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

RE: IE Team's Proposal for Cross Site Requests

From: Eric Lawrence <ericlaw@exchange.microsoft.com>
Date: Fri, 14 Mar 2008 16:59:38 -0700
To: "Web API WG (public)" <public-webapi@w3.org>, "public-appformats@w3.org" <public-appformats@w3.org>, Sunava Dutta <sunavad@windows.microsoft.com>
CC: Chris Wilson <Chris.Wilson@microsoft.com>, Zhenbin Xu <zhenbinx@windows.microsoft.com>, Gideon Cohn <gidco@windows.microsoft.com>, Sharath Udupa <Sharath.Udupa@microsoft.com>, Doug Stamper <dstamper@exchange.microsoft.com>, Marc Silbey <marcsil@windows.microsoft.com>
Message-ID: <8301DE7F96C0074C8DA98484623D7E51435C1B44CA@DF-MASTIFF-MSG.exchange.corp.microsoft.com>

Thanks for all of the great input!

=====

Jonas Sicking [jonas@sicking.cc] asked:
<<How do you define the "Intranet", "Internet", "Restricted" etc zones?  Without correct definitions for these zones it seems possible to attack intranet servers by sending unsafe (such as POST or DELETE) requests to intranet servers from internet pages.>>

The XDR design does not require zone awareness; this should be considered an optional part of the specification.  Zones are used as an attack surface reduction in IE as they are supported by the IE/Windows platform.  Other browsers could elect to implement an approach where a plain hostname (no dots) is isolated from a fully-qualified domain name (with dots) to approximate the Zones behavior.  However, non-IE browsers generally do not appear to do this today, and this is not required for a successful XDR implementation.

Note that XDR supports only the GET and POST methods, DELETE and other methods are not supported.

XDR is intended for "public" data.  We explicitly suggest that Intranet servers do not expose private data through this mechanism.  In order to ensure that no existing servers/services (in any zone) are put at risk, XDR does not send credentials of any sort, and requires that the server acknowledge the cross-domain nature of the request via the response header.

=====

Jonas Sicking [jonas@sicking.cc] asked:
<<Also, if you do have reliable definitions of the "Intranet"/"Internet"/"Restricted" zones, what is the purpose of the XDomainRequestAllowed header since presumably all servers in a zone could just read data directly from each other without regard for the XDomainRequestAllowed header.>>

If you are suggesting that XDR competes directly with server-side proxies, then yes, this is true.

The intent of XDR is to allow an AJAX application to communicate with multiple origin servers without the overhead of proxying all cross-domain requests through the server which is hosting the AJAX application.

For instance, consider an AJAX "portal" homepage hosted by AJAXPortal.example.com.  The portal page presents data from multiple providers.  In order to allow this, the developer has at least three design options:

1> Use XDR.  The clientside page retrieves data directly from the data providers.
2> Use a serverside proxy running on AJAXPortal.example.com, which uses a backend process to call out to the data provider's servers.  The proxy aggregates the data on AJAXPortal.example.com, and then supplies it to the client.
3> Include multiple IFRAMEs, each of which points to a different data provider.

Using XDR (#1) results in lower load on the AJAXPortal server as compared to using a serverside proxy (#2).  Using XDR (#1) provides increased client application flexibility as compared to using multiple IFRAMEs (#3).

=====

Jonas Sicking [jonas@sicking.cc] observed:
<<Also, the OPTIONS request is there to prevent requests that XDR simply always allows, i.e. cross site requests using unsafe methods. So I'm not sure I see how XDR is safer in that regard here.>>

XDR intentionally does not permit cross-site requests that are not already possible using plain HTML forms.  HTML permits GET/POST forms to be submitted cross-domain.

=====

Laurens Holst [lholst@students.cs.uu.nl] asked:
<<So, if I cannot set HTTP headers, how am I supposed to set an Accept
header to indicate that I e.g. want to receive application/xhtml+xml,
application/atom+xml, application/*rss*+xml, application/rdf+xml, etc.?
Your proposal is completely unfriendly to content negotiation. Also,
there are valid use cases for setting other headers.

I sincerely hope you will fix this issue by creating a blacklist of
headers instead of disallowing them entirely.>>

We absolutely agree that it is possible to define use cases that XDR does not accommodate.  We believe that XDR enables the most common cross-domain scenarios with negligible impact to the attack surface of existing servers and the browser.

Creating a "blocklist" of headers is problematic as there is no existing mechanism to determine whether a target server will interpret a given header in a particular way.

By way of example, we are aware of servers which utilize custom HTTP request headers as an anti-CRSF mechanism.  Such servers assume that, because the only mechanism currently available in the browser to send custom headers is via XMLHTTPRequest, if such custom headers are present, then the request "must" be from a same-origin XHR object.  Hence, permitting use of custom headers in XDR would expose such servers to attack.  It's absolutely reasonable to argue that such servers never should have made such assumptions, however we do not feel it is appropriate to put servers at risk.

