W3C home > Mailing lists > Public > public-webappsec@w3.org > January 2016

Re: In-browser sanitization vs. a “Safe Node” in the DOM

From: David Ross <drx@google.com>
Date: Fri, 22 Jan 2016 16:10:36 -0800
Message-ID: <CAMM+ux5MjAp3o4npBciQJd4A1dkVdLF9QYpB0kDss3ZSWW3Hbg@mail.gmail.com>
To: Jim Manico <jim.manico@owasp.org>
Cc: Michal Zalewski <lcamtuf@coredump.cx>, Chris Palmer <palmer@google.com>, Crispin Cowan <crispin@microsoft.com>, Craig Francis <craig.francis@gmail.com>, Conrad Irwin <conrad.irwin@gmail.com>, "public-webappsec@w3.org" <public-webappsec@w3.org>
Yeah I'm overdue in spending more time on the initial batch of jSanity bug
reports.  I hadn't been able to reproduce this one so far but I need to
work through it.  If it does reproduce it would be an implementation bug in
jSanity.  But I don't see what points to some deficiency in jSanity
relative to some other type of sanitizer.

Let's look at how Safe Node would perform in this case relative to
sanitization.  In the case of Safe Node, it's the browser's job to enforce
this policy:
* Disablement of script / active content

To implement this, you might imagine a check in the rendering engine at the
point new script feeds into the script engine to be executed.  This check
would validate the originating node's ancestors.  Script would only execute
if the originating node does not have a Safe Node as an ancestor.  That's a
whitelist.

Meanwhile the sanitizer on the other hand takes input X and produces output
Y, hopefully with the <scriptlet> removed.  The enforcement is separate
from the code in the browser itself, so it's just making a good guess as to
how the browser will behave when it handles the markup.

Dave


On Fri, Jan 22, 2016 at 3:33 PM, Jim Manico <jim.manico@owasp.org> wrote:

