Using a model class that has properties that are required (as marked with the [Required] attribute) and properties that are optional (as not marked with the [Required] attribute) can lead to problems if an attacker communicates a request that contains more data than is expected.

The ASP.NET MVC framework will try to bind request parameters to model properties.

Having mixed requiredness without explicitly communicating which parameters are to be model-bound may indicate that there are model properties for internal use but can be controlled by attacker.

The following code defines a possible model class that has properties that have [Required] and properties that do not have [Required]:

public class MyModel
{
    [Required]
    public String UserName { get; set; }

    [Required]
    public String Password { get; set; }

    public Boolean IsAdmin { get; set; }
}

If any optional parameters can change the behavior of an application, then an attacker may be able to actually change that behavior by communicating an optional parameter in a request.

Using a model class that has non-nullable properties that are required (as marked with the [Required] attribute) can lead to problems if an attacker communicates a request that contains less data than is expected.

The ASP.NET MVC framework will try to bind request parameters to model properties.

If a model has a required non-nullable parameter and an attacker does not communicate that required parameter in a request -- that is, the attacker uses an under-posting attack -- then the property will have the default value (usually zero) which will satisfy the [Required] validation attribute. This may produce unexpected application behavior.

The following code defines a possible model class that has a required enum, which is non-nullable:

public enum ArgumentOptions
{
    OptionA = 1,
    OptionB = 2
}

public class Model
{
    [Required]
    public String Argument { get; set; }

    [Required]
    public ArgumentOptions Rounding { get; set; }
}

If a model class has required property and is the type of an optional member of a parent model class, it may be susceptible to under-posting attacks if an attacker communicates a request that contains less data than is expected.

The ASP.NET MVC framework will try to bind request parameters to model properties, including submodels.

If a submodel is optional -- that is, the parent model has a property without the [Required] attribute -- and if an attacker does not communicate that submodel, then the parent property will have a null value and the required fields of the child model will not be asserted by model validation. This is one form of an under-posting attack.

Consider the following the model class definitions:

public class ChildModel
{
    public ChildModel()
    {
    }

    [Required]
    public String RequiredProperty { get; set; }
}

public class ParentModel
{
    public ParentModel()
    {
    }

    public ChildModel Child { get; set; }
}

If an attacker does not communicate a value for the ParentModel.Child property, then the ChildModel.RequiredProperty property will have a [Required] which is not asserted. This may produce unexpected and undesirable results.