The .NET 4.5 framework was released a couple of months ago and it included several improvements in the security area. To benefit from these improvements you need to do a few changes to you application's configuration file.
There are some important improvement in ASP.NET 4.5:

  • There are changes to the ASP.NET request validation, it now supports deferred (lazy) validation, as well as giving the option to fetch data unvalidated.
  • The AntiXSS library is included in the framework.
  • There are significant Cryptographic Improvements in ASP.NET 4.5.
  • Windows Identity Foundation is now included in the framework, referred to as WIF 4.5.

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To take advantage of these new bits you'll have to do a bit of configuration, we'll get into that right away:

Switching to 4.5
While retargeting a couple of MVC applications to the new framework version, I learned that it's not enough to install the 4.5 framework and change the "Target framework" accordingly. You'll find that a comment appears in the web.config file:
<!--
    For a description of web.config changes for .NET 4.5 see http://go.microsoft.com/fwlink/?LinkId=235367.       
    The following attributes can be set on the <httpRuntime> tag.      
     <system.Web>
        <httpRuntime targetFramework="4.5" />
     </system.Web>

-->
It's important that you set the targetFramework in your configuration file, else your application will run in "4.0" mode.

Enabling AntiXss
You'd want to set the AntiXss library as the default encoder — that can easily be done in the httpRuntime configuration element:

<httpRuntime targetFramework="4.5" encoderType="System.Web.Security.AntiXss.AntiXssEncoder,System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />

Note that there can be side effects to this, as AntiXSS takes a white list approach to encoding. That means that there may be characters that weren't encoded before, that will be encoded by AntiXSS.

Request validation
Lazy validation was introduced in ASP.NET 4.5, I just did some testing on it and it seems that lazy validation is the enabled regardless of how you set the "requestValidationMode", after you've installed the 4.5 framework. However, if you need access to any request parameters unvalidated, you'll need to set the validation mode to "4.5", as such:
<httpRuntime targetFramework="4.5" requestValidationMode="4.5" />
This will give you access to the unvalidated collections of parameters, e.g.:

Request.Unvalidated.QueryString["lastName"];
This is a much better approach than disabling request validation altogether. But use it with care, as always you should throroughly validate the input.

WIF 4.5
WIF is now part of the framework — that meant some breaking changes. It shouldn't take to much time to upgrade though,  particularly if you're concerned with RP's. There's a great article on MSDN with Guidelines for Migrating an Application Built Using WIF 3.5 to WIF 4.5.
There's two apparent changes I'd like to point out. First, you no longer need to set the "requestValidationMode" to "2.0" to cope with the request validation exceptions on the SignInResponseMessage's posted from an STS. WIF 4.5 plays nicely with the 4.5 request validation. Second, WIF now includes a MachineKeySessionSecurityTokenHandler which encrypts and MAC's WIF cookies based on the machine key. You'll find everything you need to set it up in: WIF and Web Farms.