How should JWT-based authentication and refresh-token mechanisms be implemented securely in an ASP.NET Core application?

JWT-based authentication is commonly used for securing APIs and distributed applications, but implementing JWT securely requires understanding more than simply generating a token.

A typical architecture is:

Client
   ↓
Authentication Server
   ↓
Access Token
   ↓
ASP.NET Core API

The access token contains claims that the API can validate.

Access Tokens

An access token should be relatively short-lived.

For example:

Access Token
   ↓
Short lifetime
   ↓
API authorization

If an access token is compromised, a shorter lifetime limits the potential exposure window.

Refresh Tokens

Refresh tokens solve a different problem.

Instead of forcing a user to authenticate repeatedly:

Access Token expires
       ↓
Refresh Token
       ↓
New Access Token

The refresh token can be exchanged for a new access token according to the application's authentication architecture.

Never Treat Refresh Tokens Like Access Tokens

Refresh tokens are highly sensitive credentials.

They should be:

• Protected
• Stored securely
• Revocable
• Rotated where appropriate
• Associated with the appropriate client/user/session
• Invalidated when misuse is detected

A particularly important practice is refresh-token rotation.

Conceptually:

Refresh Token A
      ↓
New Access Token
      +
Refresh Token B
      ↓
Invalidate Token A

If Token A is later reused unexpectedly, the server can detect suspicious activity.

JWT Validation

The API should validate important token properties such as:

• Signature
• Issuer
• Audience
• Expiration
• Not-before constraints where relevant
• Required claims

The API should never blindly trust claims simply because they appear inside a JWT.

Signing Keys

Signing keys must be protected carefully.

Applications should not hard-code secrets in source code.

For Azure-hosted applications, secure secret/key-management patterns can involve services such as Azure Key Vault and managed identities.

Authorization

Authentication answers:

Who are you?

Authorization answers:

What are you allowed to do?

ASP.NET Core can use policies and claims to enforce authorization.

For example:

Authenticated User
        ↓
Role/Claim
        ↓
Authorization Policy
        ↓
API Endpoint

This is much stronger than simply checking whether a user is logged in.

Token Storage

For browser-based applications, token storage deserves careful consideration because insecure storage can increase exposure to browser-based attacks.

The correct approach depends on the application architecture and client type. Modern applications should generally avoid casually placing long-lived sensitive credentials in browser-accessible storage.

Common Mistakes

Developers frequently make mistakes such as:

• Long-lived access tokens
• Poor signing-key protection
• No refresh-token revocation
• No token rotation
• Ignoring issuer/audience validation
• Storing secrets in source code
• Treating authentication as authorization
• Returning excessive claims
• Logging sensitive tokens

Enterprise Perspective

A production authentication system should also consider:

Identity Provider
       ↓
Token Issuance
       ↓
API Validation
       ↓
Authorization
       ↓
Auditing / Monitoring
       ↓
Revocation / Incident Response

Career Tip

For ASP.NET Core developers, authentication is an area where security architecture matters as much as coding ability. Understanding JWTs, OAuth/OIDC concepts, claims, policies, token lifetimes, and secure credential management can significantly strengthen an enterprise .NET developer profile.