Approaches presented herein enable restricting access to a locked computing resource in a web browser with a user-tailored CAPTCHA. More specifically, a request for a CAPTCHA is obtained from a web browser of a user attempting to access a resource. A set of terms associated with the user are selected from a pool of terms collected from an environment of the user within a pre-determined time period. A familiarity value of the selected terms indicating a recognizability of the terms by the user is determined. A CAPTCHA of the selected set of terms is generated having a level of graphic noise corresponding to the familiarity value. The generated CAPTCHA is then sent to the web browser. When a response to the CAPTCHA is received from the web browser, it is analyzed to determine whether the response matches the selected set of terms. If so, the web browser is instructed to grant the user access to the resource.