What are some common security vulnerabilities that you should be aware of when building a .NET Core Web API, and how do you prevent them?

Some common security vulnerabilities that you should be aware of when building a .NET Core Web API include:

1. Injection attacks: These are attacks where malicious code is injected into your application via input fields such as forms, query strings, and HTTP headers. To prevent this, you should always validate and sanitize user input, and use parameterized queries instead of concatenating strings to build SQL queries.
2. Cross-Site Scripting (XSS) attacks: These are attacks where an attacker injects malicious scripts into a web page, which can then be executed by unsuspecting users. To prevent this, you should always encode user input, sanitize output, and enable Content Security Policy (CSP) to restrict the types of content that can be loaded on your page.
3. Cross-Site Request Forgery (CSRF) attacks: These are attacks where an attacker tricks a user into executing an unwanted action on a website. To prevent this, you should always use anti-forgery tokens and validate the origin of each request.
4. Broken authentication and session management: These are vulnerabilities that occur when authentication and session management mechanisms are not implemented correctly. To prevent this, you should always use secure authentication protocols such as OAuth or OpenID Connect, enforce strong password policies, and ensure that sessions are properly managed and timed out.
5. Insufficient logging and monitoring: These are vulnerabilities that occur when logs are not properly configured or monitored, which can allow attackers to go undetected. To prevent this, you should always enable logging and monitoring, and use tools such as Azure Application Insights to track performance, usage, and security issues.

To prevent these security vulnerabilities and ensure the safety and security of your .NET Core Web API, it's important to follow best practices such as secure coding practices, continuous security testing, and regular security audits. You should also keep your dependencies up-to-date, use security-focused frameworks and libraries, and stay up-to-date with the latest security news and trends.