Question

Why is Json Request Behavior needed?

If I want to restrict the HttpGet requests to my action I can decorate the action with the [HttpPost] attribute

Example:

[HttpPost]
public JsonResult Foo()
{
    return Json("Secrets");
}

// Instead of:
public JsonResult Foo()
{
    return Json("Secrets", JsonRequestBehavior.AllowGet);
}

Why isn’t [HttpPost]sufficient?
Why the framework “bugs” us with the JsonRequestBehavior.AllowGet for every JsonResult that we have. If I want to deny get requests I’ll add the HttpPost attribute.

Answer 1

MVC defaults to DenyGet to protect you against a very specific attack involving JSON requests to improve the liklihood that the implications of allowing HTTP GET exposure are considered in advance of allowing them to occur.

This is opposed to afterwards when it might be too late.

Note: If your action method does not return sensitive data, then it should be safe to allow the get.

Further reading from my Wrox ASP.NET MVC3 book

By default, the ASP.NET MVC framework does not allow you to respond to
an HTTP GET request with a JSON payload. If you need to send JSON in
response to a GET, you’ll need to explicitly allow the behavior by
using JsonRequestBehavior.AllowGet as the second parameter to the Json
method. However, there is a chance a malicious user can gain access to
the JSON payload through a process known as JSON Hijacking. You do not
want to return sensitive information using JSON in a GET request. For
more details, see Phil’s post at
http://haacked.com/archive/2009/06/24/json-hijacking.aspx/ or this SO post.

Haack, Phil (2011). Professional ASP.NET MVC 3 (Wrox Programmer to
Programmer) (Kindle Locations 6014-6020). Wrox. Kindle Edition.

Related StackOverflow question

With most recents browsers (starting with Firefox 21, Chrome 27, or IE 10), this is no more a vulnerability.