[whatwg] updateWithSanitizedHTML (was Re: innerStaticHTML)

[Maciej Stachowiak]

On Nov 30, 2009, at 6:32 PM, Adam Barth wrote:

> On Mon, Nov 30, 2009 at 5:43 PM, Maciej Stachowiak <mjs at apple.com>  
> wrote:
>> 1) It seems like this API is harder to use than a sandboxed iframe.  
>> To use
>> it correctly, you need to determine a whitelist of safe elements and
>> attributes; providing an explicit whitelist at least of tags is  
>> mandatory.
>> With a sandboxed iframe, as a Web developer you can just ask the  
>> browser to
>> turn off unsafe things and not worry about designing a security  
>> policy.
>> Besides ease of use, there is also the concern that a server-side  
>> filtering
>> whitelist may be buggy, and if you apply the same whitelist on the  
>> client
>> side as backup instead of doing something high level like "disable
>> scripting" then you are less likely to benefit from defense in  
>> depth, since
>> you may just replicate the bug.
>
> I should follow up with folks in the ruby-on-rails community to see
> how they view their sanitize API.  The one person I asked had a
> positive opinion, but we should get a bigger sample size.

For server-side sanitization, this kind of explicit API is pretty much  
the only thing you can do.

>
> I think updateWithSanitizedHTML has different use cases than @sandbox.
> I think the killer applications for @sandbox are advertisements and
> gadgets.  In those cases, the developer wants most of the browser's
> functionality, but wants to turn off some dangerous stuff (like
> plug-ins).  For updateWithSanitizedHTML, the killer application is
> something like blog comments, where you basically want text with some
> formatting tags (bold, italics, and maybe images depending on the
> forum).

I can imagine use cases where allowing very open-ended but script-free  
content is desirable. For example, consider a hosted blog service that  
wants to let blog authors write nearly arbitrary HTML, but without  
allowing script. @sandbox would not be a good solution for that use  
case. In general it does not seem sensible to me that the choice of  
tag whitelisting vs high-level feature whitelisting is tied to the  
choice of embedding content directly vs. creating a frame. Is there a  
technical reason these two choices have to be tied?

>
>> 2) It seems like this API loses one of the big benefits of  
>> sanitizing HTML
>> in the browser implementation. Specifically, in theory it's safe to  
>> say
>> "allow everything except any construct that would result in script/ 
>> code
>> running". You can't do that on the server side - blacklisting is  
>> not sound
>> because you can't predict the capabilities of all browsers. But the  
>> browser
>> can predict its own capabilities. Sandboxed iframes do allow for  
>> this.
>
> The benefit is that you know you're getting the right parsing.  You're
> not going to be tripped up by <img/src=javascript: and friends.

It's true, this is a benefit. However, it seems like even if you  
whitelist tags, being able to say "no script" at a high level

> Also, this API is useful in cases where you don't have a server to  
> help you
> sanitize your input.  One example I saw recently was a GreaseMonkey
> script that wanted to add EXIF metadata to Flickr.  Basically, the
> script grabbed the EXIF data from api.flickr.com and added it to the
> current page.  Unfortunately, that meant I could use this GreaseMonkey
> script to XSS Flickr by adding HTML to my EXIF metadata.  Sure, there
> are other ways of solving the problem (I asked the developer to build
> the DOM in memory and use innerText), but you want something simple
> for these cases.

If the EXIF metadata is supposed to be text-only, it seems like  
updateWithSanitizedHTML would not be easier to use than innerText, or  
in any way superior. For cases where it is actually desirable to allow  
some markup, it's not clear to me that giving explicit whitelists of  
what is allowed is the simple choice.

>
>> I think the benefits of filtering by tag/attribute/scheme for  
>> advanced
>> experts are outweighed by these two disadvantages for basic use,  
>> compared to
>> something simple like the original staticInnerHTML idea. Another  
>> possible
>> alternative is to express how to sanitize at a higher level, using  
>> something
>> similar to sandboxed iframe feature strings.
>
> If you think of @sandbox as being optimized for rich untrusted content
> and updateWithSanitizedHTML as being optimized for poor untrusted
> content, then you'll see that's what the API does already.  The
> feature string Slashdot wants for its comments is ("a b strong i em",
> "href"), but another message board might want something different.
> For example, 4chan might want ("img", "src alt").  I don't think these
> require particularly advanced experts to understand.

updateWithSanitizedHTML and @sandbox both provide features that the  
other does not for reasons that do not seem technically necessary. For  
example, updateWithSanitizedHTML could easily have an "allow  
everything except script" mode, and @sandbox could easily allow per- 
tag whitelisting. Then the choice would be between the resource cost  
of a frame, and the sandboxing features that it's impractical to  
provide without a frame (limiting content to a bounding box while  
still allowing styling, allowing script without affecting the  
containing content, etc).

>
>> Here's a problem that exists with both this API and also  
>> innerStaticHTML:
>>
>> 3) There is no secure and efficient way to append sanitized  
>> contents to an
>> element that already has children. This may result in authors  
>> appending with
>> innerHTML +=  (inefficient and insecure!) or insertAdjecentHTML()  
>> (efficient
>> but still insecure!). I'm willing to concede that use cases other  
>> than
>> "replace existing contents" and "append to existing contents" are  
>> fairly
>> exotic.
>
> Maybe we need insertAdjecentSanitizedHTML instead or in addition.  ;)

Perhaps. The verb "update" is generic enough that it could handle  
different kinds of mutations with flags, but perhaps that means it is  
too vague for a security-sensitive API.

Regards,
Maciej