As the number of Internet users has grown, so have the security threats that they face online. Security warnings are one key strategy for trying to warn users about those threats; but recently, it has been questioned whether they are effective. We conducted a study in which 120 participants brought their own laptops to a usability test of a new academic article summary tool. They encountered a PDF download warning for one of the papers. All participants noticed the warning, but 98 (81.7%) downloaded the PDF file that triggered it. There was no significant difference between responses to a brief generic warning, and a longer specific one. The participants who heeded the warning were overwhelmingly female, and either had previous experience with viruses or lower levels of computing skills. Our analysis of the reasons for ignoring warnings shows that participants have become desensitised by frequent exposure and false alarms, and think they can recognise security risks. At the same time, their answers revealed some misunderstandings about security threats: for instance, they rely on anti-virus software to protect them from a wide range of threats, and do not believe that PDF files can infect their machine with viruses. We conclude that security warnings in their current forms are largely ineffective, and will remain so, unless the number of false positives can be reduced.
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