iOS 16 - Wikipedia
iOS 16 is the sixteenth major release of Apple's iOS mobile operating system for the iPhone. It is the successor of iOS 15, and was announced at the ...
CAPTCHAs
Private Access Tokens are a new technology that replaces <anchor>CAPTCHAs</anchor> and helps in identifying HTTP requests from legitimate devices and people without compromising their identity or personal information.[17]
CAPTCHA - Wikipedia
CAPTCHA - Wikipedia[Jump to content](#bodyContent)

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## Contents

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- (Top)

- [1Purpose](#Purpose)
- [2History](#History)Toggle History subsection  - [2.1Invention](#Invention)
- [3Characteristics](#Characteristics)
- [4Accessibility](#Accessibility)
- [5Circumvention](#Circumvention)Toggle Circumvention subsection  - [5.1Machine learning-based attacks](#Machine_learning-based_attacks)
  - [5.2Human labor](#Human_labor)
  - [5.3Outsourcing to paid services](#Outsourcing_to_paid_services)
  - [5.4Insecure implementation](#Insecure_implementation)
- [6Alternative CAPTCHAs](#Alternative_CAPTCHAs)
- [7See also](#See_also)
- [8References](#References)
- [9Further references](#Further_references)
- [10External links](#External_links)

Toggle the table of contents

# CAPTCHA

58 languages

- [العربية](https://ar.wikipedia.org/wiki/%D8%AD%D8%B1%D9%88%D9%81_%D8%AA%D8%AD%D9%82%D9%82 "حروف تحقق – Arabic")
- [Azərbaycanca](https://az.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Azerbaijani")
- [বাংলা](https://bn.wikipedia.org/wiki/%E0%A6%95%E0%A7%8D%E0%A6%AF%E0%A6%BE%E0%A6%AA%E0%A6%9A%E0%A6%BE "ক্যাপচা – Bangla")
- [閩南語 / Bân-lâm-gú](https://zh-min-nan.wikipedia.org/wiki/Captcha "Captcha – Minnan")
- [Български](https://bg.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Bulgarian")
- [Bosanski](https://bs.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Bosnian")
- [Català](https://ca.wikipedia.org/wiki/Test_de_CAPTCHA "Test de CAPTCHA – Catalan")
- [Čeština](https://cs.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Czech")
- [Dansk](https://da.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Danish")
- [Deutsch](https://de.wikipedia.org/wiki/Captcha "Captcha – German")
- [Eesti](https://et.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Estonian")
- [Ελληνικά](https://el.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Greek")
- [Español](https://es.wikipedia.org/wiki/Captcha "Captcha – Spanish")
- [Esperanto](https://eo.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Esperanto")
- [Euskara](https://eu.wikipedia.org/wiki/Captcha "Captcha – Basque")
- [فارسی](https://fa.wikipedia.org/wiki/%DA%A9%D9%BE%DA%86%D8%A7 "کپچا – Persian")
- [Français](https://fr.wikipedia.org/wiki/CAPTCHA "CAPTCHA – French")
- [Galego](https://gl.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Galician")
- [한국어](https://ko.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Korean")
- [Հայերեն](https://hy.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Armenian")
- [हिन्दी](https://hi.wikipedia.org/wiki/%E0%A4%95%E0%A5%85%E0%A4%AA%E0%A5%8D%E0%A4%9A%E0%A4%BE "कॅप्चा – Hindi")
- [Hrvatski](https://hr.wikipedia.org/wiki/Captcha "Captcha – Croatian")
- [Ilokano](https://ilo.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Iloko")
- [Bahasa Indonesia](https://id.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Indonesian")
- [Íslenska](https://is.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Icelandic")
- [Italiano](https://it.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Italian")
- [עברית](https://he.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Hebrew")
- [Қазақша](https://kk.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Kazakh")
- [Latviešu](https://lv.wikipedia.org/wiki/Cilv%C4%93ktests "Cilvēktests – Latvian")
- [Lietuvių](https://lt.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Lithuanian")
- [Magyar](https://hu.wikipedia.org/wiki/Captcha "Captcha – Hungarian")
- [മലയാളം](https://ml.wikipedia.org/wiki/%E0%B4%95%E0%B4%BE%E0%B4%AA%E0%B5%8D%E0%B4%9A%E0%B5%8D%E0%B4%9A "കാപ്ച്ച – Malayalam")
- [Bahasa Melayu](https://ms.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Malay")
- [Nederlands](https://nl.wikipedia.org/wiki/Captcha "Captcha – Dutch")
- [日本語](https://ja.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Japanese")
- [Norsk bokmål](https://no.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Norwegian Bokmål")
- [Oʻzbekcha / ўзбекча](https://uz.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Uzbek")
- [Polski](https://pl.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Polish")
- [Português](https://pt.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Portuguese")
- [Română](https://ro.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Romanian")
- [Русский](https://ru.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BF%D1%87%D0%B0 "Капча – Russian")
- [संस्कृतम्](https://sa.wikipedia.org/wiki/%E0%A4%B8%E0%A4%82%E0%A4%AD%E0%A5%87%E0%A4%AA%E0%A5%82%E0%A4%B8%E0%A5%8D%E0%A4%B5%E0%A4%B8%E0%A4%BE%E0%A4%9F%E0%A5%8D%E0%A4%AF%E0%A5%82%E0%A4%AA "संभेपूस्वसाट्यूप – Sanskrit")
- [Sicilianu](https://scn.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Sicilian")
- [සිංහල](https://si.wikipedia.org/wiki/%E0%B6%9A%E0%B7%90%E0%B6%B4%E0%B7%8A%E0%B6%A0%E0%B7%8F_%E0%B6%B4%E0%B7%8A%E2%80%8D%E0%B6%BB%E0%B7%84%E0%B7%9A%E0%B6%BD%E0%B7%92%E0%B6%9A%E0%B7%8F "කැප්චා ප්‍රහේලිකා – Sinhala")
- [Simple English](https://simple.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Simple English")
- [Slovenščina](https://sl.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Slovenian")
- [Српски / srpski](https://sr.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BF%D1%87%D0%B0 "Капча – Serbian")
- [Suomi](https://fi.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Finnish")
- [Svenska](https://sv.wikipedia.org/wiki/Robotfilter "Robotfilter – Swedish")
- [தமிழ்](https://ta.wikipedia.org/wiki/%E0%AE%95%E0%AE%BE%E0%AE%AA%E0%AF%8D%E0%AE%9A%E0%AE%BE "காப்சா – Tamil")
- [తెలుగు](https://te.wikipedia.org/wiki/%E0%B0%95%E0%B0%BE%E0%B0%AA%E0%B1%8D%E0%B0%9A%E0%B0%BE "కాప్చా – Telugu")
- [ไทย](https://th.wikipedia.org/wiki/%E0%B9%81%E0%B8%84%E0%B8%9B%E0%B8%95%E0%B9%8C%E0%B8%8A%E0%B8%B2 "แคปต์ชา – Thai")
- [Тоҷикӣ](https://tg.wikipedia.org/wiki/%D0%9A%D0%B0%D0%BF%D1%87%D0%B0 "Капча – Tajik")
- [Türkçe](https://tr.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Turkish")
- [Українська](https://uk.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Ukrainian")
- [Tiếng Việt](https://vi.wikipedia.org/wiki/CAPTCHA "CAPTCHA – Vietnamese")
- [吴语](https://wuu.wikipedia.org/wiki/%E9%AA%8C%E8%AF%81%E7%A0%81 "验证码 – Wu")
- [中文](https://zh.wikipedia.org/wiki/%E9%AA%8C%E8%AF%81%E7%A0%81 "验证码 – Chinese")

