Sounds like a security risk to me. If someone tricks you into starting this they could follow you to other websites and collect data.
A similar attack would be to override the back button. You know those sites where the back button takes you to a redirect that takes you to the same page? Take one of those, but on the redirect page save the referring URL. Then when the user hits back, have the redirect page start the session sharing and redirect to the referrer. Sounds plenty dangerous to me.
Often, it's the "sounds plenty dangerous" innovations (fire, automobile, chainsaw, theory of relativity), that provide the most utility.
If you consider this to be dangerous that's a flaw in every web browser ever, not this piece of software. This kind of thing was possible 10 years ago too.