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Table of Contents

Status

Current state: Under DiscussionApproved for 3.1.0

Discussion thread: here

JIRA: KAFKA-13202

...

When it comes to Kafka, the work done via KIP-255 (OAuth Authentication via SASL/OAUTHBEARER) introduced a framework that allowed for integration with OAuth-compliant providers. With this framework in place, Kafka clients could now pass a JWT access token to a broker when initializing the connection as a means of authentication. This means Kafka can start to leverage these standards for authorization and authentication for which many current and prospective customers are asking.

However, the KIP for the existing implementation -255 quickly notes that:

OAuth 2 is a flexible framework with multiple ways of doing the same thing.

...

The KIP sums up this intentional missing functionality by directing organizations wishing to adopt OAuth within Kafka:

Production use cases will require writing an implementation of AuthenticateCallbackHandler that can handle an instance of OAuthBearerTokenCallback

So the exact implementation is left up to each organization to implement. To fill the gap in the Apache Kafka project, a handful of open source Java implementations have been provided by the community. These implementations can be included in the class path of a OAuth-enabled Kafka client and configured appropriately to achieve the goal.

...

No changes to the public interface are anticipated; it will leverage the existing AuthenticateCallbackHandler API.

Proposed Changes

The OAuth/OIDC work will allow out-of-the-box configuration by any Apache Kafka users to connect to an external identity provider service (e.g. Okta, Auth0, Azure, etc.). The code will implement the standard OAuth clientcredentials grant type.

The proposed change is largely composed of a pair of AuthenticateCallbackHandler implementations: one to login on the client and one to validate on the broker.


Image RemovedImage Added

As show in the above diagram, the login callback is executed on the client and the validate callback is executed on the broker.

...

Here is an example call to retrieve a JWT access token using curl and jq:

CLIENT_ID=abc123
CLIENT_SECRET='S3cr3t!'
URL=https://myidp.example.com/oauth2/default/v1/token
SCOPE=sales-pipeline

base_64_string=$(echo -n "$CLIENT_ID:$CLIENT_SECRET" | base64)

access_token=$(curl \
--silent \
--request POST \
--url $URL \
--header "accept: application/json" \
--header "authorization: Basic $base_64_string" \
--header "cache-control: no-cache" \
--header "content-type: application/x-www-form-urlencoded" \
--data "grant_type=client_credentials&scope=$SCOPE" | jq -r .access_token)

...

The name of the implementation class will be org.apache.kafka.common.security.oauthbearer.internals.secured.OAuthBearerLoginCallbackHandler and it will accept instances of org.apache.kafka.common.security.oauthbearer.OAuthBearerTokenCallback and org.apache.kafka.common.security.auth.SaslExtensionsCallback. The fully-qualified class name will be provided to the client's sasl.login.callback.handler.class configuration.

Because the HTTP call made to the OAuth/OIDC provider may time out or transiently fail, there will be a retry mechanism that waits between waits between attempts. The number of attempts that are made (including the first attempt) are configured via the loginAttempts configuration setting. The retry will use an variable as it uses an exponential backoff approach; the first attempt to connect to the HTTP endpoint will be made immediately. If that first attempt fails, a second attempt will first wait a configurable number of milliseconds–loginRetryWaitMs–sasl.login.retry.backoff.ms–before trying again. If that second attempt fails, the wait time (loginRetryWaitMs) will be doubled before a third attempt. This pattern repeats as needed up to loginAttempts. However, there is also a configurable the maximum wait time between attempts–loginRetryMaxWaitMs–such that loginRetryWaitMs <= loginRetryMaxWaitMs, regardless of the number of attempts: min(currentRetryWaitMs, loginRetryMaxWaitMs)of sasl.login.retry.backoff.max.ms.

There are several configuration options for this callback handler:

...

. Sensitive configuration options and SASL extensions appear under the JAAS configuration (sasl.jaas.config) while the rest are top-level configuration.

