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RE: Need PDF of MS' input [Was Re: Seeking earlier feedback from MS]

From: Sunava Dutta <sunavad@windows.microsoft.com>
Date: Fri, 20 Jun 2008 14:59:50 -0700
To: Jonas Sicking <jonas@sicking.cc>
CC: Arthur Barstow <art.barstow@nokia.com>, Marc Silbey <marcsil@windows.microsoft.com>, public-webapps <public-webapps@w3.org>, Eric Lawrence <ericlaw@exchange.microsoft.com>, Chris Wilson <Chris.Wilson@microsoft.com>, David Ross <dross@windows.microsoft.com>, "Mark Shlimovich (SWI)" <marksh@microsoft.com>, Doug Stamper <dstamper@exchange.microsoft.com>, Zhenbin Xu <Zhenbin.Xu@microsoft.com>, Michael Champion <Michael.Champion@microsoft.com>
Message-ID: <083D18C6B9B71F4CBCA7B76D97B748310C80CAC6C4@NA-EXMSG-W601.wingroup.windeploy.ntdev.microsoft.com>

Hey Jonas, rather than seem to disappear I wanted to set expectations. (We're not ignoring you and don't want to come off like that) Zhenbin and I have been busy so far on XHR and a few other issues. We'll be starting replying to this Monday next week.
Btw, thanks for the statement:
"> Looking forward to continued discussion on these topics. There is
> definitely some interesting stuff in here so I'm glad we got this
> feedback!"

It was the longest analysis I've ever written and if this proves to be useful nothing could give me greater pleasure! (in IE8) -:)
Congrats on shipping Btw,

> -----Original Message-----
> From: Jonas Sicking [mailto:jonas@sicking.cc]
> Sent: Monday, June 16, 2008 10:00 PM
> To: Sunava Dutta
> Cc: Arthur Barstow; Marc Silbey; public-webapps; Eric Lawrence; Chris
> Wilson; David Ross; Mark Shlimovich (SWI); Doug Stamper; Zhenbin Xu;
> Michael Champion
> Subject: Re: Need PDF of MS' input [Was Re: Seeking earlier feedback
> from MS]
>
> Hi Sunava et Al,
>
> Thanks for the feedback!
>
> This is a great start for a discussion. I hope we can get to more
> concrete discussions about the various issues that microsoft is seeing
> and try to figure out ways to address them.
>
> There is a lot of experience at microsoft on these issues, especially
> as
> first deployers of the XMLHttpRequest API, so I'm greatly looking
> forward to using that experience to improve the Access-Control spec.
>
> Hopefully we can get to those meaty parts in this discussion that is
> following from your mail.
>
>
> I'll start with a mini FAQ to avoid repeating myself below:
>
> Why is the PEP in the client rather than the server?
>
>    In order to protect legacy servers some of the enforcement will have
>    to live in the client. We can't expect existing legacy servers to
> all
>    of a sudden enforce something that they haven't before.
>
>    In fact, even XDR using client side PEP. It's the client that looks
>    for the XDomainRequest header and denies the webpage access to the
>    data if the header is not there.
>
>    In fact, Access-Control does allow full PEP on the server if it so
>    chooses by providing an "Origin" header.
>
> Is Access-Control designed with "Security by design"
>
>    Yes. In many ways. For example Access-Control does not allow any
>    requests to be sent to the server that aren't already possible
> today,
>    unless the server explicitly asks to receive them.
>
>    Additionally Access-Control sets up a safe way to transfer private
>    data. This prevents sites from having to invent their own which
> risks
>    them inventing something less safe.
>
>    Thirdly, Access-Control integrates well with the existing HTTP
>    architecture of the web by supporting REST apis and the
>    Content-Type header. This allows existing security infrastructure
>    to inspect and understand Access-Control requests properly.
>
> What about DNS rebinding attacks.
>
>    Even with DNS rebinding attacks Access-Control is designed not to
>    allow any requests which are not possible already in todays web
>    platform as implemented in all major browsers.
>
>
> Especially the last point is something that seems to have been
> misunderstood at microsoft. It is not the case that DNS rebinding
> attacks affect Access-Control any different than it affects the rest of
> the web platform. Any server that wants to protect itself against DNS
> rebinding attacks in the current web platform will automatically get
> protected against Access-Control. And any site that does not protect
> itself is already vulnerable to the exact same attacks with
> Access-Control as it is on the current web platform. In fact,
> Access-Control is less vulnerable than XMLHttpRequest on its own is. So
> a server doesn't need to deploy anything extra to "defend" itself
> against Access-Control.
