WebApp Sec: Preventing cross site scripting

[Andrew Beverley <mail () andybev com>]

I am currently writing a web application that, as a small part of it,
needs to display an email message. Obviously the message is potentially
in html format, which to display could be sent straight to the browser.

I would like to know the best way of filtering out undesirable html. I
understand the best way is to only allow acceptable information, in this
case all the different html formatting tags.

However, there is a lot of tags that are acceptable. Another approach
would be to strip out all the bad stuff such as <SCRIPT>, <OBJECT>,
<APPLET>, and <EMBED> but this is far from ideal because of new tags
becoming available and so on.

Are there any functions available (for php) that will take a html page
as input and strip out all nasty stuff? Does anyone have suggestions as
to how to do this as easy as possible?

Thanks,

Andrew Beverley