Microsoft.AspNetCore.Authentication.JwtBearer.JwtBearerEvents – Invalid signature

The Microsoft.AspNetCore.Authentication.JwtBearer.JwtBearerEvents with the message “Invalid signature” is an error that occurs during JWT (JSON Web Token) authentication in ASP.NET Core when the token’s signature cannot be validated. This typically happens when the token has been tampered with, the signing key is incorrect, or the token is not properly signed.

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Key Points

• This error is specific to JWT authentication in ASP.NET Core.
• It occurs when the token’s signature does not match the expected signature, indicating the token may be invalid or tampered with.
• Common causes include mismatched signing keys, expired tokens, or incorrect token configuration.

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Common Causes

1. Mismatched Signing Key:

• The signing key used to validate the token does not match the key used to sign the token.

1. Expired Token:

• The token has expired, and the signature validation fails as a result.

1. Tampered Token:

• The token has been altered after being signed, causing the signature to become invalid.

1. Incorrect Token Configuration:

• The token is not properly configured (e.g., missing or incorrect claims, audience, or issuer).

1. Clock Skew:

• The server’s clock is not synchronized with the token issuer’s clock, causing validation issues.

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How to Fix

Client-Side Fixes

1. Use the Correct Signing Key:

• Ensure the client is using the correct key to sign the token.
• If using asymmetric encryption (e.g., RSA), ensure the public key is used for validation.

1. Check Token Expiry:

• Ensure the token has not expired before sending it to the server.

1. Validate Token Configuration:

• Ensure the token includes the correct claims, audience, and issuer.

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Server-Side Fixes

1. Configure JWT Authentication Correctly:

• Ensure the server is configured with the correct signing key and validation parameters.

   services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
       .AddJwtBearer(options =>
       {
           options.TokenValidationParameters = new TokenValidationParameters
           {
               ValidateIssuer = true,
               ValidateAudience = true,
               ValidateLifetime = true,
               ValidateIssuerSigningKey = true,
               ValidIssuer = "https://example.com",
               ValidAudience = "https://example.com",
               IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("your-secret-key"))
           };
       });

2. Handle Invalid Signature Errors:

• Use JwtBearerEvents to handle invalid signature errors and provide meaningful feedback.

   services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
       .AddJwtBearer(options =>
       {
           options.Events = new JwtBearerEvents
           {
               OnAuthenticationFailed = context =>
               {
                   if (context.Exception.GetType() == typeof(SecurityTokenInvalidSignatureException))
                   {
                       context.Response.Headers.Add("Invalid-Signature", "true");
                       context.Response.StatusCode = StatusCodes.Status401Unauthorized;
                       context.Fail("Invalid token signature.");
                   }
                   return Task.CompletedTask;
               }
           };
       });

3. Synchronize Clocks:

• Ensure the server’s clock is synchronized with the token issuer’s clock to avoid validation issues due to clock skew.

   options.TokenValidationParameters.ClockSkew = TimeSpan.FromMinutes(5); // Allow 5 minutes of clock skew

4. Log Errors:

• Log invalid signature errors for debugging and monitoring.

   logger.LogError(context.Exception, "Invalid JWT signature detected.");

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Example Scenario

Client Request

GET /api/data HTTP/1.1
Host: example.com
Authorization: Bearer invalid-token

Server Response

HTTP/1.1 401 Unauthorized
Invalid-Signature: true
Content-Type: application/json

{
  "error": "Invalid signature",
  "message": "The token's signature could not be validated."
}

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Debugging Tips

1. Inspect the Token:

• Use tools like jwt.io to decode and inspect the token’s claims and signature.

1. Check Signing Key:

• Verify that the signing key used by the server matches the key used to sign the token.

1. Test with a Valid Token:

• Test the API with a known valid token to ensure the server processes it correctly.

1. Review Token Configuration:

• Ensure the token includes the correct claims, audience, and issuer.

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