>
> >   Section 4: Secure Design Principles
> >
> >     Why Secure Design Principles Are Important__
> >
> > */"Secure by design/*/, in /software engineering
> > <http://en.wikipedia.org/wiki/Software_engineering>/, means that the
> > software has been designed from the ground up to be secure. Malicious
> > practices are assumed, and care is taken to minimize impact when a
> > security vulnerability is discovered. For instance, when dealing with
> > /user <http://en.wikipedia.org/wiki/User_%28computing%29>/ input,
> when
> > the user has to type his or her name, and that name is then used
> > elsewhere in the /program
> > <http://en.wikipedia.org/wiki/Computer_program>/, care must be taken
> > that when a user enters a blank name, the program does not break." -
> > /Secure by Design, Wikipedia
> <http://en.wikipedia.org/wiki/Secure_by_design>
> >
> > Secure design principles are key to ensuring that users, whether the
> > end-user or service provider, are protected. The increasingly hostile
> > Web and ever more clever attackers lead to the proliferation of new
> > vectors like XSS and CSRF. In the Web of today, it is critical that
> > solutions be secure-by-design /prior/ to release. This does not
> > guarantee that there will be no exploits; however it does ensure that
> > the bug trail is significantly lower and goes a long way toward
> > protecting the user. For more details on this, please read our MSDN
> > article on The Trustworthy Computing Security Development Life Cycle
> > <http://msdn.microsoft.com/en-us/library/ms995349.aspx>.
>
> This sounds great. We've been using these types of principals when
> designing Access-Control too.
>
> >     Background of Client Side Cross-Domain Proposals
> >
> > Cross-site XMLHttpRequest is essentially a combination of a cross-
> domain
> > access mechanism, Access Control
> > <http://dev.w3.org/2006/waf/access-control/> (AC), and an object to
> > enable this mechanism, in this case, a versioned XMLHttpRequest
> object
> > called XMLHttpRequest Level 2
> > <http://dev.w3.org/2006/webapi/XMLHttpRequest-2/> (XHR). This
> > cross-domain implementation will be referred to as CS-XHR.
> >
> > *NOTE: This paper is based on the AC and XHR level 2 draft on
> 3/June/08.*
>
> This is not entirely true. There is nothing that prevents Access-
> Control
> from being applied on XMLHttpRequest Level 1. Just as Access-Control
> can
> be applied to XInclude, XQuery, XSLT, XPath, <video>, CSS fonts, SVG,
> XBL etc.
>
> > XDomainRequest (XDR) is the new object that we designed for cross
> domain
> > using a "clean room" approach, one where we start with strict
> security
> > principles and a "clean slate" and add functionality only if it meets
> > those principles.
>
> We've used the same approach when designing Access-Control as well.
>
> > /"To me, it boils down to three issues: security, simplicity, and
> > architecture. I believe security concerns trump all others, and my
> > analysis is that Microsoft's security team made the right calls with
> > the XDR proposal, taking the conservative approach where no headers,
> > cookies or other credentials are transmitted to other domains, and
> the
> > policy enforcement point (PEP) is assumed to be on the server. This
> > aligns with the de facto security model for today's Web where a user
> > establishes trust with the single domain, where the user and that
> > domain share secret information only between themselves, such as the
> > information stored in cookies. At OpenAjax Alliance, we have a
> > Security Task Force which contains some industry experts on web
> > security issues and the strong consensus (different than unanimity)
> > was a preference for XDR, mainly for security reasons. On the
> > simplicity side, XDR is appropriately simple (roughly as simple as
> > JSON Request), whereas Access Control has incrementally added
> > complexity (syntax rules for allowing/denying domains, two-step dance
> > for POST requests, detailed lists of headers that are transmitted) to
> > the point that it is now a small beast. On the architecture side,
> > Access Control is just plain wrong, with the PEP on the client
> instead
> > of the server, which requires data to be sent along the pipe to the
> > client, where the client is trusted to discard the data if the user
> > isn't allowed to see the data; it is just plain architecturally wrong
> > to transmit data that is not meant to be seen. Regarding the
> criticism
> > of XDR with more complex workflows where two sites need to work in
> > coordination with each other, possibly including the use of cookies
> > from the two sites, there are lots of ways to skin that cat and for
> > security reasons (such as CSRF concerns) should not be done within
> the
> > context of the cross-domain request mechanism. For example, HTML5
> > allows postMessage(), so you can set up a web page with two IFRAMES,
> > each talking to a different server, and have them do client-side
> > communications via postMessage(); also, there are various server-side
> > alternatives to address these scenarios." - Jon Ferraiolo, Web
> > Architect, IBM & Open AJAX Alliance/
>
> This is addressed by the FAQ above.
