Spammers are using an automated method to create bogus pages on Google’s Blogger service, again highlighting the diminishing effectiveness of a security system intended to stop mass account registrations, according to security vendor Websense.
The spammers are sending coded instructions to PCs in their botnets, or networks of computers that have been infected with malicious software, wrote Sumeet Prasad, a threat analyst, on Websense’s blog.
Those sophisticated instructions tell PCs how to register a free account on Blogger. The spammers also figured out a way to solve the CAPTCHA (Completely Automated Public Turing test to Tell Computers and Humans Apart), the warped text that has to be deciphered in order to complete an account registration.
CAPTCHA was designed to prevent hackers from using automated tools for abusive purposes.
Spammers have circumvented this tool using bot-compromised PCs to send a request to an external host. The host tries to solve the CAPTCHA and then sends the answer back to the PC.
Websense estimates the process has an 8 to 13 percent success rate.
It’s unknown how exactly the CAPTCHA gets solved. It’s been theorized the process has been outsourced to real humans who get paid for every one deciphered. But researchers have successfully developed methods that enable computers to increase their success rate at solving the puzzles, indicating that hackers have also figured out how to do it.
Security vendors and researchers have seen a rapid rise in accounts used for spam on free e-mail services from Microsoft, Yahoo and Google, indicating current CAPTCHA technology has reached the end its usefulness.
Microsoft’s system to thwart automatic registrations of e-mail accounts leads to “a false sense of security,” according to two researchers who have developed a low-cost way to break the security mechanism.
Jeff Yan and Ahmad Salah El Ahmad of the School of Computing Science at Newcastle University in the U.K. wrote in a research paper that their method can solve around 60 percent of Microsoft’s CAPTCHAs used for validating registrations for its Windows Live Mail service.
Microsoft could make its CAPTCHAs harder to solve for computers by, for instance, letting letters overlap, but that also makes it harder for people, said Yan, who lectures at the University of Newcastle.
As of the last few months, CAPTCHAs have been become increasingly ineffective. The CAPTCHA systems used by free e-mail providers such as Microsoft, Google and Yahoo have been solved on a mass scale, leading to an increase in spam originating from their domains.
Details are scarce on how hackers are solving the CAPTCHAs in great numbers. It has been suspected that low-wage CAPTCHA solvers are being employed in order to get a steady stream of new e-mail accounts.
Yan and El Ahmad started their work in mid-2007. Microsoft was notified of the problems outlined in their paper in September 2007. The researchers released the paper a few days ago with Microsoft’s blessing.
Overall, Microsoft’s CAPTCHA system is well designed, and the company even holds three patents related to it, they wrote. But designing a fool-proof CAPTCHA system isn’t easy.
“To the best of our knowledge, this for the first time shows that a CAPTCHA that was carefully designed by serious professionals…is nevertheless vulnerable to novel but simple attacks,” Yah and El Ahmad wrote.
In February, it was discovered that hackers were using a method that appeared to have a 30 percent to 35 percent success rate in solving the CAPTCHA used for Windows Live Hotmail.
Using their own analysis and algorithms, Yan and El Ahmad have almost doubled the success rate of the February attacks.
One of the hardest parts of breaking CAPTCHAs is separating the letters and putting the letters in the right order, a process known as segmentation. The twisting, wispy letters are confusing to machines, and humans are much better at sorting out extraneous lines.
Yan and El Ahmad’s analysis was performed with off-the-shelf hardware: a 1.86 GHz Intel Core 2 Duo CPU (central processing unit) with 2G bytes of RAM. Their seven-step method is capable of removing “arcs” or strokes that link letters and make letters hard to isolate.
Ninety-two percent of the time, they could isolate each of the eight characters used for Microsoft’s CAPTCHA. Combined with character recognition techniques, the CAPTCHAs could be solved 61 percent of the time.
Their method also works against the latest CAPTCHAs deployed by Yahoo last month, although the success rates are not as high. Yan said he will soon release another research paper looking at Yahoo’s CAPTCHAs.
Of the big three — Yahoo, Microsoft and Google — Google seems to have the most effective CAPTCHAs right now due to the difficulty automated programs have in separating the characters, Yan said.
“Actually I think at a high level, the idea of a CAPTCHA is a good one, but the devil is in the details,” Yan said.
“Spammers include these redirecting accounts in different spam campaigns rather than including their actual spam domains,” Prasad wrote. “Spammers use this tactic to defeat a range of antispam services.”
In effect, they’re using Google’s Blogger domain as a shield, as it’s unlikely to be blocked by other security software products for being a suspicious domain.
The latest methods means a potential increase in the number of garbage pages on Blogger. But the sheer number of Blogger sites on the whole helps the spammy ones stay under the radar a bit longer, Prasad wrote.
Google has been fighting spam for a long time on Blogger. It uses automated spam classifying algorithms to keep blogs full of spam links out of its featured content. Users can also use a reporting tool to alert Google to spam blogs, but the fight continues.