The JAAS configuration options are:

  • clientId: supports OAuth clientcredentials grant type

  • clientSecret: supports OAuth's clientcredentials grant

  • scope: optional scope to reference in the call to the OAuth server

The top-level configuration options for the client login callback handler are:

  • sasl.oauthbearer.token.endpoint.url: OAuth issuer token endpoint URL
  • sasl.oauthbearer.scope.claim.namescopeClaimName: optional override name of the scope claim; defaults to scope
  • subClaimNamesasl.oauthbearer.sub.claim.name: optional override name of the sub claim; defaults to scope sub
  • sasl.login.connect.timeout.msloginConnectTimeoutMs: optional value in milliseconds for HTTPS connect timeout; defaults to 10000
  • loginReadTimeoutMssasl.login.read.timeout.ms: optional value in milliseconds for HTTPS read timeout; defaults to 10000
  • loginAttempts: optional number of attempts to make to connect to the OAuth/OIDC identity provider; defaults to 3
  • loginRetryWaitMs: optional sasl.login.retry.backoff.ms: optional value in milliseconds for the amount of time to wait between HTTPS call attempts; defaults to 250 100
  • sasl.login.retry.backoff.max.ms: optional value in loginRetryMaxWaitMs: optional value in milliseconds for the maximum wait between for HTTPS call attempts . The HTTP calls retry mechanism uses an exponential backoff approach up to the number defined by loginAttempts but not more than loginRetryWaitMs(as described above); defaults to 10000

Here's an example of the configuration as a part of a Java properties file:

...

sasl.login.callback.handler.class=...OAuthBearerLoginCallbackHandler
sasl.login.connect.jaastimeout.configms=15000
sasl.oauthbearer.token..OAuthBearerLoginModule required \
tokenEndpointUri="endpoint.url=https://myidp.example.com/oauth2/default/v1/token"
sasl.jaas.config=...OAuthBearerLoginModule required \
clientId="abc123" \
clientSecret="S3cr3t!" \
scope="sales-pipeline" \
Extension_supportFeatureX="true" \
Extensionextension_organizationId="sales-emea" ;


In the above example, the OAuth provider’s tokenEndpointUri to retrieve an access token provider’s sasl.oauthbearer.token.endpoint.url has been specified as well as an override of the default for sasl.login.connect.timeout.ms. The values for for clientId and clientSecret as as provided by the OAuth provider for an “API” or “machine-to-machine” account are required in the JAAS configuration. The optional optional scope value will allow the inclusion of a scope parameter when requesting the token.

Notice that there are two is also a SASL extension configuration values in this example too: Extension_supportFeatureX and Extension: extension_organizationId. These Extensions will be ignored during the OAuth token retrieval step, but will be passed to the broker through the existing SASL extension mechanism from KIP-342.

Validator Callback Handler (Broker)

The validation callback handler is invoked as part of the SASL Kafka server authentication process when a JWT is received. This callback handler will parse and validate the JWT using its contents along with provided configuration.

Validation Logic

The basic overview of the JWT validation that will be performed is:

Once the login has occurred for this client, the returned access token can be reused by other connections from this client. While these additional connections will not issue the token retrieval HTTP call on the client, the broker will still validate the token once for each time it is sent by each client connection.

Per KIP-368, the OAuth token re-authentication logic from the existing implementation is automatically "inherited" by this implementation, so no additional work is needed to support that feature.

Validator Callback Handler (Broker)

The validation callback handler is invoked as part of the SASL Kafka server authentication process when a JWT is received. This callback handler will parse and validate the JWT using its contents along with provided configuration.

Validation Logic

The basic overview of the JWT validation that will be performed is:

  1. Parse the JWT into separate header, payload, and signature sections

  2. Base-64 decode the header and payload

  3. Extract the necessary claims for validation

  4. Match the key ID (kid) specified in the JWT header to a JWK ID from the JWK Set

  5. Ensure the encoding algorithm (alg from the header) isn't none and matches the expected algorithm for the JWK ID

  6. Encode the header and payload sections of the original encoded JWT access token

  7. Parse the JWT into separate header, payload, and signature sections

  8. Base-64 decode the header and payload

  9. Extract the necessary claims for validation

  10. Match the key ID (kid) specified in the JWT header to a JWK ID from the JWK Set

  11. Ensure the encoding algorithm (alg from the header) isn't none and matches the expected algorithm for the JWK ID

  12. Encode the header and payload sections of the original encoded JWT access token using the public key from the JWK and ensure it matches the signature section of the JWT

  13. Extract the scope, iat, exp, and sub claims as these are needed by the OAuthBearerToken object to be passed to the OAuthBearerValidatorCallback.

