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

Re: IE Team's Proposal for Cross Site Requests

From: Maciej Stachowiak <mjs@apple.com>
Date: Fri, 14 Mar 2008 15:03:35 -0700
Cc: Chris Wilson <Chris.Wilson@microsoft.com>, "Web API WG (public)" <public-webapi@w3.org>, Eric Lawrence <ericlaw@exchange.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: <D0D00CE8-40E7-4632-B807-54551D926364@apple.com>
To: Jonas Sicking <jonas@sicking.cc>

On Mar 14, 2008, at 2:42 PM, Jonas Sicking wrote:

> 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.
> I would be very interested to hear back on the two first emails I  
> posted to this thread as they relate to this exact subject.

The attack scenario would be like this:
- evil.com binds rebind-domain.com to point to evil.com's IP
- evil.com does a cross-domain XHR to rebind-domain.com with POST or a  
custom method, making sure to allow
- evil.com rebinds rebind-domain.com to point to webstore.com's IP
- evil.com does a cross-domain XHR to rebind-domain.com with POST or a  
custom method, it is now allowed

The end result is that you send a request with a disallowed method to  
webstore.com, but the user's normal cookies or other credentials won't  
be sent since the request is sent to rebind-domain.com, not  
webstore.com, as far as the client knows.

However, after thinking about this, you could achieve the same by DNS  
rebinding using evil.com itself, making the XHR not cross-domain at  
all. So, even though there seems to be a potential gap in the OPTIONS  
pre-request here, it does not appear to be a new hole.

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. You can also use  
methods besides the ones allowed for form posting. I am not sure why  
the OPTIONS preflight check was added in the first place, I hope  
whoever came up with the pre-check design can chime in to indicate  
whether this attack subverts the purpose of the check.


> / Jonas
> Jonas Sicking wrote:
>> So the worry here is a scenario where an attacker tricks a user to  
>> go to evil.com which does an evil POST to webstore.com. And at the  
>> same time the attacker launches a DNS rebind attack on the user for  
>> the webstore.com domain name such that the OPTIONS request goes to  
>> an attacker controlled server which approves the POST, but then  
>> lets the actual post go to the real webstore.com server?
>> If so, couldn't the user simply trick the user to go to  
>> webstore.com, and use a DNS rebind attack so that when  
>> webstore.com/ is fetched it returns a HTML page that contains  
>> script that uses normal XHR to do a POST to webstore.com. When the  
>> POST happens the attacker lets that go to the real webstore.com  
>> server.
>> I.e. I don't see how Cross-site XHR in combination with DNS rebind  
>> attacks lets you do something that DNS rebind attacks doesn't  
>> already let you do on it's own.
>> XXX = Cross-site Extensions to XHR. So basically XHR+AC spec.
>> / Jonas
>> Chris Wilson wrote:
>>> Yes, DNS rebinding is one of the major attack vectors I was  
>>> talking about.  If the access controls are negotiated  
>>> independently of the actual request/response, this is nearly  
>>> always a concern.  (Yes, you could require follow-ups to go to the  
>>> same IP address; that's both a pain to actually implement (because  
>>> a high-level request needs low-level access; typically, I don't  
>>> believe we need to know about the IP address at the XHR level) and  
>>> somewhat confusing (because it will break if there's normal,  
>>> permitted DNS round-robin going on, e.g.).
>>> Maciej, does XXX = XHR L2 or XDR?
>>> -----Original Message-----
>>> From: Maciej Stachowiak [mailto:mjs@apple.com]
>>> Sent: Friday, March 14, 2008 1:25 PM
>>> To: Jonas Sicking
>>> Cc: Chris Wilson; Web API WG (public); Eric Lawrence; Zhenbin Xu;  
>>> Gideon Cohn; Sharath Udupa; Doug Stamper; Marc Silbey
>>> Subject: Re: IE Team's Proposal for Cross Site Requests
>>> On Mar 14, 2008, at 11:24 AM, Jonas Sicking wrote:
>>>> Can you describe what you mean by "persistent allow" design?
>>> Anne and I discussed this comment on IRC. One possible flaw is that
>>> the OPTIONS request to guard against an unaware server receiving  
>>> cross-
>>> domain POST or other methods is subject to a DNS rebinding attack
>>> (though this could be fixable by requiring the OPTIONS and the  
>>> follow-
>>> up request to go to the same IP or something along those lines). I'm
>>> not sure if this is the vulnerability Chris had in mind. I don't  
>>> think
>>> XXX has the same vulnerabilities as Flash though, because the  
>>> access-
>>> control headers are not an out-of-band control file so the actual
>>> access control check can't be bypassed via DNS rebinding, only the
>>> method check.
>>>  - Maciej
>>>> / Jonas
>>>> Chris Wilson wrote:
>>>>> Oops.  Obviously, this was not to go to the whole group.
>>>>> I've been asked a lot, over the last week and a half, why we
>>>>> implemented XDR rather than the current cross-domain XHR
>>>>> proposals.  The short version is, as Sunava discusses in the
>>>>> summary of this mail, that x-domain XHR (and Flash's approach, et
>>>>> al) is subject to specific x-domain injection attacks because of
>>>>> its persistent-allow design.
>>>>> *From:* Chris Wilson
>>>>> *Sent:* Friday, March 14, 2008 11:00 AM
>>>>> *To:* Sunava Dutta; Web API WG (public)
>>>>> *Cc:* Eric Lawrence; Zhenbin Xu; Gideon Cohn; Sharath Udupa; Doug
>>>>> Stamper; Marc Silbey
>>>>> *Subject:* RE: IE Team's Proposal for Cross Site Requests
>>>>> I'd move half the summary section up front to make it clear why
>>>>> we're not wild about x-domain XHR.  You need to lead with that.
>>>>> *From:* Sunava Dutta
>>>>> *Sent:* Thursday, March 13, 2008 8:47 PM
>>>>> *To:* Sunava Dutta; Web API WG (public)
>>>>> *Cc:* Eric Lawrence; Chris Wilson; Zhenbin Xu; Gideon Cohn;  
>>>>> Sharath
>>>>> Udupa; Doug Stamper; Marc Silbey
>>>>> *Subject:* IE Team's Proposal for Cross Site Requests
>>>>> 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 Friday, 14 March 2008 22:04:15 UTC

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