[whatwg] sandboxing scripts

Alexey Feldgendler alexey at feldgendler.ru
Tue Dec 27 20:29:14 PST 2005

On Wed, 28 Dec 2005 04:18:52 +0600, Hallvord R M Steen  
<hallvors at gmail.com> wrote:

> Sorry to be slow at responding, Christmas and all that..

Merry Christmas and happy New Year to you!

>> 1. The entire thing has to degrade SAFELY in existing browsers.
> I'm not absolutely sure this is a requirement, since it improves on
> today's situation which *is* no security at all once you include a
> script from another server.
> In this context "degrading safely" means "not being
> backward-compatible". You can't add a safejavascript: uri scheme
> without breaking backwards compatibility, do server-side sniffing and
> send different code to different browsers by UA-name, which in itself
> adds so many complications that it is a security problem in its own
> right.

Safe degradation means that untrusted scripts should not be executed on  
older browsers at all. We don't trust an external script, and if we can't  
execute it in such a way that it doesn't break anything, then we shouldn't  
execute it at all.

>> 2. The site author has to take care that the "sandbox" attribute is
>> included in every <script> element, even in user-supplied code.

> Yes. I agree that perhaps an element is a better idea, so that
> everything inside could live in its own environment.

>>> In all cases the limitation would apply only to the thread created by
>>> that SCRIPT tag. Functions defined in those scripts might be called
>>> later and would run with normal privileges.

>> This is dangerous, too, because a malicious script can try to redefine
>> common JS functions like window.alert() to do something bad.

> Yes, but origin-checking every function is too complex  
> implementation-wise.

JS already has origin-checking in the sense that every function is bound  
to its parent namespace (class, window, whatever). No extra  
origin-checking is required beyond that. Functions inside the sandbox are  
just bound to their isolated namespace, just like normal functions are  
bound to the window namespace.

>> 2.2. If the <sandbox> has a domain="..." attribute, then the scripts
>> inside the sandbox have access to cookies from the specified domain, can
>> interact with other sandboxes and frames from that domain, and are
>> otherwise restricted in a similar way as a regular content from that
>> domain (but not breaking out of 2.1 restriction). The "domain" attribute
>> can only specify the domain of the containing document or a subdomain
>> thereof.

> For obvious reasons we can not allow a sandbox to specify
> freely what domain it belongs to and change behaviour to for
> example allow reading cookies or sending XMLHttpRequests to
> that domain, because we have no way to verify that sandbox
> contents are related to the domain it claims to be related
> to. I basically agree with the restriction proposed above, I'm not
> sure what exactly you mean by  "subdomain" though. Would you call
> useraccount.livejournal.com a "subdomain" of www.livejournal.com ? If
> the answer is yes, would you call example.org.uk a "subdomain" of
> demo.org.uk, given that they also share two out of three labels?
> If we say that the sandbox's domain can add server names to the parent
> page's any sandbox that wants to claim it belongs to
> useraccount.livejournal.com must be served from http://livejournal.com
> without the www. Hard to impose such extra restrictions on existing
> content.
> document.domain can only be set to a dot-separated substring of
> itself. We can not use that model either because we can't let content
> on example.co.uk set document.domain to co.uk and contact all other
> .co.uk domains.

The entire thing about allowing to set document.domain to its own suffix  
is a bit broken, don't you think? I think it's there mostly because of the  
www prefix. Some day long ago it seemed wise to introduce the www prefix  
for host names of web servers. I don't know what was the rationale for  
that, but nowadays it seems clear that the www prefix is redundanty. Vast  
majority of sites have www.domain.com aliased to domain.com, which means  
that they continue to support the tradition but don't put it to anything  

Anyway, the www prefix is giving us troubles now. Maybe an exception  
should be made for the exact string "www." so that www-prefixed domains  
are considered equivalent to those without the prefix, and a page on  
www.livejournal.com can declare sandboxes for username.livejournal.com. It  
sounds ugly, but the www prefix is the only case I can think of when my  
approach doesn't work.

>> 2.3. The JS namespace in a sandbox is isolated. JS inside the sandbox
>> cannot see the variables and functions declared outside, and vice versa.
>> JS outside the sandbox can accesss JS variables and functions from  
>> inside the sandbox in an explicit way (like  
>> sandboxElement.sandbox['variable']).
>> If the outer JS needs to make several things (DOM nodes, JS variables)
>>  from the outside accessible to the inner JS, it can do so by putting
>> references to these into sandboxElement.sandbox array.

