Checking existence of firewalled web servers in Firefox viaiframe.onload

In short in Firefox 112, it is possible to check existence
of firewalled web servers. This doesn't work in Chrome and Chromium 112
for me.

If user A has tcp connection to web server B, then in the
following html:

<iframe src="http://B" onload="load()" onerror="alert('error')" id="i1" />

the javascript function load() will get executed if B serves
valid document to A's browser and will not be executed otherwise.

This work for both http and https, and for http it is allowed
B to be IP address. Under some configurations of Apache2,
it serves http despite having https configured.

In some sense, this is close to nmap via javascript in a browser.

Potential privacy implication is when the attacker guess the
range of firewalled IPs and check them all in a loop.

For online test:

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Source: 9/rpA/3202/erusolcsidlluf/gro.stsilces

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