JWT Validation

Document updated on Nov 2, 2021

Protect endpoints from public usage by validating JWT tokens generated by any industry-standard OpenID Connect (OIDC) integration.

Before digging any further, some answers to frequently asked questions:

1. KrakenD does not generate the tokens itself. Still, you can plug it into any SaaS or self-hosted OpenID Identity Provider (IdP) using industry standards (e.g., Auth0, Azure AD, Google Firebase, Keycloak, etc.)

2. KrakenD does not need to validate all calls using your IdP. KrakenD validates every incoming call’s signature and it doesn’t make token introspection (asking the IdP data about the token owner).

3. If you don’t have an identity server, you can still use your classic monolith/backend login system and adapt it to return a JWT payload (a simple JSON). From here, let KrakenD sign the token for you and start using tokens right away.

4. Your self-hosted identity server doesn’t need to be exposed to the Internet, as it can live behind KrakenD and let the token generation requests be proxied through KrakenD. If you use a SaaS solution, then it’s exposed.

5. If you are new to JWT validation, start reading the JSON Web Tokens overview

JWT header requirements

When KrakenD decodes the base64 token string passed in the Bearer or a cookie, it expects to find in its header section the following three fields:

{
    "alg": "RS256",
    "typ": "JWT",
    "kid": "MDNGMjU2M0U3RERFQUEwOUUzQUMwQ0NBN0Y1RUY0OEIxNTRDM0IxMw"
}

The alg and kid values depend on your implementation, but they must be present.

The example above used this public key. Notice how the kid matches the single key present in the JWK document and the token header.

KrakenD is built with security in mind and uses JWS (instead of plain JWT or JWE), and the kid points to the right key in the JWS. This is why this entry is mandatory to validate your tokens.

Basic JWT validation

The JWT validation must be present inside every endpoint definition needing it. If several endpoints are going to require JWT validation consider using the flexible configuration to avoid repetitive declarations.

Enable the JWT validation by adding the namespace "auth/validator" inside the extra_config of the desired endpoint.

For instance, to protect the endpoint /protected/resource:

{
    "endpoint": "/protected/resource",
    "extra_config": {
        "auth/validator": {
            "alg": "RS256",
            "audience": ["http://api.example.com"],
            "roles_key": "http://api.example.com/custom/roles",
            "roles": ["user", "admin"],
            "jwk_url": "https://albert-test.auth0.com/.well-known/jwks.json",
            "cache": true
        }
    },
    "backend": [
        {
        "url_pattern": "/"
        }
    ]
}

This configuration makes sure that:

• The token is well-formed and didn’t expire
• The token has a valid signature
• The role of the user is either user or admin (taken from a key in the JWT payload named http://api.example.com/custom/roles)
• The token is not revoked in the bloom filter (see revoking tokens)
• The key from the identity server is cached to avoid hammering it

JWT validation settings

The following settings are available for JWT validation. There are many options, although generally only the fields alg and jwk_url or jwk_local_path are mandatory, and the rest of the keys can be added or not at your best convenience or depending on other options.

These options are for the extra_config’s namespace "auth/validator" placed in every endpoint (use flexible configuration to avoid code repetition):

