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
