Tuesday 19 January 2010

Promiscuous JavaScript considered dangerous

A trick commonly used to incorporate content from one web site into pages provided by another is for the provider site to make available a piece of JavaScript which inserts the required content into any page when loaded and executed. There are lots of examples - see for example Talks.cam 'Custom views', or the Met Office's Weather widgit.

This is realy neat, and has the huge advantage that it doesn't need any server-side support to impliment (typically RSS/Atom would need work on the server to collect and format the feeds) and can be deployed by just about anyone with editing access to the pages in question and the ability to cut and paste.

But it comes at a huge price. JavaScript that you include in your pages (wherever it comes from) has largely unfettered access to those pages. So it can rewrite you content, read cookies (and so steal authentication sessions), post content back to your server, etc., etc. So consider, before doing so, that adding JavaScript to your pages that is controlled by someone else amounts to giving that 'someone else' editing rights to your page. Are you happy to do that?

Of course reputable providers of this sort of JavaScript won't do anything like this deliberatly. But you do need to consider that they might do it accidently - it's particularly easy for them to mess up other JavaScript on you pages (for example the Met Office widgit above sets the JavaScript global variable moDays without any check if it's being used elsewhere). This trick is oftern used to provide feed information that may have been originally supplied to the provider site by even more third parties. There's the danger that the provider's cross site scripting protection may be incomplete, opening the posibility of unknown third parties being able to inject JavaScript into your pages, which is bad. And if you are relying on someone else's cross site scripting protection you need to know what character encoding they are doing it for and to be sure that you are setting a compatible one (this is discussed in a very helpful and to their credit very honest page supplied by Talks.cam that describes the risks of using their 'Promiscuous JavaScript' offering).

Note that I'm NOT saying that you shouldn't use this trick (as a supplier or as a provider). I am suggesting that you should be aware of the risks and make an apropriate assement of how they affect any particular deployment before proceding.