Date: Tue, 24 Jan 2012 15:43:14 -0800
From: Neil Kandalgaonkar<neilk(a)wikimedia.org>
To: Wikimedia developers<wikitech-l(a)lists.wikimedia.org>
Subject: Re: [Wikitech-l] Escaping messages
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 1/23/12 9:59 PM, Daniel Friesen wrote:
2 - We
could ensure that the message library never emits scripts, by
applying a simple jQuery filter to the final result.
Don't delude yourself
into thinking that you can easily blacklist the
elements that would run a script.
Thanks for the pointer. You're right, I wasn't being careful enough.
Even so I think we have some reason for limited optimism in this case,
because jQuery operates on nodes in browser, not strings on the server.
Adding something to a DOM usually normalizes it, so there's less chance
of missing something due to unusual ways of encoding, escaping, or
As far as I know these are the main dangers:
- SCRIPT, STYLE tags
- LINK, IFRAME, FRAME, FRAMESET, META, OBJECT, EMBED tags
- inherently scripted attributes, such as "onclick".
'mocha:', 'livescript:', matched case-insensitively.
- hardest one: element styles with values that, once cleaned of
comments, contain the script words above or /expression(.*)/
However there are other dangers too. Yesterday I discovered that in
Chrome, a script will be executed if you .append() it to anything, even
if it's not part of the document. Annoying.
Anyway I'm not going to war on this, but some reasonable efforts can be
That's a really scary approach to security in my opinion. Well its
true that browsers may generally normalize things, are we sure every
browser ever made (including browsers not yet released) do that? Are
we sure there isn't certain weird situations (aka bugs) where the
browser would not normalize something, etc.
Things should either be escaped totally (So we know its safe), or not
escaped at all (so we know its dangerous and treat it as such). Half
measures of just stripping some tags on a blacklist will lull people
into a false senses of security.
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