In today’s interconnected world, APIs are the backbone of modern apps. Protecting these APIs and ensuring only authorised users access sensitive data is now more crucial than ever. While many authentication and authorisation methods exist, OAuth2 Introspection stands out as a robust and flexible approach. In this post, we will explore what OAuth2 Introspection is, why we should use it, and how to implement it in our .NET apps.
Before we dive into the technical details, let’s remind ourselves why API security is so important. Think about it: APIs often handle the most sensitive stuff. If those APIs are not well protected, we are basically opening the door to some nasty consequences. Data breaches? Yep. Regulatory fines (GDPR, HIPAA, you name it)? Potentially. Not to mention, losing the trust of our users. A secure API shows that we value their data and are committed to keeping it safe. And, of course, it helps prevent the bad guys from exploiting vulnerabilities to steal data or cause all sorts of trouble.
The most common method of securing APIs is using access tokens as proof of authorization. These tokens, typically in the form of JWTs (JSON Web Tokens), are passed by the client to the API with each request. The API then needs a way to validate these tokens to verify that they are legitimate and haven’t been tampered with. This is where OAuth2 Introspection comes in.
OAuth2 Introspection
OAuth2 Introspection is a mechanism for validating bearer tokens in an OAuth2 environment. We can think of it as a secure lookup service for our access tokens. It allows an API to query an auth server, which is also the “issuer” of the token, to determine the validity and attributes of a given token.
The workflow of an OAuth2 Introspection request.
To illustrate the process, the diagram above visualises the flow of an OAuth2 Introspection request. The Client sends the bearer token to the Web API, which then forwards it to the auth server via the introspection endpoint. The auth server validates the token and returns a JSON response, which is then processed by the Web API. Finally, the Web API grants (or denies) access to the requested resource based on the token validity.
Introspection vs. Direct JWT Validation
You might be thinking, “Isn’t this just how we normally validate a JWT token?” Well, yes… and no. What is the difference, and why is there a special term “Introspection” for this?
With direct JWT validation, we essentially check the token ourselves, verifying its signature, expiry, and sometimes audience. Introspection takes a different approach because it involves asking the auth server about the token status. This leads to differences in the pros and cons, which we will explore next.
With OAuth2 Introspection, we gain several key advantages. First, it works with various token formats (JWTs, opaque tokens, etc.) and auth server implementations. Furthermore, because the validation logic resides on the auth server, we get consistency and easier management of token revocation and other security policies. Most importantly, OAuth2 Introspection makes token revocation straightforward (e.g., if a user changes their password or a client is compromised). In contrast, revoking a JWT after it has been issued is significantly more complex.
.NET Implementation
Now, let’s see how to implement OAuth2 Introspection in a .NET Web API using the AddOAuth2Introspection authentication scheme.
The core configuration lives in our Program.cs file, where we set up the authentication and authorisation services.
// ... (previous code for building the app)
builder.Services.AddAuthentication("Bearer")
.AddOAuth2Introspection("Bearer", options =>
{
options.IntrospectionEndpoint = "<Auth server base URL>/connect/introspect";
options.ClientId = "<Client ID>";
options.ClientSecret = "<Client Secret>";
options.DiscoveryPolicy = new IdentityModel.Client.DiscoveryPolicy
{
RequireHttps = false,
};
});
builder.Services.AddAuthorization();
// ... (rest of the Program.cs)
This code above configures the authentication service to use the “Bearer” scheme, which is the standard for bearer tokens. AddOAuth2Introspection(…) is where the magic happens because it adds the OAuth2 Introspection authentication handler by pointing to IntrospectionEndpoint, the URL our API will use to send the token for validation.
Usually, RequireHttps needs to be true in production. However , in situations like when the API and the auth server are both deployed to the same Elastic Container Service (ECS) cluster and they communicate internally within the AWS network, we can set it to false. This is because the Application Load Balancer (ALB) handles the TLS/SSL termination and the internal communication between services happens over HTTP, we can safely disable RequireHttps in the DiscoveryPolicy for the introspection endpoint within the ECS cluster. This simplifies the setup without compromising security, as the communication from the outside world to our ALB is already secured by HTTPS.
Finally, to secure our API endpoints and require authentication, we can simply use the [Authorize] attribute, as demonstrated below.
[ApiController]
[Route("[controller]")]
[Authorize]
public class MyController : ControllerBase
{
[HttpGet("GetData")]
public IActionResult GetData()
{
...
}
}
Wrap-Up
OAuth2 Introspection is a powerful and flexible approach for securing our APIs, providing a centralised way to validate bearer tokens and manage access. By understanding the process, implementing it correctly, and following best practices, we can significantly improve the security posture of your applications and protect your valuable data.
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