Due to the frequent encountering of web URLs in various application scenarios (e.g., chatting and email reading), many mobile apps build their in-app browsing interfaces (IABIs) to provide a seamless user experience. Although this achieves user-friendliness by avoiding the constant switching between the subject app and the system built-in browser apps, we find that IABIs, if not well designed or customized, could result in usability security risks. In this paper, we conduct the first empirical study on the usability (in)security of in-app browsing interfaces in both Android and iOS apps. Specifically, we collect a dataset of 25 high-profile mobile apps from five common application categories that contain IABIs, including Facebook and Gmail, and perform a systematic analysis (not end-user study though) that comprises eight carefully designed security tests and covers the entire course of opening, displaying, and navigating an in-app web page. During this process, we obtain three major security findings: (1) about 30% of the tested apps fail to provide enough URL information for users to make informed decisions on opening an URL; (2) nearly all custom IABIs have various problems in providing sufficient indicators to faithfully display an in-app page to users, whereas ten IABIs that are based on Chrome Custom Tabs and SFSafariViewController are generally secure; and (3) only a few IABIs give warnings to remind users of the risk of inputting passwords during navigating a (potentially phishing) login page. Most developers had acknowledged our findings but their willingness and readiness to fix usability issues are rather low compared to fixing technical vulnerabilities, which is a puzzle in usability security research. Nevertheless, to help mitigate risky IABIs and guide future designs, we propose a set of secure IABI design principles.