What are the benefits and drawbacks of using IIS CORS module vs custom middleware for CORS in web API?

1 IIS CORS module

The IIS CORS module is an extension that you can install on your IIS server to enable and manage CORS settings for web applications. This module allows you to define rules and policies for CORS requests, such as which origins are allowed, what methods and headers are supported, and how long the responses can be cached. The module also handles preflight requests, which are special requests that browsers send to check the CORS compatibility before making the actual request. The advantages of using the IIS CORS module include its easy installation and configuration, as well as its reliability and security. However, it may not support advanced or custom scenarios, such as dynamic origin validation, custom response headers, or error handling. Additionally, it may introduce some performance overhead and may not be compatible with older versions of IIS or web frameworks.

2 Custom middleware

Custom middleware is a piece of code that can be inserted into the request pipeline of a web API to handle CORS requests. It gives you more flexibility and control, as you can implement any logic or behavior that you need for your web API. Additionally, it allows you to tailor CORS settings for each web API or endpoint, and adjust them dynamically based on the request context or parameters. Performance can be improved as well, as you can optimize your middleware code and avoid unnecessary processing or caching. However, custom middleware requires more coding and testing, may introduce complexity and inconsistency, and may increase the risk of errors or vulnerabilities. Therefore, you need to ensure that your middleware code is compatible with your web API framework and follows the CORS specification while handling all the possible scenarios and exceptions for CORS requests.

3 How to choose

When it comes to implementing CORS in web API hosted on IIS, there is no one-size-fits-all answer. The approach you choose depends on your specific requirements, preferences, and constraints. Factors to consider include the size and scope of your web API and the number of CORS requests you expect to handle, the level of customization and flexibility needed for your CORS settings and behavior, the availability and compatibility of the IIS CORS module or custom middleware with your IIS version and web API framework, and the trade-off between performance and security you are willing to accept. Combining both approaches is also an option - such as using the IIS CORS module for general rules and policies, while using custom middleware for specific or complex scenarios - but it is essential to test and monitor your CORS implementation to ensure it meets your web API goals and standards.

4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?