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- [View source](/w/index.php?title=CAPTCHA&action=edit "This page is protected. You can view its source [e]")
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General


- [What links here](/wiki/Special:WhatLinksHere/CAPTCHA "List of all English Wikipedia pages containing links to this page [j]")
- [Related changes](/wiki/Special:RecentChangesLinked/CAPTCHA "Recent changes in pages linked from this page [k]")
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In other projects


- [Wikimedia Commons](https://commons.wikimedia.org/wiki/Category:Captcha)
- [Wikibooks](https://en.wikibooks.org/wiki/CAPTCHA)

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[![Page semi-protected](//upload.wikimedia.org/wikipedia/en/thumb/1/1b/Semi-protection-shackle.svg/20px-Semi-protection-shackle.svg.png)](/wiki/Wikipedia:Protection_policy#semi "This article is semi-protected due to vandalism")

From Wikipedia, the free encyclopedia

Test to determine whether a user is human

[![](//upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Crystal_Clear_app_kedit.svg/40px-Crystal_Clear_app_kedit.svg.png)](/wiki/File:Crystal_Clear_app_kedit.svg)

This article **may need to be rewritten** to comply with Wikipedia's [quality standards](/wiki/Wikipedia:Manual_of_Style "Wikipedia:Manual of Style"), as It feels like an essay criticising CAPTCHA.[You can help](https://en.wikipedia.org/w/index.php?title=CAPTCHA&action=edit). The [talk page](/wiki/Talk:CAPTCHA "Talk:CAPTCHA") may contain suggestions._(November 2022)_

[![](//upload.wikimedia.org/wikipedia/commons/6/69/Captcha.jpg)](/wiki/File:Captcha.jpg) This CAPTCHA ( [reCAPTCHA v1](/wiki/ReCAPTCHA_v1 "ReCAPTCHA v1")) of "smwm" obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.

