Google Chrome and Mozilla Firefox users may have had their entire online history siphoned and stored by third-party developers.


A prominent security researcher found a popular plugin for the Google Chrome and Mozilla Firefox was recording everything users did online.


The software, which is designed to allow users to customise the appearance of how webpages appear inside the web browsers, has been hijacked by spyware.


The extension, which has more than 1.8 million users worldwide, may have been recording the browsing history of everyone who uses it.


Worse still, this browsing data could be linked to details that make users identifiable in the real world, making them vulnerable to hackers and blackmailers.


The finding was made by Robert Theaton, a software engineer from San Francisco, who discovered the software, dubbed Stylish, had been recording browser history since January 2017, when it was bought by new owners SimilarWeb.


This allows its new owner, SimilarWeb, to connect an individual with all of their online activity.


Those who have created a Stylish account on will have a unique identifier that can easily be linked to a login cookie and text files intended to help users access a website faster and more efficiently.


This means that not only does SimilarWeb own a copy of any user's complete browsing histories, they also own enough other data to theoretically tie these histories to email addresses and real-world identities.