google Docs to create spam content and including the link in the spam e-mail messages, evading traditional antispam techniques that rely on identifying known spam domains in URLs."

Social network users are also vulnerable to attack from CAPTCHA-compromised sites, says Stephan Chenette, manager of security research at Websense Security Labs.

"The newer generation doesn't use e-mail to communicate," Chenette explains. "Instead, they use social networks, and they're not too concerned about revealing their personal information on social networks or blogs where they post instead of sending e-mail. What happens is that an attacker creates a public blog of his own or sets up an account; he can then use these to publish malicious links. By exploiting the trust of the people on that community, he uses them to spread botnets and the like."

Because social networks offer such an "enormous attack surface" and "their users don't think of themselves as being vulnerable in the same way experienced e-mail or IM users are," they're especially easy to exploit, says Chenette.

Another new attack vector is coming from CAPTCHA's collapse: the quick creation of fake Web sites. According to Chenette, these sites get their content from legitimate Web sites by copying and pasting to maximize their search engine optimization and reputation to quickly gain an audience.

"Reputation is all the rage for malicious attackers. From a search engine perspective, the content is what matters. Malicious attackers will pull sites' contents and embed it in their site, and that gives them a high search-engine ranking, which gives them a higher reputation," says Chenette. "We've been seeing that quite a lot recently. Of course, search engine poisoning is quite old, but now reputation sites [such as Digg] that use CAPTCHA are being targeted."