A **CAPTCHA** ([/ˈkæp.tʃə/](/wiki/Help:IPA/English "Help:IPA/English") [_KAP-chə_](/wiki/Help:Pronunciation_respelling_key "Help:Pronunciation respelling key")) is a type of [challenge–response](/wiki/Challenge%E2%80%93response_authentication "Challenge–response authentication") test used in [computing](/wiki/Computing "Computing") to determine whether the user is human in order to deter bot attacks and spam.[\[1\]](#cite_note-1)

The term was coined in 2003 by [Luis von Ahn](/wiki/Luis_von_Ahn "Luis von Ahn"), [Manuel Blum](/wiki/Manuel_Blum "Manuel Blum"), Nicholas J. Hopper, and [John Langford](/wiki/John_Langford_(computer_scientist) "John Langford (computer scientist)").[\[2\]](#cite_note-abhl-2) It is a [contrived acronym](/wiki/Contrived_acronym "Contrived acronym") for "Completely Automated Public [Turing test](/wiki/Turing_test "Turing test") to tell Computers and Humans Apart."[\[3\]](#cite_note-3) A historically common type of CAPTCHA (displayed as [reCAPTCHA v1](/wiki/ReCAPTCHA_v1 "ReCAPTCHA v1")) was first invented in 1997 by two groups working in parallel. This form of CAPTCHA requires entering a sequence of letters or numbers in a distorted image. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, CAPTCHAs are sometimes described as [reverse Turing tests](/wiki/Reverse_Turing_test "Reverse Turing test").[\[4\]](#cite_note-4)

Two widely used CAPTCHA services are [Google](/wiki/Google "Google")'s [reCAPTCHA](/wiki/ReCAPTCHA "ReCAPTCHA")[\[5\]](#cite_note-5)[\[6\]](#cite_note-6) and the independent hCaptcha.[\[7\]](#cite_note-7)[\[8\]](#cite_note-8) It takes the average person approximately 10 seconds to solve a typical CAPTCHA.[\[9\]](#cite_note-9)

## Purpose

The purpose of CAPTCHAs is to prevent spam on websites, such as promotion spam, registration spam, and data scraping. Many websites use CAPTCHA effectively to prevent bot raiding. CAPTCHAs are designed so that humans can complete them, while most robots cannot.[\[10\]](#cite_note-10) Newer CAPTCHAs look at the user's behaviour on the internet, to prove that they are a human.[\[11\]](#cite_note-11) A normal CAPTCHA test only appears if the user acts like a bot, such as when they request webpages, or click links too fast.

## History

Since the 1980s–1990s, users have wanted to make text illegible to computers.[\[12\]](#cite_note-:1-12) The first such people were [hackers](/wiki/Hacker_culture "Hacker culture"), posting about sensitive topics to [Internet forums](/wiki/Internet_forum "Internet forum") they thought were being automatically monitored on keywords. To circumvent such filters, they replaced a word with look-alike characters. _HELLO_ could become `|-|3|_|_()` or `)-(3££0`, and others, such that a filter could not detect _all_ of them. This later became known as [leetspeak](/wiki/Leet "Leet").[\[13\]](#cite_note-13)

One of the earliest commercial uses of CAPTCHAs was in the Gausebeck–Levchin test. In 2000, idrive.com began to protect its signup page[\[14\]](#cite_note-14) with a CAPTCHA and prepared to file a patent.[\[12\]](#cite_note-:1-12) In 2001, [PayPal](/wiki/PayPal "PayPal") used such tests as part of a fraud prevention strategy in which they asked humans to "retype distorted text that programs have difficulty recognizing."[\[15\]](#cite_note-stringham2015-15) PayPal co founder and CTO [Max Levchin](/wiki/Max_Levchin "Max Levchin") helped commercialize this use.

A popular deployment of CAPTCHA technology, [reCAPTCHA](/wiki/ReCAPTCHA "ReCAPTCHA"), was acquired by Google in 2009.[\[16\]](#cite_note-16) In addition to preventing bot fraud for its users, Google used reCAPTCHA and CAPTCHA technology to digitize the archives of _[The New York Times](/wiki/The_New_York_Times "The New York Times")_ and books from Google Books in 2011.[\[17\]](#cite_note-17)

### Invention

Eran Reshef, [Gili Raanan](/wiki/Gili_Raanan "Gili Raanan") and Eilon Solan, who worked at [Sanctum](/wiki/Sanctum_(company) "Sanctum (company)") on [Application Security Firewall](/wiki/Web_application_firewall "Web application firewall"), first patented CAPTCHA in 1997. Their patent application details that "The invention is based on applying human advantage in applying sensory and cognitive skills to solving simple problems that prove to be extremely hard for computer software. Such skills include, but are not limited to processing of sensory information such as identification of objects and letters within a noisy graphical environment, signals and speech within an auditory signal, patterns and objects within a video or animation sequence".[\[18\]](#cite_note-:0-18)

## Characteristics

CAPTCHAs are automated, requiring little human maintenance or intervention to administer, producing benefits in cost and reliability.[\[19\]](#cite_note-19)

Modern text-based CAPTCHAs are designed such that they require the simultaneous use of three separate abilities—invariant recognition, [segmentation](/wiki/Image_segmentation "Image segmentation"), and parsing to complete the task.[\[20\]](#cite_note-20)

