Prevent XSS attacks and still use Html.Raw

It isn't easy and you probably don't want to do this. May I suggest you use a simpler language than HTML for end user formatted input? What about Markdown which (I believe) is used by *. Or one of the existing Wiki or other lightweight markup languages?

If you do allow Html, I would suggest the following:

• only support a fixed subset of Html
• after the user submits content, parse the Html and filter it against a whitelist of allowed tags and attributes.
• be ruthless in filtering and eliminating anything that you aren't sure about.

There are existing tools and libraries that do this. I haven't used it, but I did stumble on http://htmlpurifier.org/. I assume there are many others. Rick Strahl has posted one example for .NET, but I'm not sure if it is complete.

About ten years ago I attempted to write my own whitelist filter. It parsed and normalized the entered Html. Then it removed any elements or attributes that were not on the allowed whitelist. It worked pretty well, but you never know what vulnerabilities you've missed. That project is long dead, but if I had to do it over I would have used an existing simpler markup language rather than Html.

There are so many ways for users to inject nasty stuff into your pages, you have to be fierce to prevent this. Even CSS can be used to inject executable expressions into your page, like:

<STYLE type="text/css">BODY{background:url("javascript:alert('XSS')")}</STYLE>

Here is a page with a list of known attacks that will keep you up at night. If you can't filter and prevent all of these, you aren't ready for untrusted users to post formatted content viewable by the public.

Right around the time I was working on my own filter, MySpace (wow I'm old) was hit by an XSS Worm known as Samy. Samy used Style attributes with embedded background Url that had a javascript payload. It is all explained by the author.

Note that your example page says:

This page is meant to accept and display raw HTML by trusted editors.

The key issue here is trust. If all of your users are trusted (say employees of a web site), then the risk here is lower. However, if you are building a forum or social network or dating site or anything that allows untrusted users to enter formatted content that will be viewable by others, you have a difficult job to sanitize Html.

 

Why you should never use Html.Raw in your Razor views

 

@Html.Raw(json) cross-site scripting trap

 

 

Is there a risk in using @Html.Raw?

@Html.Raw will allow executing any script that is on the value to display. If you want to prevent that you need to use @Html.AttributeEncode