Industry-wide misunderstandings of HTTPS

Abstract

In a survey of 30 sites that serve sensitive content over an HTTPSprotected connection, we found that over 70 % of them failed to appropriately prevent disk caching, and left unencrypted sensitive content behind on endusers’ machines, at risk for later exposure. Moreover, over half of the sites that failed to prevent disk caching appeared to have attempted to do so using outdated, non-standard, or erroneous methods, some of which failed entirely, while others were only successful at preventing disk caching in certain browsers, but not all. In an effort to explain this wide-spread failure, our research has uncovered drastically inconsistent behavior across browsers, inconsistent support of standard and non-standard anti-disk caching directives, and even inconsistent and incorrect recommendations from authoritative sources in the security community. Through this history we show that web developers are not solely to blame, and that web browser developers, web server developers, security professionals and authors of online sources, and perhaps even the standards bodies should share in this failure. In this paper, we identify the disk caching behaviors of all major browsers, and describe how to reliably prevent disk caching for each of them. We present the results of our site survey, demonstrating wide-spread failures to prevent disk caching of sensitive data. We introduce a tool for Firefox users to reliably prevent disk caching of HTTPS protected content, despite failures by the web application, and we provide an online tool to help web developers identify how to reliably prevent disk caching across multiple browsers. Lastly, we make recommendations to the various parties with a hand in this failure on how to address these issues going forward.