W3C home > Mailing lists > Public > public-webapps@w3.org > October to December 2012

Re: [webcomponents] More backward-compatible templates

From: Adam Barth <w3c@adambarth.com>
Date: Thu, 1 Nov 2012 09:14:32 -0700
Message-ID: <CAJE5ia82g+hk_k=ZHHqsm9q-kJ+pCz=R=eZ3z4q-1iRJAgyj=Q@mail.gmail.com>
To: Hajime Morrita <morrita@google.com>
Cc: Maciej Stachowiak <mjs@apple.com>, Anne van Kesteren <annevk@annevk.nl>, "public-webapps@w3.org WG" <public-webapps@w3.org>
On Thu, Nov 1, 2012 at 8:42 AM, Hajime Morrita <morrita@google.com> wrote:

> A naive proposal: Can we introduce an alias of <script> element, like
> <scr>, and ask template authors to use <scr> instead of <script> inside
> <script template>?  Since <scr> is just an alias of <script>, authors can
> use it even outside <script template>.
>

The problem is then that you can't have doubly nested templates because
<scr> wouldn't be able to nest inside itself.

Adam



> I guess it won't confuse neither the tokenizer nor existing parser, and it
> will be polyfill-friendly. It isn't as clean as <template>, but not as ugly
> as <+script> IMO. Obviously "scr" sounds ugly so it is great if it has some
> unused sensible name.
>
>
>
>
> On Thu, Nov 1, 2012 at 3:14 PM, Adam Barth <w3c@adambarth.com> wrote:
>
>>
>>
>>
>> On Thu, Nov 1, 2012 at 6:33 AM, Maciej Stachowiak <mjs@apple.com> wrote:
>>
>>>
>>> On Nov 1, 2012, at 1:57 PM, Adam Barth <w3c@adambarth.com> wrote:
>>>
>>>
>>>>
>>> (5) The nested template fragment parser operates like the template
>>>> fragment parser, but with the following additional difference:
>>>>      (a) When a close tag named "+script" is encountered which does not
>>>> match any currently open script tag:
>>>>
>>>
>>> Let me try to understand what you've written here concretely:
>>>
>>> 1) We need to change the "end tag open" state to somehow recognize
>>> "</+script>" as an end tag rather than as a bogus comment.
>>> 2) When the tree builder encounter such an end tag in the ???? state(s),
>>> we execute the substeps you've outlined below.
>>>
>>> The problem with this approach is that nested templates parse
>>> differently than top-level templates.  Consider the following example:
>>>
>>> <script type=template>
>>>  <b
>>> </script>
>>>
>>> In this case, none of the nested template parser modifications apply and
>>> we'll parse this as normal for HTML.  That means the contents of the
>>> template will be "<b" (let's ignore whitespace for simplicity).
>>>
>>> <script type=template>
>>>   <h1>Inbox</h1>
>>>   <script type=template>
>>>     <b
>>>   </+script>
>>>  </script>
>>>
>>> Unfortunately, the nested template in this example parses differently
>>> than it did when it was a top-level template.  The problem is that the
>>> characters "</+script>" are not recognized by the tokenizer as an end tag
>>> because they are encountered by the nested template fragment parser in the
>>> "before attribute name" state.  That means they get treated as some sort of
>>> bogus attributes of the <b> tag rather than as an end tag.
>>>
>>>
>>> OK. Do you believe this to be a serious problem? I feel like
>>> inconsistency in the case of a malformed tag is not a very important
>>> problem, but perhaps there are cases that would be more obviously
>>> problematic, or reasons not obvious to me to be very concerned about cases
>>> exactly like this one.
>>>
>>
>> It's going to lead to subtle parsing bugs in web sites, which usually
>> means security vulnerabilities.  :(
>>
>> Also: can you think of a way to fix this problem? Or alternately, do you
>>> believe it's fundamentally not fixable? I've only spent a short amount of
>>> time thinking about this approach, and I am not nearly as much an expert on
>>> HTML parsing as you are.
>>>
>>
>> I definitely see the appeal of trying to re-use <script> for templates.
>>  Unfortunately, I couldn't figure out how to make it work sensibly with
>> nested templates, which is why I ended up recommending that we use the
>> <template> element.
>>
>> Another approach we considered was to separate out the "hide from legacy
>> user agents" and the "define a template" operations.  That approach pushes
>> you towards a design like
>>
>> <xmp>
>>   <template>
>>     <h1>Inbox</h1>
>>     <template>
>>       <h2>Folder</h2>
>>     </template>
>>   </template>
>> </xmp>
>>
>> You could do the same thing with <script type=something>, but <xmp> is
>> shorter (and currently unused).  This approach has a bunch of
>> disadvantages, including being verbose and having some unexpected parsing:
>>
>> <xmp>
>>   <template>
>>     <div data-foo="<xmp>bar</xmp>">
>>       This text is actually outside the template!
>>     </div>
>>   </template>
>> </xmp>
>>
>> The <script type=template> has similar problems, of course:
>>
>> <script type=template>
>>   <div data-foo="<script>bar</script>">
>>     This text is actually outside the template!
>>   </div>
>> </script>
>>
>> Perhaps developers have a clearer understanding of such problems from
>> having to escape </script> in JavaScript?
>>
>> All this goofiness eventually convinced me that if we want to support
>> nested templates, we ought to use the usual nesting mechanics of HTML,
>> which leads to a design like <template> that nests like a normal tag.
>>
>>           (a.i) Consume the token for the close tag named "+script".
>>>>          (a.ii) Crate a DocumentFragment containing that parsed
>>>> contents of the fragment.
