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Inaccessibility of CAPTCHA 2 via W3C

April 18, 2018


The use of CAPTCHAs in recent years has seen a number of different approaches being used to tell humans and robots apart. While the traditional identification of text in an image remains popular, other mechanisms such as multi-device authentication and the Google reCAPTCHA are gaining in prominence.

However, for people with disabilities, it often remains the case that the CAPTCHA presented determines that the user is not human. In addition, research suggests that many popular CAPTCHA techniques are no longer secure.

This document examines a number of potential solutions that allow systems to test for human users while preserving access for users with disabilities.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at

Editor: Scott Hollier

This document was published by the Accessible Platform Architectures Working Group as an Editor's Draft. Comments regarding this document are welcome. Please send them to (subscribe,archives).

Publication as an Editor's Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes containsEssential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

table of contents

  1. Abstract
  2. Status of This Document
  3. 1.The problem
  4. 2.Security effectiveness
  5. 3.Types of CAPTCHA and access implications
    1. 3.1Traditional character-based CAPTCHA
    2. 3.2Logic puzzles
    3. 3.3Sound output
    4. 3.4Limited-use accounts
    5. 3.5Non-interactive checks
      1. 3.5.1Spam filtering
      2. 3.5.2Heuristic checks and the Google ReCAPTCHA
    6. 3.6Federated identity systems
      1. 3.6.1Single sign-on
      2. 3.6.2Public-key infrastructure solutions
      3. 3.6.3Biometrics
    7. 3.7Multiple user devices
    8. 3.8Image and video
      1. 3.8.1Visual comparison CAPTCHAs
      2. 3.8.23D CAPTCHA
      3. 3.8.3Video Game CAPTCHA
  6. 4.Conclusion
  7. 5.References

1. The problem

Web sites with resources that are attractive to aggregators such as sign-up Web pages, travel and event ticket sites, Web-based email accounts and social media portals have taken measures to ensure that they can offer their service to individual users without content being harvested or exploited by Web robots.

An initially popular solution was the use of graphical representations of text in registration or comment areas. The site would attempt to verify that the user in question was in fact a human by requiring the user to complete this task commonly referred to as a Completely Automated Public Turing test to Tell Computers and Humans Apart [CAPTCHA].

The CAPTCHA was initially developed by researchers at Carnegie Mellon University and has been primarily associated with a technique whereby an individual had to identify a distorted set of characters from a bitmapped image, then enter those characters into a form. However, in recent times the types of CAPTCHA that appear on Web sites and mobile apps have changed significantly. As such, the term "CAPTCHA" is used in this document to refer to all projects which are specifically designed to differentiate a human from a computer.

While online users broadly have reported finding traditional CAPTCHAs frustrating to complete, it is generally assumed that a CAPTCHA can be resolved within a few incorrect attempts. The point of distinction for people with disabilities is that a CAPTCHA not only separates computers from humans, but also removes people with disabilities from being able to complete the processes they requested. For example, reliance on visual and textual verification comes at a huge price to users who are blind, visually impaired or dyslexic. Likewise audio CAPTCHAs represent challenges for people who are Deaf or hearing impaired, and the assumption of traditional CAPTCHAs that all Web users can read a particular character set or English-based words makes the test inaccessible to a large number of Web users.

Assistive Technology users also face challenges resulting from the CAPTCHA image due to it containing no meaningful text equivalent, as that would make it a giveaway to computerized systems. In many cases, these systems make it impossible for users with certain disabilities to create accounts, write comments, or make purchases on these sites - n essence, CAPTCHAs fail to properly recognize users with disabilities as human. Such issues also extend to situational disabilities whereby a user may not be able to effectively view a traditional CAPTCHA on a mobile device due to the small screen size or hear an audio-based CAPTCHA in a noisy environment.

2. Security effectiveness

It is important to note that the effectiveness in using a CAPTCHA as a security solution has deteriorated in recent years. Current CAPTCHA methods that rely primarily on text-based or image-based problems can be largely cracked using both complex and simple computer algorithms. Research suggests that approximately 20% of traditional CAPTCHAs can be broken using OCR algorithms (Hernández-Castro, C. J., Barrero, D. F., & R-Moreno, M. D., 2016)(Li, Q., 2015).

