Most modern malware infections happen through the browser, typically as the result of a drive-by or social engineering attack. While there have been numerous studies on measuring and defending against drive-by downloads, little attention has been dedicated to studying social engineering attacks. In this paper, we present the first systematic study of web-based social engineering (SE) attacks that successfully lure users into downloading malicious and unwanted software. To conduct this study, we collect and reconstruct more than two thousand examples of in-thewild SE download attacks from live network traffic. Via a detailed analysis of these attacks, we attain the following results: (i) we develop a categorization system to identify and organize the tactics typically employed by attackers to gain the user’s attention and deceive or persuade them into downloading malicious and unwanted applications; (ii) we reconstruct the web path followed by the victims and observe that a large fraction of SE download attacks are delivered via online advertisement, typically served from “low tier” ad networks; (iii) we measure the characteristics of the network infrastructure used to deliver such attacks and uncover a number of features that can be leveraged to distinguish between SE and benign (or non-SE) software downloads.