Quote:While web applications are bound by the Same Origin Policy (SOP) and cannot access data from other web applications unless mechanisms such as Cross-Origin Resource Sharing (CORS) are implemented into both, browser extensions are not subject to the same rule, meaning they can read and write data on web applications.

The extensions also have access to a broad range of sensitive user information, including browsing history, bookmarks, credentials (cookies) and list of installed extensions, and can download files and store them on the user’s device. 

Browser extensions and web applications are executed in separate contexts, but they can interact by exchanging messages, regardless of the browser. This allows web applications to exploit extension privileged capabilities and steal sensitive user information, Dolière Francis Somé from the Université Côte d'Azur, Inria, France, says in a research paper (PDF).

The researcher analyzed the communication interfaces exposed to web applications by Chrome, Firefox, and Opera browser extensions and discovered that many of them can be exploited for access to privileged capabilities.