• alg (recognized string): The hashing algorithm used by the issuer. See the hashing algorithms section for a comprehensive list of supported algorithms.
• jwk_url (string): The URL to the JWK endpoint with the public keys used to verify the token’s authenticity and integrity.
• jwk_local_path (string): Local path to the JWK public keys. Instead of pointing to an external URL (jwk_url), public keys are kept locally, in a plain JWK file (security alert!), or encrypted. When encrypted, also add:
  • secret_url (string): An URL with a custom scheme using one of the supported providers (e.g.: awskms://keyID) (see providers below)
  • cypher_key (string): The cyphering key.
• cache (boolean): Set this value to true to store the required keys (from the JWK descriptor) in memory for the next cache_duration period and avoid hammering the key server, as recommended for performance. The cache can store up to 100 different public keys simultaneously.
• cache_duration (int): Change the default duration to 15 minutes. Value in seconds.
• audience (list): Set when you want to reject tokens that do not contain the given audience.
• roles_key (string): When validating users through roles, provide the key name inside the JWT payload that lists their roles. If this key is nested inside another object, use the dot notation . to traverse each level. E.g.: resource_access.myclient.roles represents the payload {resource_access: { myclient: { roles: ["myrole"] } } .
• roles (list): When set, the JWT token not having at least one listed role is rejected.
• roles_key_is_nested (bool): If the roles key uses a nested object using the . dot notation, you must set it to true to traverse the object.
• scopes (list): A list of scopes to validate. Make sure to use a list [] in the config, but when passing the token, the scopes should be separated by spaces, e.g.: "my_scopes": "resource1:action1 resource3:action7".
• scopes_key: The key name where KrakenD can find the scopes. The key can be a nested object using the . dot notation, e.g.: data.data2.scopes
• scopes_matcher (string): Valid options are all or any. When you use all, every scope defined in the endpoint must be present in the token. Otherwise, any matching scope will let you pass.
• issuer (string): When set, tokens not matching the issuer are rejected.
• cookie_key (string): Add the key name of the cookie containing the token when it is not passed in the headers
• disable_jwk_security (boolean): When true, disables security of the JWK client and allows insecure connections (plain HTTP) to download the keys. Useful for development environments.
• jwk_fingerprints (strings list): A list of fingerprints (the certificate’s unique identifier) for certificate pinning and avoid man-in-the-middle attacks. Add fingerprints in base64 format.
• cipher_suites (integers list): Override the default cipher suites. Use it if you want to enforce an even higher security standard.
• jwk_local_ca (string): Path to the CA’s certificate verifying a secure connection when downloading the JWK. Use when not recognized by the system (e.g., self-signed certificates).
• propagate_claims (list): Enables passing claims in the backend’s request header (see below). You can pass nested claims using the dot . operator. E.g.: realm_access.roles.
• key_identify_strategy (string): Allows strategies other than kid to load keys. Allowed values are: kid, x5t, kid_x5t
• operation_debug (bool): When true, any JWT validation operation gets printed in the log with a level ERROR. You will see if a client does not have sufficient roles, the allowed claims, scopes, and other useful information.

For the complete list of recognized algorithms and cipher suites, scroll down to the end of the document.

Here there is an example using an external jwk_url:

{
    "endpoint": "/foo",
    "extra_config": {
        "auth/validator": {
            "alg": "RS256",
            "jwk_url": "https://url/to/jwks.json",
            "cache": true,
            "audience": [
                "audience1"
            ],
            "roles_key": "department",
            "roles_key_is_nested": false,
            "roles": [
                "sales",
                "development"
            ],
            "scopes_key": "my_scopes",
            "scopes_matcher": "any",
            "scopes": [
                "resource1:action1",
                "resource2:action1",
                "resource1:action2"
            ],
            "issuer": "http://my.api.com",
            "cookie_key": "TOKEN",
            "disable_jwk_security": true,
            "jwk_fingerprints": [
                "S3Jha2VuRCBpcyB0aGUgYmVzdCBnYXRld2F5LCBhbmQgeW91IGtub3cgaXQ=="
            ],
            "cipher_suites": [
                10, 47, 53
            ],
            "operation_debug": true
    }
}
}

Validation process

KrakenD does the following validation to let users hit protected endpoints:

• The jwk_url must be accessible by KrakenD at all times (caching is available)
• The token is well formed
• The kid in the header is listed in the jwk_url or jwk_local_path.
• The content of the JWK Keys (k) is base64 urlencoded
• The algorithm alg is supported by KrakenD and matches exactly the one used in the endpoint definition.
• The token hasn’t expired
• The signature is valid.
• The given issuer matches (if present in the configuration)
• The given audience matches (if present in the configuration)
• The given claims are within the endpoint accepted roles (if present in the configuration))

The configuration allows you to define the set of required roles. For example, a user who passes a token with roles A and B, can access an endpoint requiring "roles": ["A","C"] as it has one of the required options (A).

If the token is expired, the signature doesn’t match, the required claims do not match, or the token is revoked, a 401 Unauthorized is returned.

When the token doesn’t include the defined ACL’s required roles, a 403 Forbidden is returned.

When you generate tokens for end-users, make sure to set a low expiration. Tokens are supposed to have short lives and should expire in a few minutes or hours.

Accepted providers for encrypting payloads

When using a jwk_local_path, the secret_url scheme accepts different providers:

Local secrets

The local secrets require an URL with the following scheme:

base64key://base64content

The URL host must be base64 encoded and must decode to exactly 32 bytes. Here is an example of the extra_config:

{
    "jwk_local_path":"./jwk.txt",
    "secret_url":"base64key://smGbjm71Nxd1Ig5FS0wj9SlbzAIrnolCz9bQQ6uAhl4=",
    "cypher_key":"gCERmfqHMoEu3+utqBa/R1oMZYIvh0OOKtJmnX/hDPDxbXCGXGvO3SF7B5FWxrJnRW7rnjGIV4eP2VLrYX2q9pJM49BpP+A9"
}

This config will use the key smGbjm71Nxd1Ig5FS0wj9SlbzAIrnolCz9bQQ6uAhl4= for decrypting de cypher_key and then decrypting the content of the file ./jwt.txt.

