Re: <BASE> tag used for hijacking external resources (XSS)

[Bouke van Laethem <vanlaethem@xxxxxxxxx>]

On Fri, Dec 16, 2011 at 9:59 PM, Mario Vilas <mvilas@xxxxxxxxx> wrote:
> Makes sense as a trick to bypass some crappy XSS filters that look for
> strings like "javascript:", but I don't think it's a vulnerability in
> itself.

I would consider it a browser bug (although I agree it would mostly be
abused through bypassing what you refer to as "crappy XSS filters"),
because the browsers are going too far out of their way to parse
invalid html.

From http://www.w3.org/TR/html4/struct/links.html#h-12.4:
When present, the BASE element must appear in the HEAD section of an
HTML document, before any element that refers to an external source.
The path information specified by the BASE element only affects URIs
in the document where the element appears.

w3.org doc also refers to RFC1080, http://www.ietf.org/rfc/rfc1808.txt:
10, Appendix:
[...]
HTML defines a special element "BASE" which, when present in the
"HEAD" portion of a document, signals that the parser should use the
BASE element's "HREF" attribute as the base URL for resolving any
relative URLs.  The "HREF" attribute must be an absolute URL.