Server-Side Browsers: Exploring the Web’s Hidden Attack Surface

As websites grow ever more dynamic and load more of their content on the fly, automatically interacting with them via simple tools like curl is getting less of an option. Instead, headless browsers with JavaScript support, such as PhantomJS and Puppeteer, have gained traction on the Web over the last few years. For various use cases like messengers and social networks that display link previews, these browsers visit arbitrary, user-controlled URLs. To avoid compromise through known vulnerabilities, these browsers need to be diligently kept up-to-date.In this talk, we investigate the phenomenon of what we coin 'server-side browsers' at scale and find that many websites are running severely outdated browsers on the server-side. Remarkably, the majority of them had not been updated for more than 6 months and over 60% of the discovered implementations were found to be vulnerable to publicly available proof-of-concept exploits.