While HTTP-based content-negotiation is certainly well-defined by the HTTP specifications, for operational reasons it is relatively uncommon in the wild.  Different content types are usually served from different URLs; for instance, you can see that Yahoo!-based services use a URL parameter to pivot between JSON and XML format (http://developer.yahoo.com/common/json.html).

=====

Maciej Stachowiak [mjs@apple.com] asked:
<<How does this compare to the Cross-Site Extensions for XMLHttpRequest standard that is being developed by Web API and Web App Formats (and as implemented in Firefox betas)? From Apple's point of view we would like to have a unified standard in this area.>>

We believe that the XDR proposal exposes a smaller surface of attack than the Cross-Site extensions for XHR.  Specifically, it can be demonstrated that the capabilities exposed by XDR are virtually identical to the capabilities exposed by existing HTML tags.  The one exception (obviously) is that the XDR object allows examination of response bodies cross-domain if and only if the server explicitly indicates that such access is permissible via the XDomainRequestAllowed header.

=====

Maciej Stachowiak [mjs@apple.com] asked, in part:
<<I am also not sure if a DNS rebound cross-domain XHR with POST or some other method can do anything that you can't do with a cross-domain form submission. You can set custom headers, but that seems unlikely to make the difference between safe and unsafe.>>

It's certainly a possibility.  For instance, consider a device which accepts SOAP XML as input  The designers of the device were wise to note that a cross-domain form submission could be made (encType = text/plain) that contains XML-formatted content, and thus they devised an anti-CSRF mechanism of rejecting requests that do not bear a proper SOAPAction header.  Such restriction properly blocks CSRF via HTML forms, but is put at risk if a cross-domain XHR request is able to send arbitrary headers.


