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Lack of data validation - Source Code

Need​

Implementation of robust data validation mechanisms in the source code

Context​

  • Usage of C# for building robust and scalable applications
  • Usage of Microsoft.AspNetCore.Mvc for building web applications with ASP.NET Core MVC
  • Usage of System.Text.RegularExpressions for pattern matching and regular expression operations

Description​

Non compliant code​

public class UserController : Controller
{
    [HttpPost]
    public IActionResult Login(string username, string password)
    {
        string pattern = @"^([a-z0-9_\\.-]+)@([\\da-z\\.-]+)\\.([a-z\\.]{2,6})$";
        Regex regex = new Regex(pattern);
        if (regex.IsMatch(username))
        {
            // Proceed with login
        }
        else
        {
            return BadRequest("Invalid username");
        }
    }
}

In this code snippet, we have a Login method in the UserController class that accepts a username and password as parameters. The method uses a regular expression to validate the username input.

The regular expression pattern @"^([a-z0-9_\\.-]+)@([\\da-z\\.-]+)\\.([a-z\\.]{2,6})$" is used to validate the username, which is expected to be an email address. This pattern is complex and may cause the server to crash when evaluating certain strings.

The Regex.IsMatch(username) method is used to check if the username matches the regular expression pattern. If the username is a match, the method proceeds with the login process. If not, it returns a BadRequest response with an "Invalid username" message.

This code is vulnerable to Regular Expression Denial of Service (ReDoS) attacks. An attacker can send a specially crafted string as the username that will cause the regular expression engine to consume excessive resources, potentially causing the server to crash or become unresponsive. This is because the regular expression used is complex and can take a long time to evaluate certain strings.

Steps​

  • Replace the use of dangerous regular expressions with a more secure and efficient method of data validation.
  • Consider using built-in validation attributes provided by ASP.NET Core, such as [EmailAddress] for validating email addresses.
  • If custom validation is required, use simpler and more specific regular expressions that are less prone to causing crashes or vulnerabilities.
  • Ensure that input data is properly sanitized and validated before using it in any operations or queries.
  • Implement input validation at multiple layers of the application, including client-side validation, server-side validation, and database validation.
  • Regularly update and patch the application and its dependencies to ensure that any known vulnerabilities are addressed.
  • Perform security testing and code reviews to identify and fix any other potential vulnerabilities in the application.

Compliant code​

public class UserController : Controller
{
    [HttpPost]
    public IActionResult Login([EmailAddress]string username, string password)
    {
        if (ModelState.IsValid)
        {
            // Proceed with login
        }
        else
        {
            return BadRequest("Invalid username");
        }
    }
}

The updated code now uses the built-in [EmailAddress] attribute provided by ASP.NET Core to validate the username parameter. This attribute checks if the input is in a valid email format, eliminating the need for a potentially dangerous regular expression.

The ModelState.IsValid property is used to check if the input data satisfies all validation rules. If the username is not a valid email address, ModelState.IsValid will be false, and the method will return a BadRequest response with an error message.

This approach provides a more secure and efficient method of data validation, as it leverages built-in features of the framework and does not rely on potentially dangerous regular expressions. It also simplifies the code and makes it easier to understand and maintain.

Remember to always sanitize and validate input data at multiple layers of the application, including client-side validation, server-side validation, and database validation. Regularly update and patch the application and its dependencies to address any known vulnerabilities, and perform security testing and code reviews to identify and fix any other potential vulnerabilities.

References​

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