Making Defeating CAPTCHAs Harder for Bots

TL;DR

This paper reviews and introduces safeguards to strengthen CAPTCHAs against bot circumvention, emphasizing usability and layered protection to improve security without compromising user experience.

Contribution

It presents five new safeguards and compares their effectiveness alongside existing ones, enhancing CAPTCHA resilience against automated attacks.

Findings

Seven existing safeguards analyzed

Five novel safeguards proposed

Layered safeguards improve CAPTCHA security

Abstract

For a number of years, many websites have used CAPTCHAs to filter out interactions by bots. However, attackers have found ways to circumvent CAPTCHAs by programming bots to solve or bypass them, or even relay them for humans to solve. In order to reduce the chances of success of such attacks, CAPTCHAs can be strengthened by the addition of certain safeguards. In this paper, we discuss seven existing safeguards as well as five novel safeguards designed to make circumventing CAPTCHAs harder. These safeguards are not mutually exclusive and can add multiple layers of protection to a CAPTCHA. We further provide a high-level comparison of their effectiveness in addressing the threat posed by CAPTCHA-defeating techniques. In order to focus on safeguards that are usable, we restrict our attention to those which have minimal adverse effect on the user experience.