W3C home > Mailing lists > Public > public-appformats@w3.org > January 2008

Re: FAQ & Use Cases

From: Jon Ferraiolo <jferrai@us.ibm.com>
Date: Fri, 18 Jan 2008 16:37:41 -0800
To: Jonas Sicking <jonas@sicking.cc>
Cc: Anne van Kesteren <annevk@opera.com>, "WAF WG (public)" <public-appformats@w3.org>, public-appformats-request@w3.org
Message-ID: <OF0BA381BF.93161A44-ON882573D4.007DFB9A-882573D5.00037365@us.ibm.com>

Hi Jonas,
Thanks for taking the time to provide in-depth responses. See below.
Jon

Jonas Sicking <jonas@sicking.cc> wrote on 01/18/2008 11:08:12 AM:

> Jon Ferraiolo wrote:
> > Hi Anne,
> > The FAQ was very helpful in documenting the rationale behind some of
the
> > approaches in the Access Control spec. However, I disagree with most of

> > the reasoning, as shown below. The use case are a step in the right
> > direction, and I have some feedback on them. (Further down)
> >
> >  >Access Control for Cross-site Requests DESIGN DECISION FAQ
> >  >
> >  >** Why is there a second check for non-GET requests?
> >  >
> >  >For non-GET requests two checks with Access-Control HTTP headers and
> >  ><?access-control?> processing instructions are performed. Initially a
> >  >"permission to make the request" check is done on the response to the
> >  >authorization request. And then a "permission to read" check is done
> > on the
> >  >response of the actual request. Both of these checks need to succeed
in
> >  >order for success to be relayed to the protocol (e.g.
XMLHttpRequest).
> >
> > I appreciate the attention that the WG has put into trying to promote a

> > secure approach to deliverying POST requests, but IMO it would be
better
> > to pursue a different technical strategy, such as what we see in
> > JSONRequest or something derivative of that. With JSONRequest, POSTs
are
> > allowed without requiring a prior GET (or HEAD or OPTIONS). JSONRequest

> > also can be made available via a JavaScript implementation that works
in
> > today's browsers. Its approach is simpler for everyone involved, both
> > server-side developers and client-side (i.e., JavaScript) developers.
> > I'm not sure if JSONRequest is perfect as is (e.g., only supports JSON
> > and not XML), but it was designed by Doug Crockford, who gives talks at

> > Ajax conferences on web security, and IMO he has a good handle on
> > security issues.
>
> Using JSONRequest here would not fulfill requirement 7 from the spec as
> JSONRequest can only be used to POST JSON data.

This has been pointed out before, but
(1) JSON can wrap other types of data, such as {xml:"<foo>foo</foo>"}
(2) I have claimed in prior email that JSONRequest would be better if it
supported XML natively and have suggested how JSONRequest might be changed
to do this

Regarding requirement #7, it would be good to include a specific list of
target datatypes, such as JSON, XML, plain text and HTML fragments, rather
than phrasing it as "we should not limit ourselves to content of a
particular type", which is phrased more to exclude JSONRequest as an
alternative rather than help guide the WG towards designing appropriate
technology.

>
> Maybe we need to clarify that 7 applies to both POSTing and GETtin data.
> Anne, would you mind adding text to state that? We should probably
> expand 5 to include sending of data too.
>
> I don't see how JSONRequest can be expanded to include posting XML data
> without violating requirment 1, especially the second pullet point.

As designed today, JSONRequest requires wrapping XML inside of JSON. If
JSONRequest were extended to support XML, then define this enhancement to
JSONRequest such that it takes into account that second bullet.

>
> >  >** Why are cookies and authentication information sent in the
request?
> >  >
> >  >Sending cookies and authentication information enables user-specific
> >  >cross-site widgets (external XBL file). It also allows for a user
> >  >authenticated data storage API that services can use to store data
in.
> >
> > As I have said previously, I disagree that XBL (and XSLT) should impact

> > decisions about how to provide the best technology for cross-site data
> > access. The XBL and XSLT specs can say that user agents must allow
> > cross-domain access, just as is allowed for CSS stylesheets.
>
> I don't understand why you think XML data is unimportant? I don't
> believe that one-format-to-rule-them-all is ever going to work. It
> hasn't in the past.

See above.

>
> > Where did the idea of user-specific widgets come from, anyway? IMO,
that
> > would be a very, very low priority (approaching zero).
>
> Why? User specific data seems very useful. It would be great to be able
> to create mashups that pull my calender data from google calender as
> well as my addressbook from yahoo mail.