- Invariant recognition refers to the ability to recognize letters despite a large amount of variation in their shapes.[\[21\]](#cite_note-21)
- Segmentation is the ability to separate one letter from another, made difficult in CAPTCHAs.
- Parsing refers to the ability to understand the CAPTCHA holistically, in order to correctly identify each character.[\[22\]](#cite_note-22)

Each of these problems poses a significant challenge for a computer, even in isolation. Therefore, these three techniques in tandem make CAPTCHAs difficult for computers to solve.[\[23\]](#cite_note-bursz-23)

Whilst primarily used for security reasons, CAPTCHAs can also serve as a benchmark task for artificial intelligence technologies. According to an article by Ahn, Blum and Langford,[\[24\]](#cite_note-Ahn2003-24) "any program that passes the tests generated by a CAPTCHA can be used to solve a hard unsolved AI problem."[\[25\]](#cite_note-25) They argue that the advantages of using [hard AI](/wiki/AI-complete "AI-complete") problems as a means for security are twofold. Either the problem goes unsolved and there remains a reliable method for distinguishing humans from computers, or the problem is solved and a difficult AI problem is resolved along with it.[\[24\]](#cite_note-Ahn2003-24)

## Accessibility

See also: [Web accessibility](/wiki/Web_accessibility "Web accessibility")

[![](//upload.wikimedia.org/wikipedia/commons/thumb/9/9d/FancyCaptcha_screenshot.png/260px-FancyCaptcha_screenshot.png)](/wiki/File:FancyCaptcha_screenshot.png) Many websites require typing a CAPTCHA when creating an account to prevent spam. This image contains a user trying to type the CAPTCHA word "sepalbeam" to protect against automated spam.

CAPTCHAs based on reading text—or other visual-perception tasks—prevent [blind](/wiki/Blindness "Blindness") or [visually impaired](/wiki/Visual_impairment "Visual impairment") users from accessing the protected resource.[\[26\]](#cite_note-w3c_inaccessibility-26)[\[27\]](#cite_note-27) Because CAPTCHAs are designed to be unreadable by machines, common [assistive technology](/wiki/Assistive_technology "Assistive technology") tools such as [screen readers](/wiki/Screen_readers "Screen readers") cannot interpret them. The use of CAPTCHA thus excludes a small percentage of users from using significant subsets of such common Web-based services as PayPal, Gmail, Orkut, Yahoo!, many forum and weblog systems, etc.[\[28\]](#cite_note-28) In certain jurisdictions, site owners could become targets of litigation if they are using CAPTCHAs that discriminate against certain people with disabilities. For example, a CAPTCHA may make a site incompatible with [Section 508](/wiki/Section_508 "Section 508") in the United States.

CAPTCHAs do not have to be visual. Any hard [artificial intelligence](/wiki/Artificial_intelligence "Artificial intelligence") problem, such as [speech recognition](/wiki/Speech_recognition "Speech recognition"), can be used as CAPTCHA. Some implementations of CAPTCHAs permit users to opt for an audio CAPTCHA, such as reCAPTCHA, though a 2011 paper demonstrated a technique for defeating the popular schemes at the time.[\[29\]](#cite_note-29)

A method of improving CAPTCHA to ease the work with it was proposed by ProtectWebForm and named "Smart CAPTCHA".[\[30\]](#cite_note-30) Developers are advised to combine CAPTCHA with JavaScript. Since it is hard for most bots to parse and execute JavaScript, a combinatory method which fills the CAPTCHA fields and hides both the image and the field from human eyes was proposed.[\[31\]](#cite_note-31)

One alternative method involves displaying to the user a simple mathematical equation and requiring the user to enter the solution as verification. Although these are much easier to defeat using software, they are suitable for scenarios where graphical imagery is not appropriate, and they provide a much higher level of accessibility for blind users than the image-based CAPTCHAs. These are sometimes referred to as MAPTCHAs (M = "mathematical"). However, these may be difficult for users with a cognitive disorder, such as [dyscalculia](/wiki/Dyscalculia "Dyscalculia").[\[32\]](#cite_note-32)

Challenges such as a logic puzzle, or trivia question can also be used as a CAPTCHA. There is research into their resistance against countermeasures.[\[33\]](#cite_note-33)

## Circumvention

Two main ways to bypass CAPTCHA include using cheap human labor to recognize them, and using [machine learning](/wiki/Machine_learning "Machine learning") to build an automated solver.[\[34\]](#cite_note-34) According to former Google " _[click fraud](/wiki/Click_fraud "Click fraud") czar_" [Shuman Ghosemajumder](/wiki/Shuman_Ghosemajumder "Shuman Ghosemajumder"), there are numerous services which solve CAPTCHAs automatically.[\[35\]](#cite_note-ai-security-35)