>
> >   Section 5: Security Concerns with Web API WG Proposal on Cross-
> Domain
> >   XMLHttpRequest
> >
>
> >  In this section, I'll demonstrate a few of these that could be
> critical
> > blockers to implementation by browsers and security minded
> developers.
> > Mozilla echoed our sentiments here by removing CS-XHR support from
> the
> > Beta <https://bugzilla.mozilla.org/show_bug.cgi?id=424923#c14> until
> the
> > specification addressed further security concerns.
>
> This is wholly false. The reasons we dropped support for Access-Control
> in FF3 was very different from the concerns that microsoft has
> expressed. In fact IE8 is vulnerable to the concerns that we had. The
> debate we have had (and are still having) is whether these concerns can
> be addressed without sacrificing security too much elsewhere.
>
> >     Extending XHR for Cross-Domain Access
> >
> > *XHR has a history of bugs and extending it for cross-domain access
> does
> > not build confidence.*
> >
> >         Recommendation
> >
> > Rather than working backwards to secure an object with a poor
> security
> > record, it makes more sense to start from a basic architecture and
> > adding functionality incrementally, securely, and only as necessary.
> >
> >         Discussion
> >
> > XHR has a poor security record across all the major browsers ranging
> > from header spoofing attacks to re-direction attacks. Header spoofing
> > attacks now are even more scary given that CS-XHR uses headers to
> > determine which sites can access resources as well as what actions
> they
> > can do (HTTP Verbs and headers).
>
> So I want to try to understand this comment. Is the concern here over
> the specific API that is used, or about what features that API allows?
>
> It would be trivial to restrict XHR when used cross site such that the
> security model is exactly that of XDR. By disallowing headers to be set
> or retrieved, and by restricting the method to only "GET" and "POST"
> cross-site XHR would have exactly the same feature set as XDR.
>
> The discussion about which API to use is orthogonal to the discussion
> about what features to allow for cross-site requests.
>
> So is the concern here somehow related to security, or is it related to
> concern about confusion of reusing the same object for both same-site
> requests and cross-site requests (which might have different security
> restrictions applied to it).
>
> I'm not saying that API discussions aren't important. They most
> certainly are. And it is an interesting discussion of if having the
> same
> API for same-site and cross-site (whatever that API is) is a good idea
> or not. So I'm just trying to understand what the exact concern is
> here.
>
> >     XHR Behaves Differently in Cross-Domain Mode and Same-Site Mode.
> >
> > *XHR behaves differently in cross-domain mode and same-site mode
> leading
> > to unnecessary confusion for the web developer by being the same API
> > only in name.*
> >
> >
> >         Recommendation
> >
> > XHR is a widely used object. Consequently, it is difficult to reverse
> > engineer without breaking existing deployments, adding complexity,
> and
> > confusing developers. In the process this may introduce new holes
> that
> > require further patching. This different cross domain behavior means
> > that it has all the disadvantages of XMLHttpRequest like its security
> > flaws without any clear benefit. Having a new object here without
> these
> > redundant cross domain properties like getAllResponseHeaders will
> > mitigate a number of these worries.
>
> Again, I'm confused about what the exact concern is here.
>
> The only way I could see that reusing the same API for cross-site
> requests would break existing deployments would be if such deployments
> rely on that calls to XHR.open throw if the requested URI is a
> cross-site one, now it will result in an error event being fired
> instead.
>
> Is that the concern?
>
> Regarding complexity, that sounds like complexity for implementors?
> Same
> thing with "may introduce new holes that require further patching",
> that
> sounds like a concern about implementation bugs? Is that correct?
>
> Again, all of these things are important, i'm just trying to understand
> what the exact concern is here. Complexity for implementors is
> definitely important in order to reduce bugs in the implementation.
>
> >     Access-Control Rules that Allow Wildcards
> >
> > *Requiring implementers to maintain access control rules that allow
> > wildcards can lead to deployment errors.*
>
> Isn't the XDomainRequest:1 the same thing as a wildcard? Doesn't that
> mean that anyone can load the resource on that URI?
