ASP.NET request validation?

I am using WIF 3.51 with a ASP.NET 3.51 web app and getting the following error.  How do I globally disable request validation or set a custom request validator in ASP.NET 3.51?

[HttpException (0x80004005): A potentially dangerous Request.Path value was detected from the client (:).]
   System.Web.HttpRequest.ValidateInputIfRequiredByConfig() +713
   System.Web.PipelineStepManager.ValidateHelper(HttpContext context) +184