### Machine learning-based attacks

There was not a systematic methodology for designing or evaluating early CAPTCHAs.[\[23\]](#cite_note-bursz-23) As a result, there were many instances in which CAPTCHAs were of a fixed length and therefore automated tasks could be constructed to successfully make educated guesses about where segmentation should take place. Other early CAPTCHAs contained limited sets of words, which made the test much easier to game. Still others\[ _[example needed](/wiki/Wikipedia:AUDIENCE "Wikipedia:AUDIENCE")_\] made the mistake of relying too heavily on background confusion in the image. In each case, algorithms were created that were successfully able to complete the task by exploiting these design flaws. However, light changes to the CAPTCHA could thwart them. Modern CAPTCHAs like [reCAPTCHA](/wiki/ReCAPTCHA "ReCAPTCHA") rely on present variations of characters that are collapsed together, making them hard to segment, and they have warded off automated tasks.[\[36\]](#cite_note-bursz2-36)

[![](//upload.wikimedia.org/wikipedia/commons/thumb/b/b6/Modern-captcha.jpg/220px-Modern-captcha.jpg)](/wiki/File:Modern-captcha.jpg) An example of a [reCAPTCHA](/wiki/ReCAPTCHA "ReCAPTCHA") challenge from 2007, containing the words "following finding". The waviness and horizontal stroke were added to increase the difficulty of breaking the CAPTCHA with a computer program.[![](//upload.wikimedia.org/wikipedia/commons/thumb/4/49/Captchacat.png/220px-Captchacat.png)](/wiki/File:Captchacat.png) A CAPTCHA usually has a text box directly underneath where the user should fill out the text that they see. In this case, "sclt ..was here".

In October 2013, artificial intelligence company [Vicarious](/wiki/Vicarious_(Company) "Vicarious (Company)") claimed that it had developed a generic CAPTCHA-solving algorithm that was able to solve modern CAPTCHAs with character recognition rates of up to 90%.[\[37\]](#cite_note-37) However, [Luis von Ahn](/wiki/Luis_von_Ahn "Luis von Ahn"), a pioneer of early CAPTCHA and founder of reCAPTCHA, said: "It's hard for me to be impressed since I see these every few months." 50 similar claims to that of Vicarious had been made since 2003.[\[38\]](#cite_note-38)

In August 2014 at Usenix WoOT conference, [Bursztein](/wiki/Elie_Bursztein "Elie Bursztein") et al. presented the first generic CAPTCHA-solving algorithm based on reinforcement learning and demonstrated its efficiency against many popular CAPTCHA schemas.[\[36\]](#cite_note-bursz2-36)

In October 2018 at [ACM](/wiki/Association_for_Computing_Machinery "Association for Computing Machinery") CCS'18 conference, Ye et al. presented a deep learning-based attack that could consistently solve all 11 text captcha schemes used by the top-50 popular websites in 2018. An effective CAPTCHA solver can be trained using as few as 500 real CAPTCHAs.[\[39\]](#cite_note-39)

### Human labor

It is possible to subvert CAPTCHAs by relaying them to a [sweatshop](/wiki/Sweatshop "Sweatshop") of human operators who are employed to decode CAPTCHAs. A 2005 paper from a [W3C](/wiki/W3C "W3C") working group said that they could verify hundreds per hour.[\[26\]](#cite_note-w3c_inaccessibility-26) In 2010, the [University of California at San Diego](/wiki/University_of_California,_San_Diego "University of California, San Diego") conducted a large scale study of CAPTCHA farms. The retail price for solving one million CAPTCHAs was as low as $1,000.[\[40\]](#cite_note-motoyama-40)

Another technique consists of using a script to re-post the target site's CAPTCHA as a CAPTCHA to the attacker's site, which unsuspecting humans visit and solve within a short while for the script to use.[\[41\]](#cite_note-41)[\[42\]](#cite_note-42)

In 2023, [ChatGPT](/wiki/ChatGPT "ChatGPT") tricked a [TaskRabbit](/wiki/Taskrabbit "Taskrabbit") worker into solving a CAPTCHA by telling the worker it was not a robot and had impaired vision.[\[43\]](#cite_note-43)

### Outsourcing to paid services

There are multiple Internet companies like _2Captcha_ and _DeathByCaptcha_ that offer human and machine backed CAPTCHA solving services for as low as US$0.50 per 1000 solved CAPTCHAs.[\[44\]](#cite_note-44) These services offer APIs and libraries that enable users to integrate CAPTCHA circumvention into the tools that CAPTCHAs were designed to block in the first place.[\[45\]](#cite_note-45)

### Insecure implementation

Howard Yeend has identified two implementation issues with poorly designed CAPTCHA systems:[\[46\]](#cite_note-46) reusing the session ID of a known CAPTCHA image, and CAPTCHAs residing on shared servers.

Sometimes, if part of the software generating the CAPTCHA is [client-side](/wiki/Client-side "Client-side") (the validation is done on a server but the text that the user is required to identify is rendered on the client side), then users can modify the client to display the un-rendered text. Some CAPTCHA systems use [MD5](/wiki/MD5 "MD5") hashes stored client-side, which may leave the CAPTCHA vulnerable to a [brute-force attack](/wiki/Brute-force_attack "Brute-force attack").[\[47\]](#cite_note-47)

## Alternative CAPTCHAs

Some researchers have proposed alternatives including image recognition CAPTCHAs which require users to identify simple objects in the images presented. The argument in favor of these schemes is that tasks like object recognition are more complex to perform than text recognition and therefore should be more resilient to machine learning based attacks.

