Feeds

Report: Popular CAPTCHAs easily defeated

Researchers build CAPTCHA dispatcher Decaptcha ...

Top 5 reasons to deploy VMware with Tegile

Security researchers have discovered the vast majority of text-based anti-spam tests are easily defeated.

Computer scientists from Stanford University discovered 13 of 15 CAPTCHA schemes from popular websites were vulnerable to automated attacks. The CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) has been used for several years to prevent automated sign-ups to webmail accounts or online forums in order to block spam bots. Surfers are typically asked during a registration process to identify distorted letters as depicted in an image. A variety of other approaches – including pictures of cats, audio clips and calculus puzzles – have been applied to the problem over the years.

Cybercrooks have responded to the challenge posed by CAPTCHAs by devising techniques that typically involve semi-automatically signing up for new accounts, while relying on the human cogs in 21st century sweatshops – typically located in India – to solve the CAPTCHA puzzles themselves.

The Stanford team, by contrast, looked at whether it was possible to fully automate the process of breaking CAPTCHAs. Their techniques including removing deliberately introduced image background noise and breaking text strings into single characters for easier recognition. The team built an automated tool, called Decaptcha, that applied these various tricks. The approach was partially inspired by techniques used to orientate robots in unknown environments.

Decaptcha was turned against the challenge response CAPTCHAs used by 15 high-profile websites, enjoying excellent bowling figures against the majority.

For example, Visa's Authorize.net payment gateway CAPTCHA was defeated 66 per cent of the time. eBay's CAPTCHA was sidestepped 43 per cent of the time. Lower, but still workable, bypass rates were achieved against Wikipedia, Digg and CNN.

Google and reCAPTCHA were the only two CAPTCHA systems that consistently thwarted Decaptcha during the tests.

Authorize.net and Digg have both switched to reCAPTCHA since these tests were run, Computerworld adds.

In a research paper (PDF), the Stanford team suggest several approaches towards making CAPTCHAs harder to beat, including making the length of a text string changeable and randomising character font and size. Lines in the background of CAPTCHAs might also prove effective. In addition, the Stanford team highlighted features that are ineffective against automated attacks but may counter the activities of humans.

The researchers, Elie Bursztein, Matthieu Martin and John C Mitchel, who previously developed techniques for breaking audio CAPTCHAs, presented their latest research at the recent ACM Conference On Computer and Communication Security in Chicago. ®

Internet Security Threat Report 2014

More from The Register

next story
'Kim Kardashian snaps naked selfies with a BLACKBERRY'. *Twitterati gasps*
More alleged private, nude celeb pics appear online
Home Depot ignored staff warnings of security fail laundry list
'Just use cash', former security staffer warns friends
Hackers pop Brazil newspaper to root home routers
Step One: try default passwords. Step Two: Repeat Step One until success
UK.gov lobs another fistful of change at SME infosec nightmares
Senior Lib Dem in 'trying to be relevant' shocker. It's only taxpayers' money, after all
Who.is does the Harlem Shake
Blame it on LOLing XSS terroristas
Snowden, Dotcom, throw bombs into NZ election campaign
Claim of tapped undersea cable refuted by Kiwi PM as Kim claims extradition plot
Freenode IRC users told to change passwords after securo-breach
Miscreants probably got in, you guys know the drill by now
THREE QUARTERS of Android mobes open to web page spy bug
Metasploit module gobbles KitKat SOP slop
BitTorrent's peer-to-peer chat app Bleep goes live as public alpha
A good day for privacy as invisble.im also reveals its approach to untraceable chats
prev story

Whitepapers

Secure remote control for conventional and virtual desktops
Balancing user privacy and privileged access, in accordance with compliance frameworks and legislation. Evaluating any potential remote control choice.
Intelligent flash storage arrays
Tegile Intelligent Storage Arrays with IntelliFlash helps IT boost storage utilization and effciency while delivering unmatched storage savings and performance.
WIN a very cool portable ZX Spectrum
Win a one-off portable Spectrum built by legendary hardware hacker Ben Heck
High Performance for All
While HPC is not new, it has traditionally been seen as a specialist area – is it now geared up to meet more mainstream requirements?
Beginner's guide to SSL certificates
De-mystify the technology involved and give you the information you need to make the best decision when considering your online security options.