Thanks again,

Eric Lawrence
Program Manager - IE Security

Sunava Dutta wrote:
> Purpose
>
> XDR helps web developers to create secure mashups, replacing less secure
> or non-performant approaches, including SCRIPT SRC'ing content or IFRAME
> injection.
>
>
>
> Microsoft would like to submit XDR to the W3C for standardization so
> that other browsers can benefit from this technology.
>
>
>
>
>
> XDomainRequest (XDR)
>
>
>     Table of Contents
>
> 1.0   Summary
>
> 2.0   Background: /Overview of how XDR allows cross site requests/
>
> 3.0   API Documentation: /Lists the programming
> interface/methods/properties/
>
> 4.0   Security Model Flowchart: /Highlights the security checks that IE8
> makes for an XDR Request./
>
> 5.0   Sample Site and Script: /For developers wishing to create an XDR
> page./
>
> 6.0   Developer Benefits of using XDR: /Covers XDR's strengths by
> demonstrating XDR's goals of security and simplicity./
>
> 7.0   Developer Release Notes: /A short bulleted list of issues
> developers should we aware of when using the object and a summary of
> what XDR cannot do./
>
> 1.0 Summary
>
> /With* Cross Domain Request* *(XDR)* developers can create cross site
> data aggregation scenarios. Similar to the XMLHttpRequest object  but
> with a simpler programming model, this request, called XDomainRequest,
> is an easy way to make anonymous requests to third party sites that
> support XDR and opt in to making their data available across domains.
> Three lines of code will have you making basic cross site requests. This
> will ensure data aggregation for public sites such as blogs etc will be
> simple, secure and fast. XDR is an approach designed from the grounds up
> with a focus on security. We understand the current cross domain
> XMLHTTPRequest proposal and recognize its ability to provide a broader
> set of services particularly around declarative auditing for access
> control based scenarios and authenticated connections. It does however
> come at the risk of more complexity and surface area of attack. While
> these are certainly compelling scenarios we realize that existing
> implementations have bugs (linked 1
> <http://www.adobe.com/devnet/flashplayer/articles/fplayer9_security.html>,
> 2 <https://bugzilla.mozilla.org/show_bug.cgi?id=389508>), some of which
> are resolved from the past like TOUCTOU and others like DNS Rebinding
> remain mostly unaddressed. In addition, maintaining configuration is
> challenging post deployment as Flash has encountered
> <http://blog.monstuff.com/archives/000302.html> (wildcarding) in the
> past. The IE team is not comfortable implementing a feature with a high
> surface area of attack and open/incoming security issues and proposes
> XDR as a safer alternative.///
>
>
>
> 2.0 Background
>
>
>
>
>
> Browsers enforce the same site origin policy, which blocks web pages
> from accessing data from another domain. Websites often work around this
> policy by having their server request content from another site's server
> in the backend, thus circumventing the check within the browser.
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>       Text Box: Figure 1 - IE7 and below need to make a request to the mashup
> server which then needs to be proxied to the web server.
>
>
>
>
> In IE8 web pages can simply make a cross domain data request within the
> browser using the new /XDomainRequest/ object instead of a
> server-to-server requests.
>
> Cross domain requests require mutual consent between the webpage and
> server. You can initiate a cross domain request in your webpage by
> creating a /xdomainrequest /object off the window object and opening a
> connection to a particular domain. The browser will request data from
> the domain's server by sending a /XDomainRequest: 1 /header. It will
> only complete the connection if the server responds with a
> XDomainRequestAllowed header with the value "1" for true.
>
>
>
> For example, a server's asp page includes the following response header:
>
> Response.AppendHeader("XDomainRequestAllowed","1");
>
>
>
>
>
>
>
> *Security note: *Cross domain requests are anonymous to protect user
> data, which means that servers cannot easily find out who is requesting
> data. As a result, you only want to request and respond with cross
> domain data that is not sensitive or personally identifiable.
>
>
>
>
>
>
> 3.0 API Documentation
>
>
>
> * *
>
> *Methods*
>
> Once you create a xdomainrequest object, you can use the /open()/ method
> to open a connection with a domain's server. This method supports the
> GET and POST HTTP methods and takes the URL to connect to as a
> parameter. Once you've opened a connection, you can use the /send()/
> method to send a data string to the server for processing if needed. For
> example:
>
>
>
> // 1. Create XDR object
>
> xdr = new XDomainRequest();
>
>
>
> //2. Open connection with server using POST method
>
> xdr.open("POST", "http://www.contoso.com/xdr.txt")
>
>
>
> //3. Send string data to server
>
> xdr.send("data to be processed")
>
>
>
> XDR also has an /abort() /method to cancel an active request, which
> takes no parameters. Data is not available on an abort.
>
> * *
>
> *Properties*
>
> *         *responseText - *After the server responds, you can retrieve
> the data string through the read-only /responseText /property.
>
> *         *timeout - *You can use the /timeout /property to set or
> retrieve the number of milliseconds the browser should wait for a server
> to respond.   IE defaults to no timeout if this property is not
> explicitly set. If the request times out, data is not available.
>
> *         *contentType *- If you are posting data to the server, use the
> /contentType /property to define the content type string that will be
> sent to the server. If you are using a GET then this property will allow
> you to read the content type.
>
>
>
> *Events*
>
> XDR has the following events:
>
> *         *onerror* - this event fires when there is an error and the
> request cannot be completed. For example, the network is not available
>
> *         *ontimeout *- this event fires when the request reaches its
> timeout as defined by the above timeOut property. If the request times
> out data is not available.
>
> *         *onprogress -* this event fires while the server responds to
> the request by streaming data back to the browser.
>
> *         *onload *- this event fires when the cross domain request is
> complete and data is available.
>
>
>
> *Security note: *Cross domain requests can only be sent and received
> from a web page to URLs in the following IE zones. We discourage
> Intranet sites from making XDR data available to help prevent intranet
