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Using JSON Web Tokens (JWTs) to secure your APIs has become an ad-hoc standard in the IT-industry. However, the underlying concepts and mechanisms can be complex and are often misunderstood.

The general advice below pertains to both clients (id tokens) and API-s (access tokens). The specific guidelines regarding access tokens should be followed by API providers.

Info

General advice when using JWTs

  • Using JWTs to handle sessions introduces new risks. Allways do a thorough risk assessment before you deploy to your production environment.

  • Using JWTs assumes that all involved secrets are handled in a proper way in both API-clients and APIs. Secrets could be both passwords and cryptographic key material. Always make sure your secrets are kept safe!

  • Do not accept unsafe JWTs with the JOSE header {"alg": none; }. JWTs should always be signed and should use a valid and correct signature scheme

  • If a JWT contains sensitive data it should be encrypted

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Things to consider when using Access Tokens

  • A "self contained" JWT can not be revoked until it expires. Consider using referenced tokens if the token provides access to sensitive or high-risk data.

  • Access Tokens should be opaque to the client, they are meant for the API.

    • The lifetime of an access token is returned as a separate value to the client.

  • Access Tokens should have a life-time that is as short as possible.
    As a general principle: the more sensitive data an API exposes the shorter the life-time should be.

  • To enhance the user experience sessions with a long life-span should be combined with Access Tokens that have short life-span. You can acheive this by using refresh tokens on the client.

Note

Always verify the integrity of the JWT

  • Always validate the token signature

  • Make sure your libraries and frameworks always validates the token signature

  • HelseID only offers signatures based on asymmetric algorithms

Note

DO NOT BLINDLY TRUST THE VALUE IN CLAIMS

Examples:

  • “kid” - do not perform certificate requests before the values are validated and verified

  • “jku” or “x5u” are header values that point to a URL. Do NOT follow these URLs before they are validated in order to avoid SSRF attacks

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Always validate claims in the Access Token

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Always verify the "exp" claim to make sure the token hasn’t expired

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Always verify the claim "nbf" to make sure the token actually is "active"

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When verifying claims with timestamps, take into account that there may be some time skew between the token issuer and your internal systems.

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Always verify the claim "iss" to make sure that the token issuer is someone you actually trust

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Always verify the claim "aud" to make sure that the token is intended for you

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Reject the request if the "aud" claim is missing

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Consider rejecting tokens with multiple values in the “aud” claim.

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If you use "scopes" for access control you must always make sure that the values in the “scope” claims are valid and correct.

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Always validate and verify that the value in the claim “helseid://claims/identity/assurance_level“, alternatively “helseid://claims/identity/security_level”, corresponds with the requirements for your API.

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Consider validating and verifying the value in the claim "helseid://claims/identity/pid".

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Consider validating and verifying the value in the claim "helseid://claims/hpr/hpr_number".

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