Today, I aim to share with you valuable insights on how to secure your ASP.NET Core API applications effectively. The increasing reliance on digital solutions has made securing these systems more important than ever before.

Here are some essential tips and best practices to ensure your ASP.NET Core API applications stand firm against cyber threats:

1. Always Use HTTPS: HTTPS provides a secure connection that is both encrypted and authenticated. It is the bare minimum requirement for any data exchange and can prevent man-in-the-middle attacks. In ASP.NET Core, enforcing HTTPS is as simple as adding the [RequireHttps] attribute to your controllers.

2. Enable CORS cautiously: Cross-Origin Resource Sharing (CORS) is a security feature that can prevent your API from being used by malicious websites. However, if misconfigured, it can give away too many permissions and make your API vulnerable. Always explicitly define which sites are allowed to access your API and restrict it to trusted origins only.

3. Use Authentication and Authorization: Authentication and Authorization are crucial for securing your API. Use authentication to verify who the user is, and authorization to manage what a user can do. ASP.NET Core has several mechanisms you can use such as cookies, JWT (Json Web Tokens), or OAuth 2.0.

4. Keep Your Secrets Secret: Never store sensitive data like database connection strings, API keys, or other secrets in your code or configuration files. ASP.NET Core provides Secret Manager and Azure Key Vault for development and production environments, respectively.

5. Implement Rate Limiting: Rate limiting is a useful technique to prevent DoS (Denial of Service) attacks. It works by limiting the number of requests that a client can make to your API in a given amount of time. Libraries like AspNetCoreRateLimit can help you achieve this.

6. Validate Input and Encode Output: Never trust user input. Always validate and sanitize it to protect your API from attacks like SQL injection and XSS (Cross-Site Scripting). Likewise, always encode your output to prevent unwanted execution of code.

7. Use Updated Software: Ensure you are always using the latest versions of the .NET Core and other software you have in your stack. Updates often come with security patches that protect your API from known vulnerabilities.

8. Log and Monitor Your API: Keep track of your API usage to detect anomalies. If a client is making too many requests or making suspicious requests, it might be a sign of an attack. Services like Application Insights or ELK (Elasticsearch, Logstash, and Kibana) stack can help you monitor your application.

9. Use HTTP Headers for Additional Security: There are several HTTP headers like X-Content-Type-Options, Content-Security-Policy, and X-Frame-Options that can provide an extra layer of security to your API.

10. Regularly Test Your API: Lastly, always remember that security is not a one-time thing but an ongoing process. Regularly test your API using penetration testing or vulnerability scanning tools to ensure it remains secure against evolving threats.

Remember, in a world where cyber threats are continuously evolving, no security measure is too small. By implementing these tips and best practices, you can make your ASP.NET Core API applications significantly more secure and keep your users’ data safer.