I want to create an extension that adds content on third party sites.
I will not show any advertising and I will not hide the site's content.
I'd like to add my related content to 5 or 6 third party websites, something like this http://i.imgur.com/jQx1r.jpg Obviuosly, only my users, if they have installed my browser extension, will see my contents on third party websites.
Are there legal policy problems about the developing of this extension or could i proceed without any problem?
Can the site owner ban my extension or sue me?
If people choose / authorize to install an extension that adds / augments content on specific sites, I don't see the problem with it. At a higher level Alexa, Google, AOL, Yahoo (and adblock for that matter) extensions all do this in some way.
How you get the user to install, how you choose to disclose what data is available and what you do with such data is typically where extension providers have problems with their users.
Of course, IANAL so if you want a true representation of what your exposure is, consult with your lawyer.
This is the attack vector used by a scam that attacks HSBC bank customers. The customers think they are "safe", as they have one of those hardware keys that changes code every few seconds. The bank think that the two-part authentication is sufficient. They all forget the browser is easily compromised. Locks don't matter if the thief is invited in.
No doubt the same trick applies to other banks too, not just HSBC.
I spoke to one of the bank's so-called security "experts" and they were so proud of rolling out these hardware key generators, until I pointed out that they made their customers a lot less secure, by giving a false sense of security.