Many websites utilise CAPTCHA (Completely Automatic Public Turing tests to tell Computers and Humans Apart) schemes as human interaction proofs to grant access to their services only to people rather than spam bots. In this paper, we examine the security of six widely used types of CAPTCHA and present novel attacks against all of them, achieving success rates of up to 88%. We made improvements ...

CAPTCHAs are an important security tool for preventing automated attacks against online systems. However, they can be an undesirable source of friction in the user experience. In this poster, we propose a novel approach to dynamically improve the CAPTCHA experience for users by individually learning which types of CAPTCHA tests each user is best at solving and adapting future tests presented ba...

Web access is affected by a great amount of accessibility issues that do not allow some users to access all information presented. Therefore, Web accessibility is an important issue because everybody should access Web content independently of their access features. Among these accessibility issues, a Web content element that interferes with Web accessibility is a CAPTCHA. A CAPTCHA is a challen...

The multimedia research community has, for long, attempted to recognize generic images, make them searchable by content, annotate them, and associate them with linguistic indexes. In the course of these attempts, the limitations of state-of-the-art algorithms in mimicking human vision has become wellknown. In this paper, we explore the exploitation of this limitation for solving a security prob...

CAPTCHAs have become a pervasive method for protecting against automated submissions to web forums and registration to web based email services. The CAPTCHAs are usually image-based, but voice CAPTCHAs have also emerged as an alternative. In this short note, we discuss our ongoing efforts on designing accessible voice CAPTCHAs for Internet Telephony. We have implemented a testbed for Skype to a...

1 ABSTRACT Tor is the most widely used anonymity network, currently serving millions of users each day. However, there is no access control in place for all these users, leaving the network vulnerable to botnet abuse and attacks. For example, criminals frequently use exit relays as stepping stones for attacks, causing service providers to serve CAPTCHAs to exit relay IP addresses or blacklistin...

With the growing and prevalent usage of Internet, threat for the service providers has arisen. One of the most important aspects of modern security concerns is to deal with users that are computer programs or bots. These programs pretend to be human users and exploit servers by submitting data automatically, thereby, hindering the services of the server. Security mechanism dealing with such att...

Completely Automated Public Turing tests to tell Computers and Humans Apart (captchas) are challenge-response tests used as a security mechanism on the web to distinguish human users from automated programs. While captchas are often necessary to stop abuse of resources, most existing schemes are intended for traditional desktop computing environments rather than for mobile device usage. In this...