Message: 10 Date: Tue, 24 Jan 2012 15:43:14 -0800 From: Neil Kandalgaonkar neilk@wikimedia.org To: Wikimedia developers wikitech-l@lists.wikimedia.org Subject: Re: [Wikitech-l] Escaping messages Message-ID: 4F1F4212.9050003@wikimedia.org 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. http://ha.ckers.org/xss.html
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 delimiting input.
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". - attribute values beginning with 'javascript:', 'vbscript:', '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 made.
-- Neil Kandalgaonkar neilk@wikimedia.org
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.
-bawolff