> David,
>
> I think your work on JSanity is excellent, but again it's a cat and mouse
> game that is hard to win. Even recently we see a significant evasion for
> JSanity https://github.com/Microsoft/JSanity/issues/6 that would not be a
> problem for a rule based HTML sanitizer.
>
> I am not saying drop your proposal, I would just like to see programmatic,
> rule based sanitization in addition to generalized blacklist
> policies/sandboxes like you are suggesting.
>
> Aloha,
> Jim
>
>
>
>
> On 1/22/16 6:17 PM, David Ross wrote:
>
>> There is a handful of examples where the rigidity basically
>>> ruled out adoption (e.g., MSIE's old <iframe> sandbox).
>>>
>> This: https://msdn.microsoft.com/en-us/library/ms534622(v=vs.85).aspx
>> It came in for Hotmail, but it was never put to use AFAIK, exactly for
>> the reason you describe.
>>
>> There is a finite list of "unsafe" things that markup / CSS can do
>> when rendered on a page.  (Essential reference, of course:
>> http://lcamtuf.coredump.cx/postxss/)  It is possible there are a
>> couple things missing from the initial list of Safe Node policies
>> requiring enforcement.  (E.g.: Link targeting is covered but we
>> probably also need a way to regulate navigation more generally.)  But
>> the problem is tractable.  And I don't think that sanitization baked
>> into the browser provides a better approach in this regard.
>>
>> Another key thing here is that with either a sanitizer or Safe Node,
>> it's important to pick a good set of secure defaults.  That way the
>> policy problems Michal described are less likely to occur as custom
>> configuration tends to be minimal.  With the sandbox attribute for
>> frames, I think the use cases vary to such an extent that it would
>> have been hard to set secure defaults.  E.g.: allow-scripts and
>> allow-same-origin are OK independently, but not when combined.
>> There's no safe default there because there are many use cases for
>> either approach.  I don't see that Safe Node policies interfere with
>> each other in this way and so we probably dodged this bullet.
>>
>> Jim said:
>>
>>> I have an aversion to different policy packages not being
>>> flexible enough to be useful.
>>>
>> FWIW, as per earlier in the thread, the Safe Node approach addresses
>> scenarios around CSS where _sanitization_ is inflexible.  (Caveat: If
>> a sanitizer is baked into the browser, all of a sudden it can pursue
>> the same approach.)
>>
>> Perhaps support both of these approaches? HTML
>>> Programmatic sanitization and several pre-built policies?
>>> That would provide both easy of use for some, and deep
>>> flexibility for others. Win win win, and win?
>>>
>> My argument is that Safe Node has advantages relative to sanitization
>> baked into the browser.  If you can identify a legit use case that
>> Safe Node can't support cleanly, but browser-based sanitization does,
>> I'd probably jump right back on the sanitization bandwagon.  I wrote a
>> client-side sanitizer not that long ago and I enjoy working on them.
>> =)
>>
>> Dave
>>
>> On Fri, Jan 22, 2016 at 2:40 PM, Jim Manico <jim.manico@owasp.org> wrote:
>>
>>> Thank you Michal. I'll give David's proposal a closer read and comment
>>> shortly.
>>>
>>> I remember Microsoft and their AntiXSS library providing an HTML
>>> Sanitizer
>>> API for untrusted HTML input. It was one of the first in any major
>>> language
>>> or framework. The first version was very permissive and useful but
>>> unfortunately was vulnerable to HTML hacking and of course XSS. The
>>> latest
>>> incarnation was fixed to be very secure, but unfortunately was not at all
>>> useful because it was so restrictive. And MS is now deprecating it with
>>> no
>>> commitment to maintain it.
>>>
>>> I have an aversion to different policy packages not being flexible
>>> enough to
>>> be useful. But I will give David's proposal a deeper read and provide
>>> comments more specific to his proposal.
>>>
>>> Perhaps support both of these approaches? HTML Programmatic sanitization
>>> and
>>> several pre-built policies? That would provide both easy of use for some,
>>> and deep flexibility for others. Win win win, and win?
>>>
>>> Aloha,
>>> Jim
>>>
>>>
>>>
>>> On 1/22/16 5:29 PM, Michal Zalewski wrote:
>>>
>>>> The need to inject untrusted markup into the DOM comes up all the time
>>>>> and
>>>>> is critical (WYSIWYG editors ,etc). But any "safe node" that limits
>>>>> what
>>>>> can
>>>>> render and execute will limit innovation. Each developer needs to
>>>>> support
>>>>> a
>>>>> different markup subset for their app, which is why policy based
>>>>> sanitization is so critical to this use case.
>>>>>
>>>>> Take a look at CAJA JS's sanitizer, Angulars $sanitize,  and other JS
>>>>> centric HTML sanitizers. They all allow the developer to set a policy
>>>>> of
>>>>> what tags and attributes should be supported, and all other markup gets
>>>>> stripped out.
>>>>>
>>>>> This is the kind of native defensive pattern we need in JavaScript,
>>>>> IMO!
>>>>>
>>>> I think there are interesting trade-offs, and I wouldn't be too quick
>>>> to praise one approach over the other. If you design use-centric
>>>> "policy packages" (akin to what's captured in David's proposal), you
>>>> offer safe and consistent choices to developers. The big unknown is
>>>> whether the policies will be sufficiently flexible and future-proof -
>>>> for example, will there be some next-gen communication app that
>>>> requires a paradigm completely different from discussion forums or
>>>> e-mail?
>>>>
>>>> There is a handful of examples where the rigidity basically ruled out
>>>> adoption (e.g., MSIE's old <iframe> sandbox).
>>>>
>>>> The other alternative is the Lego-style policy building approach taken
>>>> with CSP. Out of the countless number of CSP policies you can create,
>>>> most will have inconsistent or self-defeating security properties, and
>>>> building watertight ones requires a fair amount of expertise. Indeed,
>>>> most CSP deployments we see today probably don't provide much in term
>>>> of security. But CSP is certainly a lot more flexible and future-proof
>>>> than the prepackaged approach.
>>>>
>>>> At the same time treating flexibility as a goal in itself can lead to
>>>> absurd outcomes, too: a logical conclusion is to just provide
>>>> programmatic hooks for flexible, dynamic filtering of markup, instead
>>>> of any static, declarative policies. One frequently-cited approach
>>>> here was Microsoft's Mutation-Event Transforms [1], and I don't think
>>>> it was a step in the right direction (perhaps except as a finicky
>>>> building block for more developer-friendly sanitizers).
>>>>
>>>> [1]
>>>>
>>>> http://research.microsoft.com/en-us/um/people/livshits/papers/pdf/hotos07.pdf
>>>>
>>>
>>>
>
Received on Saturday, 23 January 2016 00:11:30 UTC

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