> data from leaking to malicious Internet sites.
>
>
>
>
>
>
>
>
>
>
>
> *Webpage equests data from a URL in the following zone:*
>
>
>
>
>
>
>
>
>
> Local
>
>
>
> Intranet
>
>
>
> Trusted (Intranet)
>
>
>
> Trusted (Internet)
>
>
>
> Internet
>
>
>
> Restricted
>
> *Webpage is in the following zone:*
>
>
>
> Local
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Block
>
> Intranet
>
>
>
> Block
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Block
>
> Trusted (Intranet)
>
>
>
> Block
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Block
>
> Trusted (Internet)
>
>
>
> Block
>
>
>
> Block
>
>
>
> Block
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Block
>
> Internet
>
>
>
> Block
>
>
>
> Block
>
>
>
> Block
>
>
>
> Allow
>
>
>
> Allow
>
>
>
> Block
>
> Restricted
>
>
>
> Block
>
>
>
> Block
>
>
>
> Block
>
>
>
> Block
>
>
>
> Block
>
>
>
> Block
>
> * *
>
> *Security note: *When using these XDR, safely handling data provided by
> another web application is a critical operation.
>
>
>
> For instance, the response could be parsed directly by Javascript, or it
> could be evaluated with a freely available JSON parser (see
> http://www.json.org/) or it could be inserted into a DOM as static text
> (using .innerText).
>
> * *
>
> * *
>
> * *
>
> *Server Side*
>
> The browser will request data from the domain's server by sending a
> /XDomainRequest: 1 /header. It will only complete the connection if the
> server responds with an XDomainRequestAllowed header with the value "1"
> for true.
>
> For example, a server's asp page includes the following response header:
>
> *Response.AppendHeader("XDomainRequestAllowed","1");*
>
> This can be done in IIS, for example, using an ASP.NET page. The line of
> code below can be embedded in your ASP page to return the header.
>
>
>
> *<<% Response.AddHeader  "XDomainRequestAllowed","1" %>Data*
>
> * *
>
> * *
>
> 4.0 Security Model Flowchart
>
> XDR Flowchart
>
> 5.0 Sample Site and Script
>
>
>
> Please refer to the AJAX Hands on Labs
> <http://code.msdn.microsoft.com/iemix08labs/Release/ProjectReleases.aspx?ReleaseId=590>
> on MSDN for demo script. This will need to be set up on your machine
> from the resource files.
>
>
>
> 6.0 Other Developer Benefits of Using XDR
>
> 1.        Simple development model.
>
> a.        On the server, the server operator must simply add one new
> header to his HTTP response indicating that cross-domain sources may
> receive the data.  HTTP Headers can be added by any CGI-style process
> (PHP/ASPNET/etc) or by the web server software (Apache/IIS/etc) itself.
>
> b.        On the client, the XDR object is all about
> cross-domain-requests.  Because XDR is a new object we are not forced to
> "bolt on" cross-domain security.  For example, XDR has no means of
> adding a custom header, because custom headers are dangerous for
> cross-domain security as the current web model does not expect a custom
> header being sent across domains. We've encountered experiences when web
> applications in the past if encountering a custom header using XHR
> assume it's coming from the same site.
>
>
>
> 2.        Provably secure
>
> a.        The XDR security model is simple.  The client sends a request
> that clearly identifies its cross-domain nature, and the server must
> respond in kind for the Same-Origin-Policy to be relaxed such that the
> client can read the response.  If the server does not set the response
> header (a "non-participating" server), the client script is not
> permitted to read the response or determine anything about the target
> server.
>
>
>
> b.        XDR is very tightly scoped to minimize the risk of increasing
> security exposure of the browser.
>
> 1.        Specifically, any request sent by XDR could also be emitted by
> a properly coded HTML FORM object.  Hence, any "non-participating" web
> server put at risk by XDR is also at risk from simple HTML.
>
>
>
> Note: The only additional exposure XDR adds is the ability of the client
> to set a specific Content-Type header.
>
>
>
> 2.        As XDR strips all credentials and cookies, it prevents even
> less attack surface for use in a Cross-Site-Request-Forgery (CSRF)
> <http://en.wikipedia.org/wiki/Cross-site_request_forgery> attack than a
> HTML Form.
>
>
>
> c.        XDR attempts to block cross-zone/protocol requests, an ASR
> which exceeds that undertaken elsewhere in the browser (e.g. SCRIPT SRC)
> due to compatibility concerns.
>
>
>
> 3.        Improved Access  Control "Locality"
>
> a.        Unlike policy file-based security, the XDR handshake is a part
> of the HTTP request and response.  This means that XDR is not at risk
> from DNS-Rebinding <http://en.wikipedia.org/wiki/DNS_rebinding> or
> Time-of-Check-Time-of-Use
> <http://en.wikipedia.org/wiki/Time-of-check-to-time-of-use> attacks.
>
> b.        Policy files must be located in a particular location on the
> server, which may cause operational problems for users with limited
> permissions on the server.  For example, consider the shared hosting
> case, where only one admin may write to the server root, but many users
> have permissions to write to sub-folders.  The users must petition the
> admin for an update to the policy file.
>
>
>
> 4.        Access-Control Flexibility
>
> a.        As Access-Control is based on a per-response basis, the server
> may choose to allow or deny access based upon any criteria desired.  For
> instance, Referer of client, time of day, number of requests per hour,
> etc, etc.
>
> b.        The XDR security model prevents attackers from easily
> determining the access control rules of the server.  The server may keep
> their rules as a trade secret.
>
>
>
> 7.0 Developer Release Notes
>
> *         Not yet available across browsers; not a W3C standard.
>
> *         Services must be explicitly coded to operate with XDR.
>
> *         As HTTP Methods are deliberately limited, standard REST-based
> interop is not possible.
>
> *         As credentials are not provided by the browser, the client
> must transmit them in the request body.  This typically should not be a
> problem but this could prevent use of the HttpOnly attribute on cookies
> that must be sent for credentials.
>
> *         The XDR handshake is HTTP-specific and cannot be directly
> translated for reuse in other protocols or situations (E.g. raw socket
> access).
>
>
>
>
>
>
>
> --
> *Sunava D*utta
> Program Manager (AJAX) - Developer Experience Team, Internet Explorer
>
> One Microsoft Way, Redmond WA 98052
> TEL# (425) 705-1418
>
> FAX# (425) 936-7329
>
>
>
Received on Saturday, 15 March 2008 19:08:41 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Saturday, 15 March 2008 19:08:42 GMT