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I have a very open ended question. Are there any security concerns with being able to run javascript from anywhere on any website?
What I mean is, let's say you go to any webpage, and after it loads you load your own JS on the fly that can be customized and can manipulate the DOM of the current page. Could that do any harm?
Thanks in advance.
Well, it depends on the content of the javascript itself, if it means harm it might only affect the user in the system where the javascript file is at or the website if it's able to inject code through javascript in which case it depends not only in the functions used by the javascript file but also the security of the website.
Nevertheless, you can do it if you want to, for instante Opera offers that by letting users to use their own javascript files to manipulate the content of the webpage they're visiting
Have a look at GreaseMonkey.