In addition, pattern-matching algorithms in some instances can achieve an even higher success rate of cracking CAPTCHAs (Yan, J., & El Ahmad, A. S., 2009)(Sano, S., Otsuka, T., Itoyama, K., & Okuno, H. G., 2015). While efforts are being made to strengthen traditional CAPTCHA security, more robust security solutions run the risk of reducing the abilities for typical users to understand the CATPCHA that needs to be resolved (Nakaguro, Y., Dailey, M. N., Marukatat, S., & Makhanov, S. S., 2013). Furthermore, captchas have been solved for good and ill, and the solutions sometimes stored for reuse, through crowd sourcing.

As such, it is highly recommended that for both security and accessibility reasons, alternative security methods are considered in preference to the use of a traditional image-based CAPTCHA such as two-step or multi-device verification methods.

3. Types of CAPTCHA and access implications

There are many techniques available to users to discourage or eliminate fraudulent account creations or uses. Several of them may be as effective as the visual verification technique while being more accessible to people with disabilities. Others may be overlaid as an accommodation for the purposes of accessibility. The following list highlights common CAPTCHA types and their respective accessibility implications.

3.1 Traditional character-based CAPTCHA

The traditional character-based CAPTCHA, as previously discussed, is largely inaccessible and insecure. It focuses on the presentation of letters or words presented in an image and designed to be difficult for robots to identify. The user is then asked to enter the CAPTCHA information into a form.

The use of a traditional CAPTCHA is particularly problematic for people who are blind or visually impaired as the assistive technology cannot process the image, therefore preventing users from entering the results in the form. Due to the mechanisms used to prevent the CAPTCHA from being read by robots, the characters are often distorted or have other characters in close proximity making it difficult to read visually. The common CAPTCHA technique has also been found to be less reliably solved by users with learning disabilities (Gafni & Nagar).

In addition, there is currently a dominant assumption that all web users can understand English, which is not the case. Examples such as Arabic and Thai demonstrate the barriers associated with CAPTCHAs based on written English and related language character sets (Tangmanee, C., 2016).

3.2 Logic puzzles

The goal of visual verification is to separate human from machine. One reasonable way to do this is to test for logic. Simple mathematical word puzzles, trivia, and the like may raise the bar for robots, at least to the point where using them is more attractive elsewhere.

Problems: Users with cognitive disabilities may still have trouble. Answers may need to be handled flexibly, if they require free-form text. A system would have to maintain a vast number of questions, or shift them around programmatically, in order to keep spiders from capturing them all. This approach is also subject to defeat by human operators.

3.3 Sound output

To reframe the problem, text is easy to manipulate, which is good for assistive technologies, but just as good for robots. So, a logical means of trying to solve this problem is to offer another non-textual method of using the same content. To achieve this, audio is played that contains a series of numbers, letters or words being read out which the user then needs to enter into a form..

However, according to the CNet article [NEWSCOM], if the sound output, which is itself distorted to avoid the same programmatic abuse, can render the CAPTCHA difficult to hear. There can also be confusion in understanding whether a number is to be entered as a numerical value or as a word, e.g. ‘7’ or ‘seven’. There are also temporal issues in that if it an element of an audio CAPTCHA is not understood, the entire CAPTCHA needs to be replayed. Currently not all audio CAPTCHAs provide a replay option, meaning it is often the case that an entirely new audio CAPTCHA has to be played if any part of it is difficult to understand.

Users who are deaf-blind, don't have or use a sound card, work in noisy environments, or don't have required sound plugins are likewise left in the lurch. Since this content is auditory in nature, users often have to write down the code before entering it, which is very inconvenient.

Although auditory forms of CAPTCHA that present distorted speech create recognition difficulties for screen reader users, the accuracy with which such users can complete the CAPTCHA tasks is increased if the user interface is carefully designed to prevent screen reader audio and CAPTCHA audio from being intermixed. This can be achieved by implementing functions for controlling the audio that do not require the user to move focus away from the text response field (Bigham, J. P., & Cavender, A. C. 2009).

