Microsoft.AspNetCore.Authentication.JwtBearer.JwtBearerEvents – Token has expired

The Microsoft.AspNetCore.Authentication.JwtBearer.JwtBearerEvents – Token has expired error occurs when an expired JWT (JSON Web Token) is used for authentication in an ASP.NET Core application. The JWT contains an exp (expiration) claim, which indicates when the token is no longer valid. If the token is used after its expiration time, the authentication process will fail with this error.

Common Causes of “Token has expired” Error:

1. Expired Token:
   • The most common cause is that the exp claim in the JWT has passed. When this happens, the token is no longer valid, and the authentication system will reject it.
2. Token Lifetime Configuration:
   • If the token’s expiration time is set too short or not configured correctly, tokens might expire before the user can complete their actions, causing this error.
3. Clock Skew:
   • A small difference between the client’s and server’s system clocks (clock skew) can lead to an issue where the token appears to be expired when it technically isn’t, due to slight timing discrepancies.
4. Token Not Refreshed:
   • If a refresh token flow isn’t implemented, or the user fails to refresh the token within the allowed timeframe, the JWT will expire and cause this error.

Steps to Diagnose and Fix:

1. Check the Token’s Expiration:

The exp claim in the JWT defines the expiration time in Unix timestamp format (seconds since the Unix epoch). You can manually decode the token (using JWT.io) to check the expiration time and compare it with the current time to see if the token has expired.

2. Check Token Lifetime Configuration:

Ensure that the exp claim is being set correctly when issuing the token. Also, make sure the token is not set to expire too quickly.

Example of setting token expiration:

var tokenHandler = new JwtSecurityTokenHandler();
var key = Encoding.ASCII.GetBytes("your-secret-key");
var tokenDescriptor = new SecurityTokenDescriptor
{
    Subject = new ClaimsIdentity(new Claim[]
    {
        new Claim(ClaimTypes.Name, "username")
    }),
    Expires = DateTime.UtcNow.AddMinutes(30),  // Set expiration time
    SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
};
var token = tokenHandler.CreateToken(tokenDescriptor);
var tokenString = tokenHandler.WriteToken(token);

3. Handle Token Expiration in the Application:

When a token expires, the application should handle this gracefully. You can implement a mechanism to request a new token, such as using a refresh token.

Example of refresh token flow:

// When the token has expired, call an API to get a new token using the refresh token.

4. Allow for Clock Skew:

When validating JWT tokens, ASP.NET Core allows you to configure clock skew, which gives some leeway in case the client’s clock is slightly out of sync with the server’s clock.

Example of allowing clock skew:

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("your-secret-key")),
            ClockSkew = TimeSpan.FromMinutes(5)  // Allow 5 minutes of clock skew
        };
    });

5. Logging Expired Token Errors:

To diagnose expired token errors, ensure that logging is enabled to capture the error details, including the time when the token was issued and its expiration.

Example:

services.AddLogging(builder =>
{
    builder.AddConsole();
    builder.AddDebug();
});

This will help you verify whether the token’s expiration is happening sooner than expected or if there is a timing issue.

6. Check Token Issuer and Audience:

Sometimes, the issue might not be the expiration, but rather the token being issued for a different audience or issuer than expected. Ensure that the issuer (iss) and audience (aud) claims match the expected values.

Example of configuring token validation:

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidIssuer = "your-issuer",
            ValidAudience = "your-audience",
            ValidateLifetime = true,
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("your-secret-key")),
            ClockSkew = TimeSpan.FromMinutes(5) // Allow some clock skew
        };
    });

Example Error Handling:

You can catch the expired token error and handle it appropriately, such as prompting the user to log in again or refresh their token.

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.Events = new JwtBearerEvents
        {
            OnAuthenticationFailed = context =>
            {
                if (context.Exception is SecurityTokenExpiredException)
                {
                    // Handle expired token error here (e.g., refresh the token or ask the user to log in again)
                    context.Response.StatusCode = 401; // Unauthorized
                    context.Response.ContentType = "application/json";
                    return context.Response.WriteAsync("{\"error\":\"Token has expired.\"}");
                }
                return Task.CompletedTask;
            }
        };
    });