  14. Optional claim validation that ensures that issuer, audience, or other claims match a given value

The extensions validation will be executed the same as in org.apache.kafka.common.security.oauthbearer.internals.unsecured.OAuthBearerUnsecuredValidatorCallbackHandler today.

A new key It's possible that a key ID (kid) could appear in the header of an incoming JWT access token . Code that can retrieve the JWKS from the OAuth provider on demand will be implemented. The common case will be that the key ID is one that has been accessed recently, so it shouldn’t need to reach out to the JWKS endpoint often. The code will need to have a means to expunge old JWKs that are no longer needed.

Broker Configuration

that does not appear in the JWKS cached by a broker. 

Broker Configuration

The name of the implementation class will be org.apache.kafka.common.security.oauthbearer.secured.OAuthBearerValidatorCallbackHandler and it will accept instances of org.apache.kafka.common.security.oauthbearer.OAuthBearerValidatorCallback and The name of the implementation class will be org.apache.kafka.common.security.oauthbearer.internals.secured.OAuthBearerValidatorCallbackHandler and it will accept instances of org.apache.kafka.common.security.oauthbearer.OAuthBearerValidatorCallback and org.apache.kafka.common.security.oauthbearer.OAuthBearerExtensionsValidatorCallback. The fullyOAuthBearerExtensionsValidatorCallback. The fully-qualified class name will be provided to the broker's listener.name.<listener name>.oauthbearer.sasl.server.callback.handler.class configuration.

It may be that the names of the claims used by the OAuth provider differ from what is expected. For example, the security principal for which the token is valid is usually contained in the sub (subject) JWT claim. There may be cases where the value of that claim may not be valid or usable, and instead the value will need to be extracted from, for example, the email claim.

There are a handful of several configuration options for this callback handler:. Since there are no sensitive configuration options, they are all in the top-level configuration. The configuration can be top-level or scoped to a specific listener with the listener prefix listener.name.<listener name>.oauthbearer. Here are the configuration options:

  • sasl.oauthbearer.jwks.endpoint.url: OAuth issuer's JWK Set endpoint URL from jwksEndpointUri: OAuth issuer's JWK Set endpoint URI from which to retrieve the set of JWKs managed managed by the provider; mutually exclusive with jwksFile

  • jwksEndpointRefreshIntervalMs: optional value in milliseconds for how often to refresh the JWKS from the URL pointed to by jwksEndpointUri. Only used when using jwksEndpointUri. Defaults to 3600000 (1 hour)

  • jwksFile: specifies a locally-accessible file name that holds a file

    this can be a file://-based URL that points to a broker file system-accessible file-based copy of the JWKS data. This allows the JWKS data to be updated on the file system and refreshed on the broker when the file is updated, thus avoiding any HTTP(S) communication with the OAuth/OIDC provider

    ; mutually exclusive with jwksEndpointUri

  • principalClaimName: name of the scope from which to extract the subject claim from the JWT; defaults to sub

  • scopeClaimName: name of the scope from which to extract the scope claim from the JWT; defaults to scope

  • clockSkew: optional value in seconds for the clock skew between the OAuth/OIDC provider and the broker. Only used when using jwksEndpointUri. Defaults to 30
  • expectedAudience
  • expectedIssuer

Here's an example of the configuration as a part of a Java properties file:

...