Chew et al. published their work in the 7th International Information Security Conference, ISC'04, proposing three different versions of image recognition CAPTCHAs, and validating the proposal with user studies. It is suggested that one of the versions, the anomaly CAPTCHA, is best with 100% of human users being able to pass an anomaly CAPTCHA with at least 90% probability in 42 seconds.[\[48\]](#cite_note-48) Datta et al. published their paper in the [ACM](/wiki/Association_for_Computing_Machinery "Association for Computing Machinery") [Multimedia](/wiki/Multimedia "Multimedia") '05 Conference, named IMAGINATION (IMAge Generation for INternet AuthenticaTION), proposing a systematic way to image recognition CAPTCHAs. Images are distorted so image recognition approaches cannot recognise them.[\[49\]](#cite_note-49)

Microsoft (Jeremy Elson, John R. Douceur, Jon Howell, and Jared Saul) claim to have developed Animal Species Image Recognition for Restricting Access (ASIRRA) which ask users to distinguish cats from dogs. Microsoft had a beta version of this for websites to use.[\[50\]](#cite_note-50) They claim "Asirra is easy for users; it can be solved by humans 99.6% of the time in under 30 seconds. Anecdotally, users seemed to find the experience of using Asirra much more enjoyable than a text-based CAPTCHA." This solution was described in a 2007 paper to Proceedings of 14th ACM Conference on Computer and Communications Security (CCS).[\[51\]](#cite_note-51) It was closed in October 2014.[\[52\]](#cite_note-52)

## See also

- [Bot prevention](/wiki/Bot_prevention "Bot prevention")
- [Defense strategy (computing)](/wiki/Defense_strategy_(computing) "Defense strategy (computing)")
- [NuCaptcha](/wiki/NuCaptcha "NuCaptcha")
- [Proof of personhood](/wiki/Proof_of_personhood "Proof of personhood")
- [Proof of work](/wiki/Proof_of_work "Proof of work")
- [reCAPTCHA](/wiki/ReCAPTCHA "ReCAPTCHA")