>
> >         Recommendation
> >
> > *         For access where AC is important, other architectures like
> > Server Side Proxying
> > <http://developer.yahoo.com/javascript/howto-proxy.html> for service
> > providers who are interested in maintaining access control rules and
> the
> > HTML 5.0's WG's Cross Document Messaging are recommended.
> >
> > *         If you are going to use CS-XHR, we recommend avoiding
> > wildcards, auditing access control rules regularly, and avoiding
> hosting
> > sensitive data from domains that expose data to CS-XHR.
>
> The first seem very drastic, and isn't really a viable replacement in
> many cases. And the second doesn't seem like a recommendation for the
> spec, but rather for someone deploying the spec?
>
> Wouldn't it be better to recommend that the spec disallows wildcarding
> together with transferring cookies? That is the kind of input I was
> hoping for from microsoft, and something that sounds like we should
> take
> a serious look at.
>
>
> >         Future Work
> >
> > Permitting the end user to decide whether the web application they're
> > using should be able to make a cross-domain request may be worth
> > investigating. There are significant user experience challenges
> because
> > the user may not understand the implications of such access.
>
> Exactly, this hasn't worked very well in the past, which is why we've
> chosen to avoid that path.
>
> >         Discussion
> >
> > The service provider who sets the access permissions and returns the
> > requested content is another key player here. Providing a simple
> > scalable solution here will ensure that mistakes in permissions don't
> > unravel as services are deployed and maintained. For example, Flash
> has
> > an access control mechanism similar to the one in CS-XHR and this has
> > been vulnerable to wildcarding attacks. Wildcarding attacks occur
> when
> > access controls are set in error (a distinct possibility as the
> number
> > of rules to filter cross domain requestors increases and becomes
> > complex) and allow for unintended access. This is especially scary
> given
> > that AC can send cookies and credentials in requests. This also
> violates
> > the AC drafts requirement
> > <http://www.w3.org/TR/access-control/#requirements> that it /"should
> > reduce the risk of inadvertently allowing access when it is not
> > intended. That is, it should be clear to the content provider when
> > access is granted and when it is not."/**
>
> I'm not sure I agree here. Saying "Access-Control: allow <*>" makes it
> pretty clear that everyone can read this resource, so we seem to pass
> the spec requirement fine.
>
> >         Community Comments
> ...
> > /Flickr was vulnerable to this exploit, because it hosted an "allow
> all"
> > policy file in its main domains: flickr.com and /www.flickr.com
> > <http://www.flickr.com>/. We notified Flickr and they fixed the hole
> > promptly by moving their APIs to a //separate domain/
> > <http://api.flickr.com/crossdomain.xml>/ and removing the
> > //crossdomain.xml file on their main domain/
> > <http://www.flickr.com/crossdomain.xml>/ (now 404). - Julien
> Couvreur,/
> > http://blog.monstuff.com/archives/000302.html//
>
> (just commenting on the flickr one here as that's the one I know of.)
>
> This wasn't technically a problem due to wildcarding. The solution they
> use still uses wildcards. This was a problem of them applying the
> policy
> too broadly across their URI space.
>
> >     Access-Control Rules Visible on the Client
> >
> > *Allowing Access Control Rules to be visible on the client leads to
> > information disclosure.*
>
> Only if the server so desires. The server has all information needed to
> make this a pure server-side policy, so this doesn't appear to be an
> issue.
>
> >         Recommendation
> >
> > *         XDR ensures that servers regulate access to individual
> > requests and that rules are not available to the client.**
>
> Actually, the XDR doesn't allow neither server or client side rules,
> it's purely all or nothing. XDR currently relies on the 'referer'
> header
> which due to firewall filtering is unreliable to the point that it's
> not
> useful for security checks.
>
> >         Discussion
> >
> > The access control rules need not be exposed to the world as this
> > information could potentially be sensitive. For example, your Bank
> may
> > maintain a list of allowed partners based on your other frequently
> > accessed bank accounts. Making these rules available on the client
> can
> > lead to profiling attacks if this data is intercepted. While AC and
> XDR
> > allow servers to use the Access-Control-Origin header to make
> > access-control decisions preventing them from being viewed on the
> > client, the reality is that in practice web developers are likely to
> opt
> > in for what's easiest and will not leverage this given the
> alternative
> > available for AC.
>
> You think that sites will knowingly broadcast their rules to the world,
> while being concerned that the world will read it? That seems like a
> far
> stretch to me.
>
> >     Access-Control Rules in Headers
> >
> > *Sending Access Control Rules in Headers can lead to inadvertent
> access.*
> >
> >
> >         Recommendation
> >
> > *         Enable users to restrict site-to-site access. This has its
> own
> > set of challenges that need to be investigated like UI.
