Abstract
The objective of this review paper is to discuss about the strengths and weaknesses in the visual and audio CAPTCHAs. The basic techniques in breaking visual CAPTCHAs are discussed in detail. The issues regarding usability features are studied by choosing the user affinity by means of reaching the users through questionnaire. In order to avoid the risks in text-based CAPTCHAs, random images based on human appearance characteristics are generated. This again is limited as visually challenged users cannot approach these types of CAPTCHAs henceforth preventing them to approach resources available. As technology improves professional programmers find means to break audio CAPTCHAs too. The suggestions to avoid these vulnerabilities are also discussed briefly.
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