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A CAPTCHA is a type of challenge-response system designed to differentiate humans from robotic software programs. CAPTCHAs are used as security checks to deter spammers and hackers from using forms on web pages to insert malicious or frivolous code.

CAPTCHAs, which are a type of Turing test, often use images, because while bots can identify the existence of an image by reading source code, they cannot tell what the image depicts. Because some CAPTCHA images are difficult to interpret, users are usually given the option to request a new test.

The most common type of CAPTCH is the text CAPTCHA, which requires the user to view a distorted string of alphanumeric characters in an image and enter the characters in an attached form. Text CAPTCHAS are also rendered as MP3 audio recordings to meet the needs of the visually impaired. Just as with images, bots can detect the presence of an audio file, but only a human can listen and know the information the file contains.

Picture recognition CAPTCHAs, which are also commonly used, ask users to identify a subset of images within a larger set of images. For instance, the user may be given a set of images and asked to click on all the ones that have cars in them.

Other types of CAPTCHAs include:

  • Math CAPTCHAs - require the user to solve a basic math problem, such as adding or subtracting two numbers.
  • I am not a robot CAPTCHA - requires the user to check a box.
  • Marketing CAPTCHAs - require the user to type a particular word or phrase related to the sponsor's brand.


Users who prefer not to solve CAPTCHAs can use any of several browser add-ons that allow users to bypass CAPTCHAs. Popular browser add-ons include AntiCapture, CAPTCHA Be Gone and Rumola. Because CAPTCHA bypass add-ons are created by third parties, end users should be aware that the browser extension could expose their browsing activity to untrusted sources.


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