On 28/05/2008, at 12:28 PM, Miguel Mitrofanov wrote:
I am taking comments on a web forum from arbitrary people. The interpretation of the HTML occurs at the user's browser. A lot of people will be using outdated browsers (IE 5.5 / 6), ergo security (at the source) becomes my problem. I cannot force them to upgrade their browsers.
I think this is very wrong for two reasons. First of all, the more web sites care of old browsers, the later people will upgrade them, therefore preventing the progress in Web (though IE 5.5 is not THAT old and bad, so this argument is not so strong). In Russia we some times say that a user with an outdated browser is an EPTH (Evil Pinocchio To Himself, don't ask me about source of this term).
I am not encouraging people to stick with IE 5.5, I am trying to prevent them from being exploited when visiting my site. It is a good- faith-best-effort, not something I will formally prove.
Secondly, I don't think that filtering HTML coming from an arbitrary user is a good idea. HTML is not very human-readable and too complex to achieve real safety without lots of work. My suggestion is to use some home-grown wiki-like syntax - it's easier to enter (*bold* instead of <b>bold</b>), easier to read (and your users would sometimes read their comments before posting - to check correctness), and easier to process, since it can't have security holes you're not aware of.
Did you read my post? I validate the XHTML against a restricted variant of the XHTML 1.0 Strict DTD. I want to ensure that if it validates, it is "safe", as I explained before. I think the "style" attribute is unsafe, so I can remove it from the DTD. (We can simulate the effect of "style" by providing pre-made CSS classes and vetting the "class" attribute.) I am sure you can generalise from here. As for some other kind of markup: if my users were sophisticated enough to use something else, then I would use it. The target audience is not very literate, let alone computer literate.
But you're right, we are off topic.
Sorry to reply to your post then, I couldn't resist. :-/ cheers peter