All about uncertainties and traps: Statistical oracle-based attacks on a new CAPTCHA protection against oracle attacks

CAPTCHAs are security mechanisms that try to prevent automated abuse of computer services. Many CAPTCHAs have been proposed but most have known security flaws against advanced attacks. In order to avoid a kind of oracle attacks in which the attacker learns about ground truth labels via active interactions with the CAPTCHA service as an oracle, Kwon and Cha proposed a new CAPTCHA scheme that employ uncertainties and trap images to generate adaptive CAPTCHA challenges, which we call “Uncertainty and Trap Strengthened CAPTCHA” (UTS-CAPTCHA) in this paper. Adaptive CAPTCHA challenges are used widely (either explicitly or implicitly) but the role of such adaptive mechanisms in the security of CAPTCHAs has received little attention from researchers. In this paper we present a statistical fundamental design flaw of UTS-CAPTCHA. This flaw leaks information regarding ground truth labels of images used. Exploiting this flaw, an attacker can use the UTS-CAPTCHA service as an oracle, and perform several different statistical learning-based attacks against UTS-CAPTCHA, increasing any reasonable initial success rate up to 100% according to our theoretical estimation and experimental simulations. Based on our proposed attacks, we discuss how the fundamental idea behind our attacks may be generalized to attack other CAPTCHA schemes and propose a new principle and a number of concrete guidelines for designing new CAPTCHA schemes in the future.