The user authentication on the security applications is one of the most important process. Because character based password is commonly used for user authentication, it is most important to protect the keyboard. Due to the reason, several software solutions for keyboard security have been applied to critical sites. This paper introduces vulnerabilities to the commonly used USB keyboard, implements a sample code using the vulnerabilities and evaluates the possibility for the keyboard data to be stolen in the guarded environment. Through the comparison of the result, a countermeasure to the vulnerabilities is proposed.