> Perhaps unlimited access from parent to sandboxElement.contentDocument
> would do? Or should we be more concerned about limiting access from
> parent to sandbox?

What do you mean under sandbox.contentDocument?

Anyway, the parent should have full access to anything inside the sandbox,  
why not?

>> 3. Sandboxes can be nested, with each inner one being additionally
>> restricted by the outer.

> Not entirely sure what you mean by "additionally restricted". We
> either keep JS environments separate or not..? :-)

Sorry, I wasn't clear enough. Under "additionally restricted" I mean that  
the inner sandbox can onlybe declared to a subdomain of the outer  
sandbox's domain.

>> 5. There should be a discussion about what a sandboxed script can do.  
>> Can it set window.location? Can it do window.open()? Maybe these
>> permissions should be governed by additional attributes to <sandbox>.

> Perhaps but I would rather not add too much complexity on permissions.
> I'd be inclined to just set a restrictive but usable policy.
> I'd disallow both window.location and window.open, and prevent sandbox
> from targetting main window with form submits, link targets etc.

About link targets: by default, each link is targeted to the current  
window, with the new page replacing the current one. Does your last  
sentence mean that a sandbox can't contain such "regular" hyperlinks?

>> 6. A sandbox can specify a single JS error handler for all enclosed
>> scripts (to address known cases of scripts which are not ready for the
>> unusual environment they are in).

> Unsure, not all browsers support window.onerror and I'm not sure if it
> is good design.

Otherwise, a malicious script can deliberately cause JS errors, which in  
some browsers will prevent other (legitimate) scripts on the same page  
 from running. This can be regarded as a DoS. The sandbox concept in  
software security usually includes some sort of graceful error handling  
for sandboxed code.

>> 7. Backward compatibility. The current browsers will ignore the unknown
>> <sandbox> element and give the enclosed scripts full access to  
>> everything.
>> This is not acceptable. As there is no way to disable scripting inside a
>> certain element in HTML 4, the HTML cleaners usually found on sites live
>> LiveJournal.com are still required. Here's what they should do.
>> 7.1. There are new elements: <safe-script>, <safe-object>, <safe-iframe>
>> (did I forget something?). They are equivalent to their "unsafe"
>> counterparts, except that the existing browsers simply ignore them. HTML
>> cleaners should replace <script> with <safe-script> and likewise.
> As I said above, this is IMO not ideal because it requires browser
> sniffing and different code for different UAs.

No, it doesn't, and that's the point: older browsers receive <safe-script>  
that they don't know about, and they simply ignore them. This prevents  
untrusted scripts from running in these browsers. Consider the case of  
LiveJournal. If they allow scripts, they can't just serve them as <script>  
to everyone. But as <safe-script> they can, because in older browsers the  
scripts just won't run, and it's not a big deal because, at present, no  
scripts are allowed at all.

> Perhaps we should go for the simpler use cases like including untrusted
> advertising SCRIPT tags before tackling the harder ones like securing
> user input on LiveJournal :-)

I should stress my point once more: bad security is worse than no  
security. If the sandbox mechanism is introduced with just the regular  
<script> element (not <safe-script>), then it would be too dangerous to  
use it. Sites like LiveJournal won't start using it at all because it's  
unsafe for the users of older browsers. And those sites who do adopt it  
will endanger their users' security because untrusted scripts will appear  
in places where they weren't allowed before.

I agree that there are cases when external scripts are "almost trusted",  
like advertisement scripts from well-known sources. They could be  
malicious in theory, but actually you don't believe it's possible. When  
given a choice: serve such script as unrestricted to an older browser, or  
don't serve it at all, -- for such scripts, the former choice will be  
reasonable. In this case, nothing stops the author from using a regular  
<script> element, not the <safe-script>, inside <sandbox>. The script will  
be restricted in newer browsers, but it will still run (unrestricted) in  
older ones.

>> -- Opera M2 9.0 TP1 on Debian Linux 2.6.12-1-k7
> I like your taste in browser and E-mail software :-)

Thanks for giving me a lot of good reasons for my choice.

-- Opera M2 9.0 TP1 on Debian Linux 2.6.12-1-k7
* Origin: X-Man's Station at SW-Soft, Inc. [ICQ: 115226275]  
<alexey at feldgendler.ru>

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