Need

Implementation of input length validation

Context

• Usage of C# for building robust and scalable applications
• Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core
• Usage of System for managing and interacting with the operating system

Description

Non compliant code

        using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace VulnerableApp.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class DataController : ControllerBase
    {
        [HttpPost]
        public IActionResult Post([FromBody] string value)
        {
            // Do something with the value
            return Ok();
        }
    }
}
        
        

The above code represents a simple API endpoint in an ASP.NET Core application. It is vulnerable due to the lack of input length validation.

The Post method in the DataController class accepts a string value from the body of the request. However, there's no validation to check the length of the incoming string. This means that an attacker could potentially send a massive amount of characters as value, which could lead to several issues such as:

- Buffer overflow: If the application tries to store the input in a buffer that's not large enough to hold it, it could result in a buffer overflow. This could corrupt data, crash the program, or even allow the execution of malicious code.
- Denial of Service (DoS): If the application tries to process a very large input, it could consume excessive resources (CPU, memory), leading to a slowdown or even a complete denial of service.
- Injection attacks: If the input is used in a context where its length isn't properly handled, it could lead to injection attacks. For instance, if the input is inserted into a database query without proper sanitization, an excessively long input could alter the query's structure and lead to SQL injection.

Therefore, to prevent these potential vulnerabilities, it's crucial to add validation to check the length of the incoming string and reject inputs that are excessively long.

Steps

• Implement input validation to check the length of the input data.
• Define a maximum allowed length for the input data.
• Validate the length of the input data before processing it.
• Return an appropriate error response if the input data exceeds the maximum allowed length.

Compliant code

        using Microsoft.AspNetCore.Mvc;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace VulnerableApp.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class DataController : ControllerBase
    {
        private const int MaxInputLength = 5000; // Define a maximum allowed length for the input data

        [HttpPost]
        public IActionResult Post([FromBody] string value)
        {
            // Validate the length of the input data before processing it
            if (value.Length > MaxInputLength)
            {
                return BadRequest("Input data exceeds maximum allowed length");
            }

            // Do something with the value
            return Ok();
        }
    }
}
        
        

The updated code now includes a constant MaxInputLength that defines the maximum allowed length for the input data. This value can be adjusted according to the specific requirements of your application.

In the Post method, before processing the input data, we now check if the length of the input data exceeds the maximum allowed length. If it does, we immediately return a BadRequest response with an appropriate error message. This prevents the application from processing overly long input data, thus mitigating the risk of potential attacks that exploit this vulnerability.

This is a simple yet effective way to implement input validation in your ASP.NET Core application. It helps ensure that your application only processes valid and appropriately-sized data, thereby enhancing its security and reliability.

References

• 194. Lack of data validation - Input Length