What is Cross Site Script Inclusion (XSSI)?

Question

I\'ve recently seen XSSI mentioned on multiple pages, e.g. Web Application Exploits and Defenses:

Browsers prevent pages of one domain from reading pages

Answer 1

XSSI is a fancy way of saying: you are including in your program, someone elses code; You don't have any control over what is in that code, and you don't have any control over the security of the server on which it is hosted.

For example, let's say i include in my html page

That script will run in my webapp with the same level of trust as any of my own javascript code. It will have access to the the full page content and DOM, it will be able to read all my app's cookies and read the users keypresses and mouse movements, and everything else that javascript can do.

If my mate dave, then decides to put something malicious in his cool widget (say, a sniffer/keylogger that sends all the user's cookies, form data and keypresses to his server) then I won't necessarily know. Also, the security of my app now depends on the security of dave's server. If dave's server gets compromised and coolwidget.js is replaced by the attacker, i again won't necessarily know and the malicious code will run as part of my app.