> >
> > *         If you are using CS-XHR, we recommend not using it to send
> > sensitive data so that if Access Control (AC) rules are compromised,
> the
> > impact of the data disclosed is minimal. When AC rules are audited
> and
> > maintained, if the rules are spoofed (a possibility because XHR has
> been
> > subject to header spoofing attacks and AC rules are maintained in
> > headers), the data may be compromised.
> >
> >
> >         Discussion
> >
> > *         The Web API Cross Site XMLHttpRequest plan allows access
> > control rules to be in headers. It is especially dangerous given that
> > XMLHttpRequest has had header spoofing attacks in the past on
> multiple
> > browsers. This could cause cross domain access to legacy sites not
> opted
> > in to cross domain or change access control rules for existing sites
> > using CS-XHR.
> >
> > *         To make things even more confusing, an XML file and headers
> > can be used to control access control in cross site XMLHttpRequest.
> >
> >
> >         Community Comments
> >
> > /"(Description Provided by CVE) : Firefox before 1.0.7 and Mozilla
> Suite
> > before 1.7.12 allows remote attackers to modify HTTP headers of XML
> HTTP
> > requests via XMLHttpRequest, and possibly use the client to exploit
> > vulnerabilities in servers or proxies, including HTTP request
> smuggling
> > and HTTP request splitting." /http://osvdb.org/osvdb/show/19645//
> >
> > /"//That the XDR proposal enables cross-domain requests with minimal
> complexity and in a way which is unlikely to cause IT administrators to
> disable the feature, is, in my opinion, reason enough to be
> enthusiastic. The XDR proposal seems like something that could be a
> stable platform on which to start building new kinds of applications./
>
> I'm not following this section at all. The first section talks about
> allowing inadvertent access. I first guess that that was inadvertent
> access to content. However the Discussion section talks about reading
> headers, so is that the concern?
>
> Then the first Community comment talks about inserting custom request
> headers, which seems to be something different? Then the second comment
> talks about that XDR is good which seems totally unrelated to security
> comments about the Access-Control spec.
>
> Can you rephrase the concern here, i'm just not understanding it.
>
>
> >     Maintaining Access Control Based on a Header
> >
> > *Maintaining Access Control based on a header that instructs the
> client
> > to serve the response to a particular domain/path instead of an
> > individual request leads to the potential for inadvertent access.*
>
> Isn't XDR also header based? Or is this in general a concern about
> header based solutions, Access-Control and XDR alike?
>
> >         Recommendation
> >
> > *         Ensure proper and complete URL canonicalization if
> > Access-Control is ever granted by path.
>
> Hmm.. this seems like a switch of topic. But yes, URL canonicalization
> (or some people has preferred to call this the process of mapping a URI
> to a file path) is a problem.
>
> > *         Enforcing access control on a per-request basis. Do not
> permit
> > policy from one URL to regulate access to another URL.
>
> Hmm.. again a topic switch. Can you please expand on this? Seems
> unrelated to the header concern in the original title and the URL
> canonicalization concern in the previous comment.
>
> >         Discussion
> >
> > This can lead to vulnerabilities that occur when  the path of the
> request
> > can be modified by an attacker using special characters, a flaw that
> we
> > pointed out to Mozilla on a teleconference on cross origin requests.
> A
> > solution here is currently being discussed by the Web API WG (See
> > right).
>
> I assume here that "This" in the first sentence refers to the "can be
> granted by path" issue?
>
> If so, yes. Based on experience from Adobes Crossdomain.xml deployment
> this seems to cover existing attacks.
>
> > Note the AC draft can be demonstrated to need the access control
> > implementers to take additional security measures
> > <http://lists.w3.org/Archives/Public/public-webapi/2008May/0435.html>
> > although this is against the draft's own requirement
> > <http://www.w3.org/TR/access-control/#requirements> of /"Must not
> > require content authors or site maintainers to implement new or
> > additional security protections to preserve their existing level of
> > security protection." /and/ /"/Must not introduce attack vectors to
> > servers that are only protected only by a firewall." /
>
> How so? Access-Control is secure by design by requiring servers to opt
> in. So if a server does nothing different from what it does today
> Access-Control will always bail early and apply the existing Same-
> Origin
> policy that UAs do today. That applies always, even for content that is
> currently only protected by a firewall.
>
> That seems to cover both the requirements that you list above?
