S6964 — Value type property used as input in a controller action should be nullable, required or annotated with the JsonRequiredAttribute to avoid under-posting.

Language
C#
Type
Code smell
Severity
Major
Tags
asp.net

"Under-posting" refers to a situation where a client sends less data than expected to the server during an HTTP request, for example when the client omits some properties from the request body that the server expects to receive.

Why is this an issue?

One of the main issues that under-posting can cause is data inconsistency. If the client sends less data than expected, the application might fill any value type properties with their default values, leading to inaccurate or inconsistent data in your database. Additionally, there might be unexpected behavior if there are certain data expected that are not provided and even security issues; for example, if a user omits a role or permission field from a POST request, and the server fills in a default value, it could inadvertently grant more access than intended.

A model class (in this case the Product class) can be an input of an HTTP handler method:


public class ProductsController : Controller
{
    [HttpPost]
    public IActionResult Create([FromBody]Product product)
    {
        // Process product data...
    }
}

Exceptions

This rule does not raise an issue when properties are decorated with the following attributes:

Additionally, this rule does not raise for properties in model classes that are not in the same project as the Controller class that references them. This is due to a limitation of Roslyn (see here).

How to fix it

You should mark any model value-type property as nullable, required or JsonRequired. Thus when a client underposts, you ensure that the missing properties can be detected on the server side rather than being auto-filled, and therefore, incoming data meets the application’s expectations.

Code examples

Noncompliant code example


public class Product
{
    public int Id { get; set; }             // Noncompliant
    public string Name { get; set; }
    public int NumberOfItems { get; set; }  // Noncompliant
    public decimal Price { get; set; }      // Noncompliant
}

If the client sends a request without setting the NumberOfItems or Price properties, they will default to 0. In the request handler method, there’s no way to determine whether they were intentionally set to 0 or omitted by mistake.

Compliant solution


public class Product
{
    public required int Id { get; set; }
    public string Name { get; set; }
    public int? NumberOfItems { get; set; }            // Compliant - property is optional
    [JsonRequired] public decimal Price { get; set; }  // Compliant - property must have a value
}

In this example, the request handler method can


public class ProductsController : Controller
{
    [HttpPost]
    public IActionResult Create(Product product)
    {
        if (!ModelState.IsValid)    // if product.Price is missing then the model state will not be valid
        {
            return View(product);
        }

        if (product.NumberOfItems.HasValue)
        {
            // ...
        }
        // Process input...
    }
}

Recommended Secure Coding Practices

Resources

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