Verifying a JSON Web Token - Amazon Cognito

Verifying a JSON Web Token

These steps describe verifying a user pool JSON Web Token (JWT).

Prerequisites

Your library, SDK, or software framework might already handle the tasks in this section. AWS SDKs provide tools for Amazon Cognito user pool token handling and management in your app. AWS Amplify includes functions to retrieve and refresh Amazon Cognito tokens.

For more information, see the following pages.

Many libraries are available for decoding and verifying a JSON Web Token (JWT). If you want to manually process tokens for server-side API processing, or if you are using other programming languages, these libraries can help. See the OpenID foundation list of libraries for working with JWT tokens.

Validate tokens with aws-jwt-verify

In a Node.js app, AWS recommends the aws-jwt-verify library to validate the parameters in the token that your user passes to your app. With aws-jwt-verify, you can populate a CognitoJwtVerifier with the claim values that you want to verify for one or more user pools. Some of the values that it can check include the following.

For more information and example code that you can use in a Node.js app or a AWS Lambda authorizer, see aws-jwt-verify on GitHub.

Understanding and inspecting tokens

Before you integrate token inspection with your app, consider how Amazon Cognito assembles JWTs. Retrieve example tokens from your user pool. Decode and examine them in detail to understand their characteristics, and determine what you want to verify and when. For example, you might want to examine group membership in one scenario, and scopes in another.

The following sections describe a process to manually inspect Amazon Cognito JWTs as you prepare your app.

Confirm the structure of the JWT

A JSON Web Token (JWT) includes three sections with a . (dot) delimiter between them.

Header

The key ID, kid, and the RSA algorithm, alg, that Amazon Cognito used to sign the token. Amazon Cognito signs tokens with an alg of RS256. The kid is a truncated reference to a 2048-bit RSA private signing key held by your user pool.

Payload

Token claims. In an ID token, the claims include user attributes and information about the user pool, iss, and app client, aud. In an access token, the payload includes scopes, group membership, your user pool as iss, and your app client as client_id.

Signature

The signature isn't decodable base64 like the header and payload. It's an RSA256 identifier derived from a signing key and parameters that you can observe at your JWKS URI.

The header and payload are base64-encoded JSON. You can identify them by the opening characters eyJ that decode to the starting character {. If your user presents a base64-encoded JWT to your app and it's not in the format [JSON Header].[JSON Payload].[Signature], it's not a valid Amazon Cognito token and you can discard it.

Validate the JWT

The JWT signature is a hashed combination of the header and the payload. Amazon Cognito generates two pairs of RSA cryptographic keys for each user pool. One private key signs access tokens, and the other signs ID tokens.

To verify the signature of a JWT token
  1. Decode the ID token.

    The OpenID Foundation also maintains a list of libraries for working with JWT tokens.

    You can also use AWS Lambda to decode user pool JWTs. For more information, see Decode and verify Amazon Cognito JWT tokens using AWS Lambda.

  2. Compare the local key ID (kid) to the public kid.

    1. Download and store the corresponding public JSON Web Key (JWK) for your user pool. It is available as part of a JSON Web Key Set (JWKS). You can locate it by constructing the following jwks_uri URI for your environment:

      https://cognito-idp.<Region>.amazonaws.com/<userPoolId>/.well-known/jwks.json

      For more information on JWK and JWK sets, see JSON Web Key (JWK).

      Note

      Amazon Cognito might rotate signing keys in your user pool. As a best practice, cache public keys in your app, using the kid as a cache key, and refresh the cache periodically. Compare the kid in the tokens that your app receives to your cache.

      If you receive a token with the correct issuer but a different kid, Amazon Cognito might have rotated the signing key. Refresh the cache from your user pool jwks_uri endpoint.

      This is a sample jwks.json file:

      { "keys": [{ "kid": "1234example=", "alg": "RS256", "kty": "RSA", "e": "AQAB", "n": "1234567890", "use": "sig" }, { "kid": "5678example=", "alg": "RS256", "kty": "RSA", "e": "AQAB", "n": "987654321", "use": "sig" }] }
      Key ID (kid)

      The kid is a hint that indicates which key was used to secure the JSON Web Signature (JWS) of the token.

      Algorithm (alg)

      The alg header parameter represents the cryptographic algorithm that is used to secure the ID token. User pools use an RS256 cryptographic algorithm, which is an RSA signature with SHA-256. For more information on RSA, see RSA cryptography.

      Key type (kty)

      The kty parameter identifies the cryptographic algorithm family that is used with the key, such as "RSA" in this example.

      RSA exponent (e)

      The e parameter contains the exponent value for the RSA public key. It is represented as a Base64urlUInt-encoded value.

      RSA modulus (n)

      The n parameter contains the modulus value for the RSA public key. It is represented as a Base64urlUInt-encoded value.

      Use (use)

      The use parameter describes the intended use of the public key. For this example, the use value sig represents signature.

    2. Search the public JSON Web Key for a kid that matches the kid of your JWT.

  3. Use a JWT library to compare the signature of the issuer to the signature in the token. The issuer signature is derived from the public key (the RSA modulus "n") of the kid in jwks.json that matches the token kid. You might need to convert the JWK to PEM format first. The following example takes the JWT and JWK and uses the Node.js library, jsonwebtoken, to verify the JWT signature:

    Node.js
    var jwt = require('jsonwebtoken'); var jwkToPem = require('jwk-to-pem'); var pem = jwkToPem(jwk); jwt.verify(token, pem, { algorithms: ['RS256'] }, function(err, decodedToken) { });

Verify the claims

To verify JWT claims
  1. By one of the following methods, verify that the token hasn't expired.

    1. Decode the token and compare the exp claim to the current time.

    2. If your access token includes an aws.cognito.signin.user.admin claim, send a request to an API like GetUser. API requests that you authorize with an access token return an error if your token has expired.

    3. Present your access token in a request to the UserInfo endpoint. Your request returns an error if your token has expired.

  2. The aud claim in an ID token and the client_id claim in an access token should match the app client ID that was created in the Amazon Cognito user pool.

  3. The issuer (iss) claim should match your user pool. For example, a user pool created in the us-east-1 Region will have the following iss value:

    https://cognito-idp.us-east-1.amazonaws.com/<userpoolID>.

  4. Check the token_use claim.

    • If you are only accepting the access token in your web API operations, its value must be access.

    • If you are only using the ID token, its value must be id.

    • If you are using both ID and access tokens, the token_use claim must be either id or access.

You can now trust the claims inside the token.