>
> >     Sending Cookies and Credentials Cross Domain
> >
> > *The Access Control sends cookies and credentials cross domain in a
> way
> > that increases the possibilities of information disclosure and
> > unauthorized actions on the user's behalf.*
>
> It is not the sending of the cookies that is the concern here, it's the
> sending of the users private data as a reply to the request that is the
> concern, no?
>
> Web developers want to transfer user-private data. They are going to do
> so whether UAs provide official APIs to do so or not.
>
> So I do think that we need to provide a safe solution for transmitting
> private data, simply saying "never send cookies" is not a viable
> solution unless an alternative is presented.
>
> So far the solutions that sites seem to use is to ask the user for the
> username/password for the third party site, and then employ a server to
> server connection. This is a really bad design security wise since it
> teaches users to give out their credentials. This is especially bad if
> users use the same username/password on multiple sites, something that
> is very common.
>
> >         Recommendation
> >
> > *         Preventing cookies and other credentials from being sent
> cross
> > domain will help ensure that private data is not inadvertently leaked
> > across domains.
> >
> > *         The HTML 5.0 feature called Cross Document Messaging,
> combined
> > with the same-origin XMLHttpRequest, enables regulated cross-domain
> > access on the client without requiring potentially dangerous
> > functionality (e.g., cross-domain submission of headers).
>
> This is definitely a decent alternative. The concern is that sites will
> want to communicate directly with other servers rather than proxy
> everything through javascript and iframes. This can be demonstrated by
> the number of such solutions that are deployed today, despite the fact
> that iframe communication (albeit cumbersome such) has been available
> for some time.
>
> Additionally there is a risk that sites will stick to current <script
> src=> solutions which has a lot of security concerns apart from the
> ones
> discussed here.
>
> >         Future Work
> >
> > Future designs may include:
> >
> > *         The user could enter credentials while making a proper
> trust
> > decision about whom ultimately gets the credentials and who this
> grants
> > access to. Any user trust decision needs to be properly understood as
> > there is the possibility that poor UI design or spoofing may lead to
> the
> > user making the wrong decision. If done correctly this does provide
> the
> > benefit of having the user's explicit assent and a number of existing
> > software dialog warnings are currently based on this mechanism.
> >
> > *         The browser could send an XDomainRequestCookie header
> > <http://lists.w3.org/Archives/Public/public-
> webapi/2008May/0284.html>.
> > This would allow cookies to be sent in a header with a new name, so
> that
> > existing sites would not inadvertently get a cookie and assume that
> this
> > is cross domain. Sites could then ignore this header and not take
> action
> > based on the use's session identifier. Aware servers on the other
> hand
> > could read the new header and provide useful, user-specific services
> > based on its contents. This of course requires the server frameworks
> to
> > need updates to look for such cookies and parse them properly. In
> > addition, any intermediary proxy that behaves differently based on
> > cookies would break, but these are issues that are definitely worth a
> > further look.
>
> This doesn't scale very well though. You'd also need to introduce a
> XDomainRequestAuthorization header and a XDomainRequestFutureAuth
> header
> and so on any time a new way of transmitting authorization data is
> invented.
>
> This is extra bad considering that you are violating the HTTP spec, so
> any existing security infrastructure that deals with auth and cookie
> headers will not recognize the new headers. So you are in fact reducing
> certain aspects of security by going against the HTTP spec.
>
> >         Discussion
> >
> > The way AC does these increases the potential for Cross-Site Request
> > Forgeries as requests will be automatically authenticated and may
> > contain headers otherwise impossible to send via script. For example,
> a
> > user may be authenticated via CS-XHR to his or her bank from their
> > online tax preparation site. If they subsequently visit an evil site,
> it
> > could craft CS-XHR requests to the Bank Site and send a token to
> > authorize actions. Even though CS-XHR requires an OPT-in model from
> the
> > server (this is good), if there is an XSS vuln, AC header spoof, or
> > wildcard accidently set, this opens up another channel for unwanted
> > authenticated actions.
> >
> > In addition, a number of sites may assume and rely on cookies being
> sent
> > with cross-site requests and this could become a third party problem
> if
> > cookies are sent by default. As the Web API WG members note, a large
> > number of sites will not understand cookie authorization and will
> wind
> > up susceptible to CSRF.
>
> (I assume there is a "not" missing in the first sentence in the above
> paragraph?)
>
> I don't understand the CSRF risks at all. CSRF is a problem with sites
> thinking they are getting same-site requests, but forget that HTML
> allows for cross site <form>s. CSRF forces sites to explicitly have to
> opt out of getting certain requests.
