CAPTCHAs have been widely used as an anti-bot means for well over a decade. Unfortunately, they are often hard and annoying to use, and human errors have been blamed mainly on overly complex challenges, or poor challenge design. However, errors can also occur because of ambient sensory distractions, and performance impact of these distractions has not been thoroughly examined. The goal of our work is to explore the impact of auditory distractions on CAPTCHA performance. To this end, we conducted a comprehensive user study. Its results, discussed in this paper, show that various types of auditory stimuli impact performance differently. Generally, simple and less dynamic stimuli sometimes improve subject performance, while highly dynamic stimuli have a negative impact. This is troublesome since CAPTCHAs are often used to protect web sites offering tickets for limited-quantity events, that sell out very quickly, i.e., within seconds. In such settings, introduction of even a small delay can make the difference between obtaining tickets from the primary source, and being forced to use a secondary market. Our study was conducted in a fully automated experimental environment to foster uniform and scalable experiments. We discuss both benefits and limitations of unattended automated experiment paradigm.
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