Can a website see what I am doing on other sites?

Question

If I am on two sites with a browser, can one of those sites see what I am doing on the other site?

Answer 1

Yes, if both sites have the same domain and javascript code to do that. No otherwise, unless there's a serious security flaw in your browser or system that is being exploited.

Reference: http://tools.ietf.org/html/draft-abarth-principles-of-origin-00

Answer 2

If all is well, then currently: no, not in the way you describe (simultaneous browser sessions).

But, more general given the title of your question, bugs and privacy leaks might always be exploited. Like today in some browsers, embedding links to the other sites might reveal if you visited those, by abusing an old "visited links" privacy leak. December 2010:

The 46 sites exploit a widely known vulnerability that currently exists in all production version browsers except of Apple's Safari, which earlier this year became the first major browser to insulate users against the threat. Google Chrome, which is based on the same Webkit engine, soon followed. Beta versions of Mozilla Firefox and Microsoft Internet Explorer also fix the problem, but production versions of those browsers are still wide open.

The exploit works by using JavaScript to read cascading style sheet technologies included in virtually every browser that causes visited links to appear in purple rather than blue. Developers have known of the weakness for a decade or more but until recently said it couldn't be easily repaired without removing core functionality.

Answer 3

Depends. If they share tracking code and set everything up to track behavior on both - then yes. Also there are many ways for malware to do so.