>
> Access-Control on the other hand is opt-in. I would think that any one
> that opts in to Access-Control realizes that they are going to get
> cross-site requests, that is the only reason you would opt in, that you
> want cross-site requests.
>
> As far as XSS concerns go, that would seem to almost by design be a
> problem with any solution deployed. For example postMessage has similar
> XSS concerns. If a bank site has allowed a tax preparation site to
> perform postMessage, then if the tax preparation site gets XSS
> attacked,
> the attacker could also attack the bank site.
>
> The wildcard problem you raised above, so lets discuss that there. Or
> is
> this one different in some way?
>
> The header spoof I think you raised above too, but as you could see
> there I didn't really follow you there :) But please do elaborate there
> if it's the same issue.
>
> > Privacy: Including the cookies lets sites more easily track users
> across
> > domains.
>
> Access-Control follows the UAs privacy settings. So there should be no
> more concerns here than with cross-site <img>.
>
> >         Community Comments
> >
> > *         /"sending cookies, by-default, with 'non-safe' requests. /
> >
> > o   /many of the risks that are associated with allowing cross-site
> XHR,
> > e.g. Cross-Site Request Forgery, can be mitigated by not sending
> cookies
> > with these requests. /
> >
> > *         /Jonas concerned that sites will assume and come to rely
> upon
> > browsers not sending cookies with cross-site requests, which could
> lead
> > to problems if we ever decide to start sending 3rd party cookies by
> default/
>
> This is a concern if we *don't* send cookies in the initial
> implementation.
>
> > *         /We should not send cookies and auth headers."
> >
> /http://wiki.mozilla.org/User:Sicking/Cross_Site_XHR_Review#Discussion_
> .26_Implications//
>
> For the most part these concerns would be addressed by my proposals to
> the mailing list. The remaining part is harder to solve and IE8 suffers
> from this problem too.
>
> > # <http://krijnhoetmer.nl/irc-logs/whatwg/20080221#l-85>/ [00:04]
> > <Hixie> the reasons to include cookies are simple -- if we don't have
> > them, we (Google) basically can't use xhr./
> >
> > /. . ./
> >
> > # <http://krijnhoetmer.nl/irc-logs/whatwg/20080221#l-85>/ [00:19]
> > <sicking> so the thing is that CSRF today is kind of a catastrophe.
> > There are lots and lots and lots of sites that are susceptible to it.
> If
> > we had a world where cookies weren't sent for third-party requests
> we'd
> > be in a much safer web/
>
> Yup. Fortunately Access-Control doesn't suffer from this problem since
> it requires Opt-in. So it's not susceptible to CSRF in the same way.
>
> >     Sending Arbitrary Headers Cross Domain
> >
> > *Sending arbitrary headers cross domain breaks a lot of assumptions
> that
> > sites today may make, opening them up for exploits. Creating complex
> > rules to limit the headers sent cross domain makes the spec even more
> > difficult to deploy reliably.*
>
> Headers are only sent if the server opts in under the current spec. So
> I
> don't understand how it could break any assumptions that sites make
> today.
>
> >         Recommendation
> >
> > Do not allow arbitrary headers to be sent cross domain. Avoid any
> design
> > where the list of blocked and allowed headers is likely to be
> confusing
> > and under constant revision as new attacks and interactions arise.
> >
> > If you are implementing CS-XHR, we advise you take extreme caution in
> > what headers you allow in the OPTIONS request, in addition to testing
> > the allow list when opening up your service cross domain.
> Furthermore,
> > we recommend taking extra caution by ensuring that the headers do not
> > specific actions that are dangerous if the request is compromised by
> a
> > DNS-Rebinding attack.
>
> *No* headers are allowed to be set by the website in the OPTIONS
> request. Only once a site has explicitly opted in can custom headers be
> set.
>
> For GET requests there is a very short whitelist (currently only has
> two
> entries) of headers that can be set without the site opting in.
>
> >         Discussion
> >
> > In general, browsers today can not send cross-domain GET/HEAD
> requests
> > with arbitrary headers. With AC, this now becomes possible, breaking
> > many previous assumptions. Microsoft is aware of sites dependent on
> the
> > expectation that arbitrary headers cannot be sent cross domain and
> this
> > is in accordance with HTML 4.0. This is not a good security practice
> by
> > any means but enabling this functionality in a way that compromises
> our
> > users is not an option. As an example, UPnP allows GET requests with
> a
> > SOAP Action header to perform actions on a device. If the SOAP Action
> > header is not actively blocked by a cross-site XMLHTTPRequest client,
> > attackers will be able to perform this attack against routers and
> other
> > UPnP devices. Contrast this with XDR, where arbitrary headers cannot
> be
> > supplied, by default.