Hmm. That's a much more reasonable example that others I have seen. (As I
say over and over, I don't buy the XSLT and XBL arguments.)

However, the trend in mashup space has been to put gadgets into (different
domain) IFRAMEs for security reasons. The Gadgets spec strongly encourages
developers to create gadgets that are self-contained and therefore can live
in a sandbox (i.e., IFRAME). IBM's QEDwiki does this also. My understanding
is that IFRAMEs are the technology of choice for this scenario for most of
the big mashup providers these days. Because of this, at OpenAjax we have
some mashup initiatives that embrace the IFRAME approach, and focused on
how to send messages across IFRAMEs in a secure manner. IBM Research has a
paper on an approach called SMash
(http://domino.research.ibm.com/library/cyberdig.nsf/papers/0EE2D79F8BE461CE8525731B0009404D/$File/RT0742.pdf)
 and contributed the open source for SMash to OpenAjax Alliance. Yahoo has
talked about a similar approach ("CrossFrame":
http://ajax.sys-con.com/read/473127.htm).

Note that if a gadget goes into its own IFRAME, then cookies become much
less of an issue because the gadget goes its own sandbox'd HTML page that
whose HTML and JavaScript is fully controlled by the organization that
created the gadget.

>
> > The negative with sending cookies is that it opens up CSRF attack
> > opportunities when web sites say allow "*", which I expect would be the

> > most common scenario. There are other approaches to sending data to a
> > 3rd party domain that are more secure and still achieve the same
result.
> > Once again, JSONRequest does not send cookies. I assume that Doug
> > Crockford assumes that authenticated information (if necessary) would
> > come from be sent with the payload (rather than via cookies), which
> > means that if domain FOO wants to upload data to domain BAR, then
domain
> > FOO's web page would require the user to someone enter their BAR
> > authentication information (which could be stored in FOO's cookies, not

> > BAR's cookies). With such an approach, the user will be told by FOO's
> > web page that this web page needs his BAR account information, so the
> > user gets to opt-in to allowing the cross-site POST, instead of the
> > current approach in Access Control where cookies (potentially with
> > credentials) are always sent without the user being aware.
>
> Without sending cookies we can't satisfy requirement 12. Especially in
> combination with requirement 3.

Others have criticized the second half of my paragraph above, and I agree
with some of the criticisms of that second half, but the fundamental issue
is that cookies open up the possibility of CSRF attacks, and my belief
(along with Doug Crockford's) is that cross-domain data services (Access
Control or JSONRequest or whatever) are possible without sending cookies,
or at least *always* sending cookies. Perhaps people have already thought
of this, but wouldn't it be better if cookies were not sent unless a prior
OPTIONS request said that the server wants to see the cookies; in other
words, the server has to explicitly opt-in to cookie transmission.

>
> JSONRequest requires that I give my login data to the requesting site.
> That seems scary.
>
> >  >Cookies and authentication information is already sent cross-site for
the
> >  >HTML <img>, <script>, and <form> elements so this does not introduce
a new
> >  >attack vector. It simply makes use of the Web.
> >
> > <img> and <script> only work with GET, so if a web server follows best
> > practices (i.e., only support POST when submitting data), then you
> > aren't subject to data submission attacks. There is no way to retrieve
> > data via <img>, so that doesn't allow data retrieval attacks. With
> > <script>, the only way to retrieve data is if the server supports
> > JSON-with-callback.
>
> This is exactly how access control works too. You can issue GET requests
> that include cookies and auth information to 3rd party sites, but you
> can't retrieve data by default. You can only retrieve data if the server
> explicitly allows it.
>
>  > Because of these factors, I don't think <img> and
>  > <script> should be used to say that the web is already vulnerable.
>
> I'm not sure what you mean by this. All we are saying is that it is
> already possible to issue GET requests that include cookies in a number
> of ways. Do you not agree with this?
>
> > It is true that <form> works with both GET and POST and does send
> > cookies to other domains, which means web site developers today indeed
> > need to protect against CSRF attacks via cross-domain use of <form>,
> > where CSRF protection is usually is accomplished by the server actually

> > managing a session with the user where a random token is maintained
> > across the session (without storing it within cookies). Right now,
> > Access Control does not have a good mechanism for achieving
> > server-controlled session management with such a random token, largely
> > because it uses a client-side PEP approach. In fact, Access Control
gets
> > in the way of the way most server developers would implement CSRF
> > protection.
>
> Please elaborate as I don't understand what you mean here. If you think
> access control allows for any specific attacks please describe in detail
> the attack.

