Want to evade a widely used security defense meant to ensure that a human--rather than an automated attack tool--is requesting access to a website or service? Outsource the interaction to end users in exchange for providing free porn, or pay a nominal fee to freelancers willing to manually log Captcha values.

Both of those strategies, in fact, are now being employed by attackers to help defeat Captcha tests, according to a new report from security firm Imperva, titled "A Captcha in the Rye." (That's a nod to J. D. Salinger's The Catcher In The Rye, in which protagonist Holden Caulfield refers to almost everyone he meets as a "phony.")

The inability of websites to tell whether requests are phony or authentic is an ongoing security problem, as the torrent of spam in many websites' comments sections illustrates. To help stop that spam, among other nuisances or attacks, many websites rely on a Captcha, which stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart. The test is meant to provide a challenge that's easy for a human to solve, but difficult or impossible for a machine to handle.

[ LinkedIn's security breach leads to a class action lawsuit. Read about it here: LinkedIn Security Breach Triggers $5 Million Lawsuit. ]

The traditional Captcha serves up a wavy image that's ostensibly difficult for a machine to process. Other Captcha approaches have involved video, games, and audio--not least to assist visually impaired users.

"Captchas are put in place to protect sites from automation of actions," said Rob Rachwald, director of security strategy at Imperva, in a blog post. Such automation can be used by attackers to seed blogs with comments that include links to malware, to quickly copy large amounts of data from website databases, and to create a large number of fake accounts to trick people into believing that information or links relayed via those accounts--for example, on Facebook, Google+, or Twitter--is legitimate.

Over the years, Captcha builders have continued to refine their technology to try and stay ahead of automated Captcha-guessing tools. Accordingly, some attackers have turned to a more straightforward cracking strategy: outsourcing. "Services like DeCaptcher recruit Captcha solvers from around the world and offer Captcha-solving services as a retailer," reads Imperva's report. "Having many employees allows [a] 24-7 service guarantee while handling massive amounts of Captchas in very little time. At current rates, Captcha solvers get $1 to $3 dollars for solving thousands of Captchas, and are often rewarded (or penalized) according to their speed and achieved percent of accurate responses."

How much does it cost to crack a Captcha? The "Bypass Captcha" service charges $14 per 1,000 Captchas cracked, while "Death by Captcha" charges only $1.39. Meanwhile, other sites do it themselves by offering free games or even porn to site users in exchange for their prowess at solving Captchas, which are copied in from targeted sites. "Instead of paying for a subscription, the user browsing the site gets--every now and then--a pop-up containing a Captcha, which he is required to solve in order to keep enjoying the site or be allowed to see more content," said the report.