Checking existence of firewalled URLs via javascript's script.onload

[Georgi Guninski <gguninski () gmail com>]

There is minor information disclosure vulnerability similar
to nmap in browser.

It is possible to check the existence of firewalled URL U via
the following javascript in a browser:

<script src="U"
    onload="alert('Exists')"
    onerror="alert('Does not exist')">

This might have privacy implication on potentially
"semi-blind CSRF" (XXX does this makes sense?).

Works for me in Firefox, Chrome and Chromium 112.

I believe the issue won't be fixed because it will break
stuff in the mess called internet.

For online test:

https://www.guninski.com/onload2.html

-- 
guninski:  https://j.ludost.net/resumegg.pdf