Experiments with a combined auditory and visual CAPTCHA requiring users to identify well known objects by recognizing either images or sounds, suggest that this technique is highly usable by screen reader users. However, its security-related properties remain to be explored (Sauer, G., Lazar, J., Hochheiser, H., & Feng, J. 2010).

3.4 Limited-use accounts

Users of free accounts very rarely need full and immediate access to a site's resources. For example, users who are searching for concert tickets may need to conduct only three searches a day, and new email users may only need to send a canned notification of their new address to their friends, and a few other free-form messages. Sites may create policies that limit the frequency of interaction explicitly (that is, by disabling an account for the rest of the day) or implicitly (by slowing the response times incrementally). Creating limits for new users can be an effective means of making high-value sites unattractive targets to robots.

The drawbacks to this approach include having to take a trial-and-error approach to determine a useful technique. It requires site designers to look at statistics of normal and exceptional users, and determine whether a bright line exists between them.

3.5 Non-interactive checks

While CAPTCHA and other interactive approaches to spam control are sometimes effective, they do make using a site more complex. This is often unnecessary, as a large number of non-interactive mechanisms exist to check for spam or other invalid content.

This category contains two popular non-interactive approaches: spam filtering, in which an automated tool evaluates the content of a transaction, and heuristic checks, which evaluate the behavior of the client.

3.5.1 Spam filtering

Applications that use continuous authentication and "hot words" to flag spam content, or Bayesian filtering to detect other patterns consistent with spam, are very popular, and quite effective. While such risk analysis systems may experience false negatives from time to time, properly-tuned systems can be comparable to a CAPTCHA approach, while also removing the added cognitive burden on the user.

Most major blogging software contains spam filtering capabilities, or can be fitted with a plug-in for this functionality. Many of these filters can automatically delete messages that reach a certain spam threshold, and mark questionable messages for manual moderation. More advanced systems can control attacks based on post frequency, filter content sent using the [TRACKBACK] protocol, and ban users by IP address range, temporarily or permanently.

3.5.2 Heuristic checks and the Google ReCAPTCHA

Heuristics are discoveries in a process that seem to indicate a given result. It may be possible to detect the presence of a robotic user based on the volume of data the user requests, series of common pages visited, IP addresses, data entry methods, or other signature data that can be collected.

Again, this requires a good look at the data of a site. If pattern-matching algorithms can't find good heuristics, then this is not a good solution. Also, polymorphism, or the creation of changing footprints, is apt to result, if it hasn't already, in robots, just as polymorphic ("stealth") viruses appeared to get around virus checkers looking for known viral footprints.

Another heuristic approach identified in [KILLBOTS] involves the use of CAPTCHA images, with a twist: how the user reacts to the test is as important as whether or not it was solved. This system, which was designed to thwart distributed denial of service (DDoS) attacks, bans automated attackers which make repeated attempts to retrieve a certain page, while protecting against marking humans incorrectly as automated traffic. When the server's load drops below a certain level, the authentication process is removed entirely.

An example of a CAPTCHA base don this approach is the Google ReCAPTCHA which features a checkbox labelled ‘I am not a robot’ or similar phrasing. The process works by collecting data such as mouse movement and keyboard navigation to determine if the user is a human or robot, while keeping the CAPTCHA process relatively simple.

Anecdotal evidence suggests that this CAPTCHA is currently the most accessible CAPTCHA solution and can be completed with a variety of assistive technologies. However, there is little formalised research investigating if this is indeed the case. There is also the additional concern that the inability of completing the reCATPCHA tends to default back to a traditional inaccessible CAPTCHA.

3.6 Federated identity systems

Many large companies such as Microsoft, Apple, Amazon, Google and the Kantara Initiative have created competing "federated network identity" systems, which can allow a user to create an account, set his or her preferences, payment data, etc., and have that data persist across all sites and devices that use the same service. Due to large companies now requiring a federated identity to use cloud-based services on their respective digital ecosystems, the popularity of federated identities has increased significantly. As a result, many Web sites and Web Services, allow a portable form of identification across the Web.