## References

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34. **[^](#cite_ref-34)**Jakobsson, Markus (August 2012). [_The death of the Internet_](http://eu.wiley.com/WileyCDA/WileyTitle/productCd-1118062418.html). [Archived](https://web.archive.org/web/20141015182639/http://eu.wiley.com/WileyCDA/WileyTitle/productCd-1118062418.html) from the original on 15 October 2014. Retrieved 4 April 2016.
35. **[^](#cite_ref-ai-security_35-0)**Ghosemajumder, Shuman (8 December 2015). ["The Imitation Game: The New Frontline of Security"](http://www.infoq.com/presentations/ai-security). _InfoQ_. InfoQ. [Archived](https://web.archive.org/web/20190323061742/https://www.infoq.com/presentations/ai-security) from the original on 23 March 2019. Retrieved 8 December 2015.
36. ^ [_**a**_](#cite_ref-bursz2_36-0) [_**b**_](#cite_ref-bursz2_36-1)Bursztein, Elie; Aigrain, Johnathan; Mosciki, Angelika; Michell, John C. (August 2014). [_The End is Nigh: Generic Solving of Text-based CAPTCHAs_](https://www.elie.net/publication/the-end-is-nigh-generic-solving-of-text-based-captchas). WoOT 2014: Usenix Workshop on Offensive Security. [Archived](https://web.archive.org/web/20160416212924/https://www.elie.net/publication/the-end-is-nigh-generic-solving-of-text-based-captchas) from the original on 16 April 2016. Retrieved 5 April 2016.
37. **[^](#cite_ref-37)**Summers, Nick. ["Vicarious claims its AI software can crack up to 90% of CAPTCHAs offered by Google, Yahoo and PayPal"](https://thenextweb.com/insider/2013/10/28/vicarious-claims-ai-software-can-now-crack-90-captchas-google-yahoo-paypal/). TNW. [Archived](https://web.archive.org/web/20180915002117/https://thenextweb.com/insider/2013/10/28/vicarious-claims-ai-software-can-now-crack-90-captchas-google-yahoo-paypal/) from the original on 15 September 2018. Retrieved 19 June 2018.
38. **[^](#cite_ref-38)**Hof, Robert. ["AI Startup Vicarious Claims Milestone In Quest To Build A Brain: Cracking CAPTCHA"](https://www.forbes.com/sites/roberthof/2013/10/28/ai-startup-vicarious-claims-milestone-in-quest-to-build-a-brain-craking-captcha/). _Forbes_. [Archived](https://web.archive.org/web/20180915002819/https://www.forbes.com/sites/roberthof/2013/10/28/ai-startup-vicarious-claims-milestone-in-quest-to-build-a-brain-craking-captcha/) from the original on 15 September 2018. Retrieved 25 August 2017.
39. **[^](#cite_ref-39)**["Yet Another Text Captcha Solver: A Generative Adversarial Network Based Approach"](https://eprints.lancs.ac.uk/id/eprint/126984/1/ccs18.pdf)(PDF). _25th ACM Conference on Computer and Communications Security (CCS), 2018_. [doi](/wiki/Doi_(identifier) "Doi (identifier)"): [10.1145/3243734.3243754](https://doi.org/10.1145%2F3243734.3243754). [S2CID](/wiki/S2CID_(identifier) "S2CID (identifier)") [53106794](https://api.semanticscholar.org/CorpusID:53106794). [Archived](https://web.archive.org/web/20201029202241/https://eprints.lancs.ac.uk/id/eprint/126984/1/ccs18.pdf)(PDF) from the original on 29 October 2020. Retrieved 16 March 2020.
40. **[^](#cite_ref-motoyama_40-0)**Motoyama, Marti; Levchenko, Kirill; Kanich, Chris; McCoy, Damon; Geoffrey, Voelker; Savage, Stefan (August 2010). [_Re: CAPTCHAs-Understanding CAPTCHA-Solving Services in an Economic Context.s_](http://static.usenix.org/event/sec10/tech/full_papers/Motoyama.pdf)(PDF). USENIX Security Symposium, 2010. [Archived](https://web.archive.org/web/20160529023244/http://static.usenix.org/event/sec10/tech/full_papers/Motoyama.pdf)(PDF) from the original on 29 May 2016. Retrieved 5 April 2016.
41. **[^](#cite_ref-41)**[Doctorow, Cory](/wiki/Cory_Doctorow "Cory Doctorow") (27 January 2004). ["Solving and creating captchas with free porn"](https://web.archive.org/web/20060209040456/http://www.boingboing.net/2004/01/27/solving_and_creating.html). _Boing Boing_. Archived from [the original](https://www.boingboing.net/2004/01/27/solving_and_creating.html) on 9 February 2006. Retrieved 27 April 2015.
42. **[^](#cite_ref-42)**["Hire People To Solve CAPTCHA Challenges"](http://petmail.lothar.com/design.html#auto35). _Petmail Design_. 21 July 2005. [Archived](https://web.archive.org/web/20200918050055/http://petmail.lothar.com/design.html#auto35) from the original on 18 September 2020. Retrieved 27 April 2015.
43. **[^](#cite_ref-43)**Hurler, Kevin. ["Chat-GPT Pretended to Be Blind and Tricked a Human Into Solving a CAPTCHA"](https://gizmodo.com/gpt4-open-ai-chatbot-task-rabbit-chatgpt-1850227471). _Gizmodo_. [Archived](https://web.archive.org/web/20230411200745/https://gizmodo.com/gpt4-open-ai-chatbot-task-rabbit-chatgpt-1850227471) from the original on 11 April 2023. Retrieved 11 April 2023.
44. **[^](#cite_ref-44)**["Top 10 Captcha Solving Services Compared"](http://www.prowebscraper.com/blog/top-10-captcha-solving-services-compared/). [Archived](https://web.archive.org/web/20181215172409/http://www.prowebscraper.com/blog/top-10-captcha-solving-services-compared/) from the original on 15 December 2018. Retrieved 10 December 2018.
45. **[^](#cite_ref-45)**["How Cybercriminals Bypass CAPTCHA"](https://www.f5.com/company/blog/how-cybercriminals-bypass-captcha). _www.f5.com_. [Archived](https://web.archive.org/web/20221027095027/https://www.f5.com/company/blog/how-cybercriminals-bypass-captcha) from the original on 27 October 2022. Retrieved 27 October 2022.
46. **[^](#cite_ref-46)**Yeend, Howard (2005). ["Breaking CAPTCHAs Without Using OCR"](https://web.archive.org/web/20170625165854/http://www.puremango.co.uk/2005/11/breaking_captcha_115/). _(pureMango.co.uk)_. Archived from [the original](http://www.puremango.co.uk/cm_breaking_captcha_115.php) on 25 June 2017. Retrieved 22 August 2006.
47. **[^](#cite_ref-47)**["CTFtime.org / #kksctf open 2019 / Kackers blockchained notes / Writeup"](https://ctftime.org/writeup/17833). _ctftime.org_. [Archived](https://web.archive.org/web/20221027095023/https://ctftime.org/writeup/17833) from the original on 27 October 2022. Retrieved 27 October 2022.
48. **[^](#cite_ref-48)**["Image Recognition CAPTCHAs"](https://web.archive.org/web/20130510022240/http://www.cs.berkeley.edu/~tygar/papers/Image_Recognition_CAPTCHAs/imagecaptcha.pdf)(PDF). Cs.berkeley.edu. Archived from [the original](http://www.cs.berkeley.edu/~tygar/papers/Image_Recognition_CAPTCHAs/imagecaptcha.pdf)(PDF) on 10 May 2013. Retrieved 28 September 2013.
49. **[^](#cite_ref-49)**["Imagination Paper"](http://infolab.stanford.edu/~wangz/project/imsearch/IMAGINATION/ACM05/). Infolab.stanford.edu. [Archived](https://web.archive.org/web/20131002170726/http://infolab.stanford.edu/~wangz/project/imsearch/IMAGINATION/ACM05/) from the original on 2 October 2013. Retrieved 28 September 2013.
50. **[^](#cite_ref-50)**["Asirra is a human interactive proof that asks users to identify photos of cats and dogs"](https://web.archive.org/web/20081215032402/http://research.microsoft.com/en-us/um/redmond/projects/asirra/). _[Microsoft](/wiki/Microsoft "Microsoft")_. Archived from [the original](https://www.microsoft.com/en-us/research/publication/asirra-a-captcha-that-exploits-interest-aligned-manual-image-categorization/) on 15 December 2008.
51. **[^](#cite_ref-51)**Elson, Jeremy; Douceur, John; Howell, Jon; Saul, Jared (October 2007). [_Asirra: A CAPTCHA that Exploits Interest-Aligned Manual Image Categorization_](https://www.microsoft.com/en-us/research/publication/asirra-a-captcha-that-exploits-interest-aligned-manual-image-categorization/). Proceedings of 14th ACM Conference on Computer and Communications Security. [Microsoft](/wiki/Microsoft "Microsoft"). [Archived](https://web.archive.org/web/20081215032402/http://research.microsoft.com/en-us/um/redmond/projects/asirra/) from the original on 15 December 2008. Retrieved 15 September 2017.
52. **[^](#cite_ref-52)**["After 8 years of operation, Asirra is shutting down effective October 1, 2014. Thank you to all of our users!"](https://web.archive.org/web/20150207180225/https://research.microsoft.com/en-us/projects/asirra/default.aspx). [Microsoft](/wiki/Microsoft "Microsoft"). Archived from [the original](https://research.microsoft.com/en-us/projects/asirra/default.aspx) on 7 February 2015.