There was prior criticism of my paragraph above by Thomas. See
http://lists.w3.org/Archives/Public/public-appformats/2008Jan/0191.html and
search for CSRF.

>
> >  >** Why can't cookies authentication information be provided by the
script
> >  >author for the request?
> >  >
> >  >This would allow distrubted cookie / user credentials search.
> >  >
> >  >** Why is the client the policy enforcement point?
> >  >
> >  >The client already is the policy enforcement point for these
requests. The
> >  >mechanism allows the server to opt-in to let the client expose the
data.
> >  >Something it currently does not do and something which servers rely
> > upon the
> >  >client not doing.
> >
> > This confuses "access control" with "policy enforcement point". Yes,
> > browsers today implement an access control policy that (currently)
> > prevents (among other things) XHR from talking to other domains and
> > prevents frames from different domains to see each other's DOM. But
> > "policy enforcement point" means deciding which particular users or
> > which particular domains should have the ability to retrieve data or
> > change data.
> >
> > With JSONRequest, the access control rules are loosened such that the
> > new mechanism (i.e., JSONRequest) is allowed to access other domains,
> > and therefore assumes that the policy enforcement point is the server
> > who receives the GET and POST requests. This approach makes more sense
> > and results in a simpler mechanism.
>
> Sure, saying 'anyone can access the data' is simpler. It also seems to
> open new attack vectors. Requirement 1 states that we must not allow new
> attack vectors.

Both Access Control and JSONRequest require the server to opt-in before
data can be retrieved or accepted. With Access Control, the server inserts
some control information into the HTTP headers or into a PI, but then
ACTUALLY SENDS THE DATA TO THE CLIENT, and then trusts that the client will
honor the allow/deny instructions. This amounts to "anyone can access the
data" since it is trivial to send any arbitrary HTTP request from a
malicious server (or modified browser).

I argue that Access Control is dangerous because it provides the server
developer with a false sense of security. Server-side developers will fall
into the trap and think that by saying "allow foo.com", then only foo.com
can see the data, but in fact any malicious server can construct an HTTP
request to retrieve the data. I like the approach in JSONRequest because it
doesn't pretend that the client can be trusted to manage access rights and
forces the server developer to manage access within the server.

>
> >  >Note however that the server is in full control. Based on the request

> > it can
> >  >simply opt to return no data at all or not provide the
necessaryhandshake
> >  >(in form of the Access-Control HTTP headers and <?access-control?>
> >  >processing instructions.
> >  >
> >  >** Why does the mechanism do both black- and whitelisting?
>
> This is a result of requirements 4, 8 and 9 together.

I disagree that requirement 8 (or 9) needs to be or should be satisfied
within Access Control, and therefore are non-requirements. Nearly all data
retrieval will be managed by some procedural logic, such as PHP or ASP. The
server developer can put the list of black-listed or white-listed domains
into his PHP or ASP scripts, with the result that you have a server-side
PEP, as has been requested by some of the people on this list. The server
approach allows multiple decision approaches. You can use black-listing,
white-listing, or look for a magic key within the request that only trusted
parties could possibly know.

>
> >  >In case the server and documents hosted on the server are in control
by
> >  >different people it is necessary that the server people are able to
> > override
> >  >the document people (if the document wants to share access) and vice
versa
> >  >(if the server wants to share access).
> >  >
> >
> > I think that both whitelisting and blacklisting represent a major
> > weakness in the Access Control spec. I have yet to see important use
> > cases where it makes sense for a particular domain FOO to allow access
> > to a particular domain BAR. In the real world, when would
> > http://www.facebook.com ever list a particular domain to which it would

> > provide access? As best as I can tell, the primary use cases are *
> > (i.e., allow everyone) and *.mydomain.com (i.e., allow access to all of

> > your own subdomains). For simplicity reasons, the best way to go is to
> > drop whitelisting and blacklisting entirely and therefore just support
> > *, which is what JSONRequest does.
>
> I don't think that seems very far fetched that a set of servers want to
> collaborate. For example www.livejournal.com and user.livejournal.com
> might want to allow each other to read data. They are basically the same
> site, but use different domain names for various security reasons.

Yes, there are some cases where specific domains are exactly what is
needed, but whitelisting/blacklisting is the best approach in only a small
subset of scenarios, and when needed, can be accomplished by leveraging
server side mechanisms that exist today already, such as PHP. Again, with
JSONRequest, anyone can make any request, and the server decides which
requests to honor, perhaps using blacklisting/whitelisting or maybe some
other approach.

