How to achieve a full reflected XSS attack which includes the ability to run a complete script and not just an alert popup with the least amount of characters? Some people already tried to answer this question like in here and here.
As we may imagine it’s possible to have an URL parameter echoed in a source of a script:
which would make possible to launch a full XSS attack providing a source with a short domain like //14.rs (7 chars length). A variation decreases the injection to just 5 chars if double slashes are already in native code:
However, both scenarios are very unlikely.
It uses the 0.0.0.0 IP address as the href of the anchor for demo purposes, so we can try it locally. In a real vulnerable page it would need to be a valid yet expired domain (broken link) able to be acquired, spoofed or even compromised. Of course, this alone isn’t enough to exploit the page which would invalidate our next step.
The injection is:
Which just inserts a <base tag before the <a one to hijack its href attribute (the acquired/spoofed domain). This sets the base URL of the document to something we control and then we just need a script in the page being called from a relative URL, which is not that difficult to find into a given page.
Now setting a web server or just a listener on port 80 to deliver our script is enough to pop the alert (or anything we want):
An useful dead link with the right conditions is really hard to find, but to deal with the alignment involving the injection and the anchor, the trick used here and probably some browser quirks may help.
Anyway, in the cases where only the 2nd condition (script from relative URL) is met, we can still use the <base tag but this time providing the href:
Which is a full XSS vector with IP in decimal for a local PoC with just 15 chars.
P.S.: it’s possible to use a domain name with just 4 chars like t.co and even a host name with just 1 char (“a”, for example) in an intranet attack.