3.6.1 Single sign-on

Ironically enough, the Passport system itself is one of the very same services that currently utilizes visual verification techniques. These single sign-on services will have to be among the most accessible on the Web in order to offer these benefits to people with disabilities. Additionally, use of these services will need to be ubiquitous to truly solve the problems addressed here once and for all.

3.6.2 Public-key infrastructure solutions

Another approach is to use certificates for individuals who wish to verify their identity. The certificate can be issued in such a way as to ensure something close to a one-person-one-vote system by for example issuing these identifiers in person and enabling users to develop distributed trust networks, or having the certificates issued by highly trusted authorities such as governments. These type of systems have been implemented for securing web pages, and for authenticating email.

The cost of creating fraudulent certificates needs to be high enough to destroy the value of producing them in most cases. Sites would need to use mechanisms which are widely implemented in user agents.

A subset of this concept, in which only people with disabilities who are affected by other verification systems would register, raises a privacy concern in that the user would need to telegraph to every site that she has a disability. The stigma of users with disabilities having to register themselves to receive the same services should be avoided. With that said, there are a few instances in which users may want to inform sites of their disabilities or other needs: sites such as Bookshare [BOOKSHARE] require evidence of a visual disability in order to allow users to access printed materials which are often unavailable in audio or Braille form. An American copyright provision known as the Chafee Amendment [CHAFEE] allows copyrighted materials to be reproduced in forms that are only usable by blind and visually impaired users. A public-key infrastructure system would allow Bookshare's maintainers to ensure that the site and its users are in compliance with copyright law.

3.6.3 Biometrics

A popular authentication method on mobile platforms include biometric technology whereby a fingerprint or facial recognition authentication method is used. This process effectively limits the ability of spammers to create infinite email accounts. The E.U./U.S. government requirements to section 3.5.3. Explain the growing popularity of dual-factor authentication using biometrics.

3.7 Multiple user devices

The user of multiple devices such as a computer, smartphone, tablet and/or wearable could provide additional support for user authentication. This could assist in addressing accessibility issues by using assistive technologies on each device to confirm the user is a human and is a specific user (Cetin, C., 2015). The use of biometrics as previously discussed could also be used as one such device authentication mechanism.

3.8 Image and video

3.8.1 Visual comparison CAPTCHAs

There are a number of new techniques based on the identification of still images. This can include identifying whether an image is a man or a woman, or whether an image is human-shaped or avatar-shaped among other comparison solutions (Conti, M., Guarisco, C., & Spolaor, R., 2015)( Kim, J., Kim, S., Yang, J., Ryu, J.-h., & Wohn, K., 2014)( Korayem, M., 2015).

While alternative audio comparison CAPTCHAs could be provided such as using similar or different tones for comparison, the reliance on visual comparison alone would be difficult for people with vision-related disabilities

3.8.2 3D CAPTCHA

A 3D representation of letters and numbers can make it more difficult for OCR software to identify, in turn making it more secure (Nguyen, V. D., Chow, Y.-W., & Susilo, W., 2014). However this solution has similar accessibility issues to traditional CAPTCHAs.

We recommend further exploration of the use of risk analysis techniques (as exemplified by the approach that Google have taken) to reduce the need for CAPTCHA.

3.8.3 Video Game CAPTCHA

This process suggests the completion of a basic video game as a CAPTCHA. The benefits include the removal of language barriers, and multiple interface methods could potentially make such a solution accessible (Yang, T.-I., Koong, C.-S., & Tseng, C.-C., 2015). It would also have the benefit of making CAPCHAs an enjoyable process, reducing the frustrations generally associated with traditional CAPTCHAs.

4. Conclusion

The evolution of CAPTCHA techniques has highlighted that traditional solutions such as text-based characters contained in images are not only challenging for people with disabilities, but also insecure. While a majority of CAPTHCAs in use remain challenging for people with disabilities to complete, recent additions including the Google reCAPTCHA, multi-device authentication and the increased prevalence of Federated identity systems currently provide the most accessible and flexible options in separating humans from robots.

However, while some CAPTCHA solutions are better than others, there is currently no ideal solution. As such, it is important that any implementation of a CAPTCHA is not going to prevent people with disabilities from being identified as human.

Source: W3C

Related Information

Captcha W3C WCAG