Tue 18 Apr 2023 03:56:51 PM EEST

Checking existence of firewalled web servers in Firefox via iframe.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:

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

Posted by joro | Permanent link