>
> >  >Access Control for Cross-site Requests USE CASES
> >  >
> >  >FOO in the scenarios below is a fictional person who lives in Havana
and
> >  >likes playing with Web technology that isn't implemented anywhere.
> >
> > Of course, the "isn't implemented anywhere" is a bit of humor, but it
> > does point out a weakness with Access Control. It won't provide value
to
> > the world until it is available in a majority of deployed browsers.
> > Since MS and Apple are not participating in the discussion so far,
there
> > is a worry that their browser might never support Access Control. I
have
> > no insight into what Apple is thinking regarding Access Control, but I
> > hearing security concerns from people at MS about Access Control within

> > discussion at OpenAjax. But let's suppose that Apple and MS do come
> > around and ultimately ship it, let's say in 3 years. It will then take
> > roughly another 3 years or so before the majority of deployed browsers
> > support the feature. On the other hand, JSONRequest can be implemented
> > in JavaScript and will work with today's browsers, and therefore the
> > community can use it immediately.
> >
> > What the above paragraph translates into is that I would like to see a
> > requirement that says something about it being highly desirable that
the
> > mechanism can run in today's browsers as they exist today (without
> > requiring the addition of a new browser plugin).
>
> You are arguing that we shouldn't design a new standard because the new
> standard doesn't work out of the box in all existing browsers. This
> would seem to limit progress a whole lot. If we followed that we would
> put most of W3C out of business, no?
>
> I've heard ample interest from all major browser vendors, with possible
> exception of microsoft, but I don't have as good personal connections
> with anyone on the IE team as I do with the other browser vendors.

I am saying that I believe it is possible to design a cross-domain data
access *API* that could be implemented natively in future browsers and
could be implemented in a reasonable manner via JavaScript in today's
browsers. The primary evidence that I have that this is feasible is that
Kris Zyp has done a pure JavaScript implementation of the JSONRequest *API*
that works with today's browsers, but obviously there will be at least one
feature that he can't accomplish with JavaScript. But if JSONRequest were
to become the industry standard, then JavaScript developers could use those
APIs today by including a JSONRequest JavaScript library, where that
library would check to see if the JSONRequest object is already in the
browser, if so use it, else build it. Sure, there are some issues to be
addressed with the pure JavaScript approach, but there is a hugely
compelling benefit that the community doesn't have to wait <n> years before
critical mass of deployment occurs (and maybe longer, with MS holding
effective veto power).

>
> >  >* FOO owns test.example.org and test2.example.org. FOO uses XSLT
> > extensively
> >  >on both domains and because FOO doesn't want to use a pre-processing
> > script
> >  >to duplicate XSLT files he puts them all on test.example.org and
> > includes a
> >  ><?access-control allow="test2.example.org"?> at the top of them.
> >
> > IMO opinion, the XSLT and XBL specs should simply say that user agents
> > should allow cross-site access, just like what happens today with CSS
> > and JavaScript. Don't need Access Control for that.
>
> This would seem to open very scary new attack vectors. Just because a
> spec is produced that says that it's ok to load new data types cross
> site doesn't mean that server administrators automatically protect
> themselves against that. Note that both XSLT and XBL can basically use
> any raw XML file. All XSLT needs is one magic attribute on the root
> element, and XBL might not even need that.

Bottom line: I don't buy it. XSLT and XBL should not have any impact on how
the cross-domain data retrieval feature should work.

>
> >  >* FOO has implemented the fictional OPEN DATA REST API on
test.example.org
> >  >to store data so that services that help him keep track of bookmarks,
> >  >friends, et cetera can store the info on FOO's domain instead of
their
> > own.
> >  >This allows FOO to switch to any other service provider taking his
data
> >  >easily with him. Using Access Control he enables 2del.icio.us.invalid
and
> >  >flickr2.invalid to access his data so they can store and manipulate
data.
> >  >To keep other people from messing with his data the API only works if

