Since the response of a WS-Federation sign-in request contains XML, the ASP.NET built-in request validation will trigger an exception. To solve this, request validation needs to be turned off for pages receiving such a response message.

Starting with ASP.NET 4.0 you can plug in your own request validation logic. This allows letting WS-Federation messages through, while applying all standard request validation to all other requests. The WIF SDK (v4) contains a sample validator that does exactly that:

public class WSFedRequestValidator : RequestValidator
{

    protected override bool IsValidRequestString(
      HttpContext context,
      string value,
      RequestValidationSource requestValidationSource,
      string collectionKey,
      out int validationFailureIndex)
    {
        validationFailureIndex = 0;

        if ( requestValidationSource == RequestValidationSource.Form &&
             collectionKey.Equals(
               WSFederationConstants.Parameters.Result,
               StringComparison.Ordinal ) )
        {
            SignInResponseMessage message =
              WSFederationMessage.CreateFromFormPost(context.Request)
               as SignInResponseMessage;

            if (message != null)
            {
                return true;
            }
        }

        return base.IsValidRequestString(
          context,
          value,
          requestValidationSource,
          collectionKey,
          out validationFailureIndex );
    }
}

Register this validator via web.config:

<httpRuntime requestValidationType="WSFedRequestValidator" />