Securing Cross Site XMLHttpRequest

June 23, 2008 – 5:26 PM

As I mentioned in my post on Cross Document Messaging, client side cross domain request is an important area of interest for AJAX developers looking for ways to avoid expensive server side proxying calls. While Cross Document Messaging is useful for allowing third party components or gadgets embedded in a page to communicate/converse using script on both sides, other cross domain scenarios like web services require access to cross domain content using network requests from a client side web application. For example, you may want to use your client side map based mashup to pinpoint Chinese restaurants for your current neighborhood. This could require the mashup to request a text file from Zagat.com with the locations of Zagat rated restaurants in the area which can then be superimposed on the map.

Along those lines, a few proposals and implementations exist like XDomainRequest in IE8, JSONRequest and the W3C’s Web Applications Working Group’s Cross Site XMLHttpRequest (CS-XHR) draft specification, which combines an Access control framework with XMLHttpRequest or other features. While XDomainRequest is focused on enabling anonymous access of third party public data, Cross Site XMLHttpRequest has added functionality and consequently enables a broader set of scenarios that may appeal to the developer who may choose to use cross domain authentication and access control among other features.  As can be expected with securing a large cross section of cross domain scenarios, a number of concerns have been identified with the CS-XHR draft by the web development community, the IE team members and members of the Web Apps Working Group. For a list of our recent feedback on security on CS-XHR and our take on important security principles in cross domain, please read our Security Whitepaper on Cross Domain. The paper also covers best practices and guidance for developers who will choose to build on the current draft if it’s supported by a future browser. Note that issues here are currently being discussed and some concerns may be mitigated as the draft evolves.

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