## Further references

- von Ahn, L; M. Blum and J. Langford. (2004) " [Telling humans and computers apart (automatically)](http://www.cs.cmu.edu/afs/cs/Web/People/aladdin/papers/pdfs/y2004/captcha_cacm.pdf)". _Communications of the ACM_, **47**(2):57–60.

## External links

**CAPTCHA** at Wikipedia's [sister projects](/wiki/Wikipedia:Wikimedia_sister_projects "Wikipedia:Wikimedia sister projects")

- ![](//upload.wikimedia.org/wikipedia/en/thumb/0/06/Wiktionary-logo-v2.svg/27px-Wiktionary-logo-v2.svg.png)[Definitions](https://en.wiktionary.org/wiki/Special:Search/CAPTCHA "wikt:Special:Search/CAPTCHA") from Wiktionary
- ![](//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/20px-Commons-logo.svg.png)[Media](https://commons.wikimedia.org/wiki/Category:Captcha "c:Category:Captcha") from Commons
- ![](//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Wikibooks-logo.svg/27px-Wikibooks-logo.svg.png)[Textbooks](https://en.wikibooks.org/wiki/CAPTCHA "b:CAPTCHA") from Wikibooks
- ![](//upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Wikidata-logo.svg/27px-Wikidata-logo.svg.png)[Data](https://www.wikidata.org/wiki/Q484598 "d:Q484598") from Wikidata
- ![](//upload.wikimedia.org/wikipedia/commons/thumb/a/a6/MediaWiki-2020-icon.svg/27px-MediaWiki-2020-icon.svg.png)[Documentation](https://www.mediawiki.org/wiki/CAPTCHA "mw:CAPTCHA") from MediaWiki

- [CAPTCHA](https://curlie.org/Computers/Internet/Abuse/CAPTCHA/) at [Curlie](/wiki/Curlie "Curlie")
- [Verification of a human in the loop, or Identification via the Turing Test](http://www.wisdom.weizmann.ac.il/~naor/PAPERS/human_abs.html), Moni Naor, 1996.
- [Inaccessibility of CAPTCHA: Alternatives to Visual Turing Tests on the Web](http://www.w3.org/TR/turingtest/), a [W3C](/wiki/W3C "W3C") Working Group Note.
- [CAPTCHA History](https://web.archive.org/web/20120205201803/http://www2.parc.com/istl/projects/captcha/history.htm) from [PARC](/wiki/PARC_(company) "PARC (company)").
- [Reverse Engineering CAPTCHAs](https://web.archive.org/web/20170915204258/https://pdfs.semanticscholar.org/692a/31f65e29ea3667de46933245f53bda55a65b.pdf) Abram Hindle, Michael W. Godfrey, Richard C. Holt, 2009-08-24

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- [FAST](http://id.worldcat.org/fast/1747935/)

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