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This article shows an easy and simple approach to recognize characters in CAPTCHA images, where the k-NN (k nearest neighbor) algorithm is employed. This proposal to recognize characters in CAPTCHA images has the objective of autofill these components in order to support automation of access to systems. The main aim of this article is to show the steps involved in the proposed process about automatic filling CAPTCHAs since the image's handling until the classification of the characters through a simple and low-cost (implementation) technique of pattern recognition. Experimental results and an error distribution about the characters' classification are showed, where it is demonstrated the possibility of application in real cases of the proposal presented. General TermsCompletely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA)