Microsoft’s ODATA plugin to the amazing Web API lets you expose a ODATA endpoint basically by decorating the enumerable getter in your REST interface with a [Queryable] attribute and returning IQueryable from the method. The ODATA API takes care of converting e.g.  $select and $filter ODATA parameters to LINQ expressions. Very cool stuff indeed.

Except that the method returns an array of the elements. And that opens up the JSON hijacking exploit that could leak data from the service. Basically the exploit leverages the dynamic nature of JavaScript to rewrite the defineSetter prototype and then fetch JSON data from the service using a script element on the page. The loaded JSON is run as a script, and the data is then available to the attacker.

There are several ways to avoid this. Exposing the endpoint only with POST or prepending while(1); to the result stream. Those would require changes to the client code. Or the server could verify that the incoming request did actually come from a XHR and not from a malicious script element.

A friendly XHR always requests data with the header Accept: application/json, …  Whereas a script element on the page will be requested with Accept: / (Chrome/FF) or Accept: application/javascript, … (IE)

With this information it is quite easy to extend the QueryableAttribute to perform this simple validation:

using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Web.Http;
using System.Web.Http.Filters;
public class SafeQueryableAttribute : QueryableAttribute
    private static readonly MediaTypeWithQualityHeaderValue ApplicationJson =         MediaTypeWithQualityHeaderValue.Parse("application/json");
        public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
        var res = actionExecutedContext.Response;
        if (res.StatusCode < HttpStatusCode.BadRequest
            && !actionExecutedContext.Request.Headers.Accept.Contains(ApplicationJson))
            actionExecutedContext.Response = new HttpResponseMessage(HttpStatusCode.Forbidden);