>>>>          (a.iii) [return to the parent template fragment parser] with
>>>> the result of step (a.ii) with the parent parser to resume after the
>>>> "+script" close tag.
>>>>
>>>>
>>>> This is pretty rough and I'm sure I got some details wrong. But I
>>>> believe it demonstrates the following properties:
>>>> (B) Allows for perfect fidelity polyfills, because it will manifestly
>>>> end the template in the same place that an unaware browser would close the
>>>> <script> element.
>>>> (C) Does not require multiple levels of escaping.
>>>> (A) Can be implemented without changes to the core HTML parser (though
>>>> you'd need to introduce a new fragment parsing mode).
>>>>
>>>
>>> I suspect we're quibbling over "no true Scotsman" semantics here, but
>>> you obviously need to modify both the HTML tokenizer and tree builder for
>>> this approach to work.
>>>
>>>
>>> In principle you could create a whole separate tokenizer and tree
>>> builder. But obviously that would probably be a poor choice for a native
>>> implementation compared to adding some flags and variable behavior. I'm not
>>> even necessarily claiming that all the above properties are advantages, I
>>> just wanted to show that there need not be a multi-escapting problem nor
>>> necessarily scary complicated changes to the tokenizer states for <script>.
>>>
>>> I think the biggest advantage to this kind of approach is that it can be
>>> polyfilled with full fidelity. But I am in no way wedded to this solution
>>> and I am intrigued at the mention of other approaches with this property.
>>> The others I know of (external source only, srcdoc like on iframe) seem
>>> clearly worse, but there might be other bigger ones.
>>>
>>
>> The xmp-like wrapper also can be polyfilled, as can approaches based on
>> HTML comments or attributes.  There's a trade-off, however.  In the long
>> view, it's not clear to me how important polyfillability is for a feature.
>>  It certainly makes adoption easier in the short term, but if we constrain
>> ourselves to designing only features that can be polyfilled at each step,
>> we'll end up with a contorted platform.
>>
>>
>>>  (D) Can be implemented with near-identical behavior for XHTML, except
>>>> that you'd need an XML fragment parser.
>>>>
>>>
>>> The downside is that nested templates don't parse the same as top-level
>>> templates.
>>>
>>>
>>> Indeed. That is in addition to the previously conceded downsides that
>>> the syntax is somewhat less congenial.
>>>
>>> Another issue is that you've also introduced the following security risk:
>>>
>>> Today, the following line of JavaScript is safe to include in an inline
>>> script tag:
>>>
>>> var x = "</+script><img onerror=alert(1)>";
>>>
>>> Because that line does not contain "</script>", the string "alert(1)"
>>> will be treated as the contents of a string.  However, if that line is
>>> included in an inline script inside of a template, the modifications of to
>>> the parser above will mean that alert(1) will execute as JavaScript rather
>>> than being treated as a string, introducing an XSS vector.
>>>
>>>
>>> I don't follow. Can you give a full example of how this would be
>>> included in a template and therefore be executed?
>>>
>>
>>  <script>
>> x =  "</+script><img onerror=alert(1)>"; // This is safe, there is no
>> script execution.
>> </script>
>>
>> <script type=template id=a>
>>    <script>
>>     x =  "</+script><img onerror=alert(1)>"; // This is not safe, the
>> alert(1) executes as script.
>>   </+script>
>> </script>
>>
>> <script>
>> document.body.appendChild(document.getTemplateById("a").instantiate());
>> </script>
>>
>> You should imagine, of course, the string not being written literally by
>> the developer but instead generated on the server side by some code that
>> knows how to escape strings for use in inline script tags (e.g., by
>> escaping "\" and "</script").  It's certainly possible to defend against
>> this XSS vector on the server, but it's one more XSS vector to worry about.
>>
>>  I hope this clarifies the proposal.
>>>>
>>>> Notes:
>>>> - Just because it's described this way doesn't mean it has to be
>>>> implemented this way - implementations could do template parsing in a
>>>> single pass with HTML parsing if desired. I wrote it this way mainly to
>>>> demonstrate the desired properties/
>>>>
>>>
>>> I'm not sure how we'd be able to that without running multiple copies of
>>> the tokenizer state machine in parallel.  The tokenizer states for the
>>> template fragment parser aren't going to line up in any meaningful way with
>>> the top-level tokenizer's search for an appropriate end tag to escape from
>>> the script data states.
>>>
>>>
>>> I'm pretty confident it's *possible* to do a one-pass version of this
>>> algorithm, but I am not sure if it is easy, or if it is desirable.
>>>
>>> What I actually imagined (knowing much less about HTML parsing than you)
>>> was that you'd enter a different tokenizer state after encountering a
>>> <script template> than a <script>. But defining that state would be
>>> challenging.
>>>
>>
>> Sure, but you're going to need a 17x bigger tokenizer state machine
>> because you'll need to track all 17 script data states for the top-level
>> tokenizer at the same time as you're tracking all the states for the
>> template fragment tokenzier.
>>
>> Adam
>>
>>
>
>
> --
> morrita
>
>
Received on Thursday, 1 November 2012 16:15:37 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 18:49:55 GMT