Feeds

Researchers find irreparable flaw in popular CAPTCHAs

Decaptcha pierces Live.com, Yahoo!, Digg

Securing Web Applications Made Simple and Scalable

Computer scientists have developed software that easily defeats audio CAPTCHAs offered on account registration pages of a half-dozen popular websites by exploiting inherent weaknesses in the automated tests designed to prevent fraud.

Decaptcha is a two-phase audio-CAPTCHA solver that correctly breaks the puzzles with a 41-percent to 89-percent success rate on sites including eBay, Yahoo, Digg, Authorize.net, and Microsoft's Live.com. The program works by removing background noise from the audio files, allowing only the spoken characters needed to complete the test to remain.

In virtually all of the tests, Decaptcha was able to correctly solve the puzzle at least once in every 100 attempts, making the technique suitable for botmasters with large armies of compromised computers. The high success rate was largely the result of the ease in removing sound distortions known as background noise, intermediate noise, and constant noise inserted into the background to throw off speech-recognition programs. Most audio-based CAPTHA systems are wide open to the attack with the notable exception of the Google-owned Recaptcha.net, which uses a different approach known as semantic noise.

"Our results indicate that non-continuous audio captcha schemes built using current methods (without semantic noise) are inherently insecure," the scientists wrote in a recently published research paper. "As a result, we suspect that it may not be possible to design secure audio captchas that are usable by humans using current methods. It is therefore important to explore alternative approaches."

Decaptcha uses a supervised algorithm that must be trained for each CAPTCHA scheme being targeted. Training requires feeding a set of puzzles with their answers into the program. Eventually, Decaptcha was able to identify the sound shapes in the underlying audio file by comparing them to a large sample of sounds already cataloged. The researchers generated 4.2 million audio CAPTCHAs.

The paper is only the latest reminder of the flaws in CAPTCHAs, which are designed to prevent scripts from registering email accounts, and carrying out other automated attacks, by presenting the user with a problem that's hard for computers to solve. Real-world attacks against audio-CAPTCHAs from Microsoft have already been used by the Pushdo spam botnet to create fraudulent email accounts on Live.com. More traditional CAPTCHAs, which require a user to recognize a word buried in a distorted image, have been successfully defeated for years, with one of the more recent examples being an optical character recognition attack on Google.

After attacks come to light, website operators typically make changes that block specific technique. Researchers then revise their attacks, requiring more changes to be made in the targeted CAPTCHA schemes.

The latest research suggests web developers may have to make permanent changes to the audio CAPTCHAs, which are offered for visually impaired users.

"Our experiments with commercial and synthetic captchas indicate that the present methodology for building audio captchas may not be rectifiable," they wrote. "Besides Recaptcha, all of the commercial schemes we tested used combinations of constant and regular noise as distortions. All in all, computers may actually be more resilient than humans to constant and regular noise so any schemes that rely on these distortions will be inherently insecure."

The paper was authored by Elie Bursztein, Hristo Paskov, and John Mitchell of Stanford University, Romain Beauxis of Tulane University, Daniele Perito of INRIA and Celine Fabry. A PDF of the report is here. ®

Mobile application security vulnerability report

More from The Register

next story
LibreSSL RNG bug fix: What's all the forking fuss about, ask devs
Blow to bit-spitter 'tis but a flesh wound, claim team
HIDDEN packet sniffer spy tech in MILLIONS of iPhones, iPads – expert
Don't panic though – Apple's backdoor is not wide open to all, guru tells us
Manic malware Mayhem spreads through Linux, FreeBSD web servers
And how Google could cripple infection rate in a second
NUDE SNAPS AGENCY: NSA bods love 'showing off your saucy selfies'
Swapping other people's sexts is a fringe benefit, says Snowden
Own a Cisco modem or wireless gateway? It might be owned by someone else, too
Remote code exec in HTTP server hands kit to bad guys
British data cops: We need greater powers and more money
You want data butt kicking, we need bigger boots - ICO
Crooks fling banking Trojan at Japanese smut site fans
Wait - they're doing online banking with an unpatched Windows PC?
NIST told to grow a pair and kick NSA to the curb
Lrn2crypto, oversight panel tells US govt's algorithm bods
prev story

Whitepapers

Reducing security risks from open source software
Follow a few strategies and your organization can gain the full benefits of open source and the cloud without compromising the security of your applications.
Consolidation: The Foundation for IT Business Transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.
Application security programs and practises
Follow a few strategies and your organization can gain the full benefits of open source and the cloud without compromising the security of your applications.
Boost IT visibility and business value
How building a great service catalog relieves pressure points and demonstrates the value of IT service management.
Consolidation: the foundation for IT and business transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.