> > you're
> >  >authenticated with test.example.org.
> >
> > I didn't fully understand the above workflow. This I understand: FOO
has
> > a web site at test.example.org that implements OPEN DATA REST API. But
> > how is it that del-icio.us or flickr would even know about
> > test.example.org to invoke the OPEN DATA REST APIs on that site? And
why
> > doesn't the web page at test.example.org simply invoke flickr or
> > del.icio.us APIs (probably using dynamic script tag with JSON today
> > using a particular API key) to retrieve the data and then upload it to
> > test.example.org? BTW - flickr does have a rich set of APIs today
(after
> > all, it's part of Yahoo), but all I could find for del.icio.us were a
> > small number of APIs that seemed to work only via server-side
> > facilities. (Maybe I am missing something.)
>
> The flow is that example.org inc advertises that they have implemented a
> public REST API that provide certain services. Other sites then use that
> API and build functionality on top of that.
>
> Such APIs have been announced by very many web vendors already. Here are
> some examples:
>
> www.flickr.com/services/api
> http://developer.yahoo.com/maps/simple/index.html
> www.google.com/apis/adwords/
> http://wiki.developers.facebook.com/index.php/API

I am well-aware of this industry trend and in fact mentioned the flickr
APIs in my comments. I was asking for more specifics about how data flowed
within this particular use case, and suggesting that the intended result
could be accomplished by a different approach today (i.e., without Access
Control).

>
> >  >* FOO signs up for the personaldata.example.com Web service where you
can
> >  >enter all kinds of personal information, such as your address, credit
card
> >  >information, et cetera. Every shopping site that has a contract with
> >  >personaldata.example.com can then easily extract data out of it as
long as
> >  >FOO is authenticated with personaldata.example.com which gives him a
> > better
> >  >shopping experience.
> >
> > Yikes! No way would any ecommerce site leverage the browser and access
> > control for anything involving credit card numbers. If there were such
a
> > personaldata.example.com web service, then it would implement
> > server-to-server communications to deal with authentication and passing

> > of secure information on a case-by-case basis, with various legal
> > documents among the various parties. Therefore, I do not think this is
a
> > valid use case.
>
> I agree that banking is more scary and would generally require very
> strong security checks.

Therefore I hope the above use case is either eliminated or modified.

>
> >  >* FOO enables cross-site access to his FOAF file and hopes everyone
will
> >  >follow him so that the Tabulator http://www.w3.org/2005/ajar/tab
becomes
> >  >easier to use/write/etc.
> >
> > This one needs more detail, such as would FOO allow everyone to have
GET
> > access, everyone to have POST access, or what? (Note that if everyone
is
> > given access, then there is no need for the complicated allow/deny
> > syntax for this particular use case. In fact, the only use case here
> > that might leverage allow/deny features is the OPEN DATA REST API, but
> > that one needs to be fleshed out some more.)
>
> Yep, this exact use case would not need allow/deny rules.
>
> > Each of the use cases needs a small write-up about how the given use
> > case is accomplished today (or is not possible today) and what proposed

> > alternative technologies (e.g., JSONRequest) might be used instead of
> > access control to achieve the desired result.
>
> As I've stated above, and many times before, JSONRequest fails to meet
> many of our requirements. Such as 4, 5, 7, 8 in combination with 4, 9 i
> think, 10, 11, and 12. And possibly even 1.

Thanks for taking all of that time to respond, but I have to say that I
still remained unconvinced about lots of the decisions. Regarding
JSONRequest, I don't buy the arguments that have been cited for discarding
it, either because I don't agree with particular requirements, or I
disagree with the analysis that concludes that JSONRequest is unsuitable.
But don't get me wrong. I'm not a JSONRequest zealot. It's just one
proposal for how to make the world better in the area of cross-domain data
access. I am just saying that Doug Crockford did a really good job thinking
through the issues, particular those related to security, and that I think
what he came up with is a better answer than what's in the current draft of
Access Control. However, as I have said previously, Access Control in its
current form provides positives and negatives (as done everything), and the
world will deal with those positives and negatives, but I just want to make
sure that I am on record as saying that my opinion is that there are
different approaches that would produce greater benefit to the community
with fewer negative consequences.

>
> Best Regards,
> Jonas Sicking
>
> > Jon
> >
> >
> > public-appformats-request@w3.org wrote on 01/16/2008 05:23:06 AM:
> >
> >  >
> >  > Hi,
> >  >
> >  > In the interested of moving forward and having to repeat myself a
bit
> > less
> >  > I created two documents:
> >  >
> >  >   * http://annevankesteren.nl/temp/access-control-faq
> >  >   * http://annevankesteren.nl/temp/access-control-use-cases
> >  >
> >  > Feel free to ask questions if anything is unclear.
> >  >
> >  > Kind regards,
> >  >
> >  >
> >  > --
> >  > Anne van Kesteren
> >  > <http://annevankesteren.nl/>
> >  > <http://www.opera.com/>
> >  >
> >
>
Received on Saturday, 19 January 2008 00:40:36 UTC

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