See this test to understand how to generate and encrypt payloads.

Amazon KMS

awskms://keyID

The URL Host + Path is used as the key ID, which can be an Amazon Resource Name (ARN), alias name, or alias ARN. Note that ARNs may contain “:” characters, which cannot be escaped in the Host part of a URL, so you should use the awskms:///<ARN> form.

More information about AWS KMS

Azure’s Key Vault

azurekeyvault://keyID

The credentials are taken from the environment unless the AZURE_KEYVAULT_AUTH_VIA_CLI environment variable is set to true, in which case it uses the az command line.

More information about Azure Key Vault

Google Cloud KMS

gcpkms://projects/[PROJECT_ID]/locations/[LOCATION]/keyRings/[KEY_RING]/cryptoKeys/[KEY]

You can take the URL from the GCP console.

Hashicorp’s Vault

hashivault://keyID

Environment variables VAULT_SERVER_URL and VAULT_SERVER_TOKEN are used.

Passing claims to the backend URL

Since KrakenD 1.2.0, it is possible to use data present in the claims to inject it into the backend’s final URL. The notation of the url_pattern field includes the parsing of {JWT.some_claim}, where some_claim is an attribute of your claim.

For instance, when your JWT payload is represented by something like this:

{
    "sub": "1234567890",
    "name": "Mr. KrakenD"
}

Having a backend defined with:

{
    "url_pattern": "/foo/{JWT.sub}",
    "method": "POST"
}

The call to your backend would produce the request:

POST /foo/1234567890

Keep in mind that this syntax in the url_pattern field is only available if the backend loads the extra_config "auth/validator" and that it does not work with nested attributes in the payload.

If KrakenD can’t replace the claim’s content for any reason, the backend receives a request to the literal URL /foo/{JWT.sub}.

Propagate JWT claims as request headers

It is possible to forward claims in a JWT as request headers. It is a common use case to have, for instance, the sub claim added as an X-User header to the request.

Important: The endpoint input_headers needs to be set as well, so the backend can see it.

{
    "extra_config": {
        "auth/validator": {
            "propagate_claims": [
                ["sub", "x-user"],
                ["realm_access.role", "x-role"]
            ]
        }
    }
}

In this case, the sub claim’s value will be added as x-user header to the request. If the claim does not exist, the mapping is just skipped.

In addition, the nested property role (inside realm_access) is passed as an x-role header.

A complete running example

The KrakenD Playground demonstrates how to protect endpoints using JWT and includes two examples ready to use:

• Integration with an external third party using a Single Page Application from Auth0
• Integration with an internal identity provider service (mocked) using a symmetric key algorithm and a signer middleware.

To try it, clone the playground and follow the README.

Supported hashing algorithms and cipher suites

Hashing algorithms

Accepted values for the alg field are:

• EdDSA: EdDSA
• HS256: HS256 - HMAC using SHA-256
• HS384: HS384 - HMAC using SHA-384
• HS512: HS512 - HMAC using SHA-512
• RS256: RS256 - RSASSA-PKCS-v1.5 using SHA-256
• RS384: RS384 - RSASSA-PKCS-v1.5 using SHA-384
• RS512: RS512 - RSASSA-PKCS-v1.5 using SHA-512
• ES256: ES256 - ECDSA using P-256 and SHA-256
• ES384: ES384 - ECDSA using P-384 and SHA-384
• ES512: ES512 - ECDSA using P-521 and SHA-512
• PS256: PS256 - RSASSA-PSS using SHA256 and MGF1-SHA256
• PS384: PS384 - RSASSA-PSS using SHA384 and MGF1-SHA384
• PS512: PS512 - RSASSA-PSS using SHA512 and MGF1-SHA512

Cipher suites

Accepted values for cipher suites are:

• 5: TLS_RSA_WITH_RC4_128_SHA
• 10: TLS_RSA_WITH_3DES_EDE_CBC_SHA
• 47: TLS_RSA_WITH_AES_128_CBC_SHA
• 53: TLS_RSA_WITH_AES_256_CBC_SHA
• 60: TLS_RSA_WITH_AES_128_CBC_SHA256
• 156: TLS_RSA_WITH_AES_128_GCM_SHA256
• 157: TLS_RSA_WITH_AES_256_GCM_SHA384
• 49159: TLS_ECDHE_ECDSA_WITH_RC4_128_SHA
• 49161: TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
• 49162: TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
• 49169: TLS_ECDHE_RSA_WITH_RC4_128_SHA
• 49170: TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
• 49171: TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
• 49172: TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA
• 49187: TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
• 49191: TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256

Default suites are:

• 49199: TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
• 49195: TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
• 49200: TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
• 49196: TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
• 52392: TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305
• 52393: TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305