>
> It seems like you are misunderstanding the spec. Only once a site has
> opted in can arbitrary headers be set. So any currently existing sites,
> such as UPnP devices, is unaffected.
>
> > An option here is to create a block list of bad headers. However,
> this
> > quickly adds to the complexity of this already complex proposal and
> to
> > make things worse will need continual updates to the spec once
> > implementations have shipped and more blacklisted headers are
> > discovered. This will presumably prevent the spec from stabilizing
> and
> > browsers will have to update patches to secure their implementations.
>
> Block lists are unacceptable we all agree. The block list currently in
> the spec really should be moved to the XMLHttpRequest Level 1 spec as
> that is where the issue lies, not with the Access-Control spec.
>
> > This is a lower concern but having an allow list would be another
> > option. That said, since web sites today do rely on not allowing
> > arbitrary headers across domain it is difficult to prove that the
> > headers on the allow list are not being used by sites for Same Site
> > Origin requests.
>
> Agreed 100%. The spec follows this design.
>
> > To make things even more complicated, the AC spec specifies a
> > complicated mix of allow lists, black lists, and other headers. For
> > example, if a header is not in an allow list, it needs a pre-flight
> > check. (The spec already requires pre-flight checks for non-GET HTTP
> > verbs). This of course is another addition to the multi-part request
> > that AC allows and if the server agrees there's still a blacklist to
> > filter out headers that should not be allowed. The convoluted
> approach
> > continues with XMLHttpRequest level 2 having its own set of
> blacklists
> > that filter headers out prior to cross domain. Moving on, this black
> > list in XMLHttpRequest has a SHOULD not and MUST not specification
> for
> > blocked headers, leaving the door open for different behaviors across
> > browsers.
>
> Even when taking DNS rebindings attack into account (at least the way
> you have defined them at the top of this email) Access-Control doesn't
> allow any requests other than the ones already possible, so it's secure
> by design.
>
> The blacklist of headers in the Access-Control spec really is just the
> same one as the XHR spec and really should just be covered there.
>
> > Header spoofing in XMLHttpRequest is a common vulnerability from the
> > past. Sending headers cross domain may allow for access control rules
> to
> > be changed, enabling legacy services not opting in to Cross Site
> > XMLHttpRequest to be vulnerable.
>
> I do agree there is some concern here. However do note that this only
> applies to servers that opt in. But I do personally have some concern
> for the servers that do opt in.
>
>
> >     Allowing Arbitrary HTTP Verbs
>
> For the sake of avoiding confusion, http "verbs" are often referred to
> as http "methods", so you might see me and others use that term.
>
> > *Allowing arbitrary HTTP verbs to be sent cross domain may allow
> > unauthorized actions on the server. Creating complex rules to secure
> > this opens up the possibility for other types of attacks.*
>
> It would be great with more detail here. I'm especially interested to
> hear feedback from microsoft here since I'm sure you have
> implementation
> experience since deploying XMLHttpRequest. Would be great to hear what
> types of problems you ran into at that time.
>
> >         Recommendation
> >
> > *         Do not allow non-GET and POST verbs. This is in line with
> > capabilities of HTML forms today and is specified by the HTML 4.**
> >
> > *         If verbs are sent cross domain, pin the OPTIONS request for
> > non-GET verbs to the IP address of subsequent requests. This will be
> a
> > first step toward mitigating DNS Rebinding and TOCTOU attacks.
>
> As mentioned before, even with DNS Rebinding attacks Access-Control is
> designed in such a way that it doesn't allow any types of requests to
> be
> sent that can't already be sent by the current web platform.
>
> However the pinning is an interesting idea here. One we should discuss
> further.
>
> > *         Using XMLHttpRequest to do this is inherently more
> complicated
> > as XHR has its own rules for blocking verbs.
>
> You mean that this is more complicated implementation wise? As stated
> before, implementation complexity is certainly important to take into
> consideration, I'm just trying to understand your concern.
>
>
> Looking forward to continued discussion on these topics. There is
> definitely some interesting stuff in here so I'm glad we got this
> feedback!
>
> Best Regards,
> Jonas Sicking
Received on Friday, 20 June 2008 22:00:57 GMT

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