[whatwg] element "img" with HTTP POST method

Adam Barth w3c at adambarth.com
Thu Dec 9 19:15:45 PST 2010

>>> On Thu, Dec 9, 2010 at 11:41 AM, Philipp Serafin <phil127 at gmail.com> wrote:
>>>> There are quite a number of older web forums that sanitize their HTML using black lists and would not strip new attributes like "post-data". For malicious users, it would be very easy to include e.g. <img src="./do_post.php" post-data="thread_id=42&post_content=Go visit (some spam URL)"> in their signature and have users doing involuntary posts by simply viewing a thread.
>>> Indeed.  You shouldn't be able to trigger POSTs from involuntary
>>> actions.  They should always require some sort of user input, because
>>> there is simply *far* too much naive code out there that is vulnerable
>>> to CSRF.
>> Unfortunately, the attacker can already trigger POSTs with involuntary
>> actions.  That code is already vulnerable attack, sadly.
> Via scripting, yes, which is usually stripped out by sanitizers (or
> just plain doesn't work, like javascript urls in images).  I don't
> believe there are any declarative ways to trigger involuntary POSTs,
> are there?

The attacker can always make a giant invisible button that covers the
whole page that submits a form.  Web sites can generate POST requests
without user intervention.  Anyone who's using POST as a security
feature as far bigger troubles than this attribute.


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