  • sasl.oauthbearer.jwks.endpoint.refresh.interval.ms: optional value in milliseconds for how often to refresh the JWKS from the URL pointed to by sasl.oauthbearer.jwks.endpoint.url. Only used when using an HTTP(S)-based URL for sasl.oauthbearer.jwks.endpoint.url. Defaults to 3600000 (1 hour)

  • sasl.oauthbearer.jwks.endpoint.retry.backoff.ms: optional value in milliseconds for the amount of time to wait between HTTPS call attempts to retrieve the JWKS; only used when using an HTTP(S)-based URL for sasl.oauthbearer.jwks.endpoint.url ; defaults to 100
  • sasl.oauthbearer.jwks.endpoint.retry.backoff.max.ms: optional value in milliseconds for the maximum wait for HTTPS call attempts to retrieve the JWKS; only used when using an HTTP(S)-based URL for sasl.oauthbearer.jwks.endpoint.url ; defaults to 10000

  • sasl.oauthbearer.sub.claim.name: name of the scope from which to extract the subject claim from the JWT; defaults to sub

  • sasl.oauthbearer.scope.claim.name: name of the scope from which to extract the scope claim from the JWT; defaults to scope

  • sasl.oauthbearer.clock.skew.seconds: optional value in seconds for the clock skew between the OAuth/OIDC provider and the broker. Only used when using an HTTP(S)-based URL for sasl.oauthbearer.jwks.endpoint.url. Defaults to 30
  • sasl.oauthbearer.expected.audience: The (optional) comma-delimited setting for the broker to use to verify that the JWT was issued for one of the expected audiences. The JWT will be inspected for the standard OAuth aud claim and if this configuration option is set, the broker will match the value from JWT's aud claim to see if there is an exact match. If there is no match, the broker will reject the JWT and authentication will fail.
  • sasl.oauthbearer.expected.issuer: Optional setting for the broker to use to verify that the JWT was created by the expected issuer. The JWT will be inspected for the standard OAuth iss claim and if this configuration option is set, the broker will match the value from JWT's iss claim to see if there is an exact match. If there is no match, the broker will reject the JWT and authentication will fail.

Here's an example of the configuration as a part of a Java properties file:


listener.name.<listener name>.oauthbearer.sasl.server.callback.handler.class=...OAuthBearerValidatorCallbackHandler
listener.name.<listener name>.oauthbearer.sasl.jaas.config=...OAuthBearerLoginModule required;
sasl.oauthbearer.jwks.endpoint.url=https://myidp.example.com/oauth2/default/v1/keys
sasl.oauthbearer.scope.claim.name=scp


In the above configuration the broker points to the appropriate OAuth provider sasl.oauthbearer.jwks.endpoint.url to retrieve a the set of JWKs for validation. In this example, a non-default value for sasl.oauthbearer.scope.claim.name has been provided because the provider uses scp for the name of the scope claim in the JWT it produces.

JWKS Management Logic

The JSON Web Key Set (JWKS) is a JSON document provided by the OAuth/OIDC provider that lists the keys used to sign the JWTs it issues.

Here is a sample JWKS JSON document:

{
  "keys": [
    {
      "kty": "RSA",
      "alg": "RS256",
      "kid": "abc123",
      "use": "sig",
      "e": "AQAB",
      "n": "..."
    },
    {
      "kty": "RSA",
      "alg": "RS256",
      "kid": "def456",
      "use": "sig",
      "e": "AQAB",
      "n": "..."
    }
  ]
}

Without going into too much detail, the array of keys enumerates the key data that the provider is using to sign the JWT signature. The key ID (kid) is referenced by the JWT's header in order to match up the JWT's signing key with the key in the JWKS. During the validation step, the jose4j OAuth library will use the contents of the appropriate key in the JWKS to validate the signature.

Given that the JWKS is referenced by the JWT, the JWKS must be made available by the OAuth/OIDC provider so that a JWT can be validated.

The JWKS will be kept up-to-date in two main ways:

  1. Providing a JWKS URL. In this mode, the JWKS data will be retrieved from the OAuth provider via the configured URL on broker startup. All then-current keys will be cached on the broker (per the ‘max age’; the jose4j library has a means to keep these-up-to-date when they age out) for incoming requests. If an authentication request is received for a JWT that includes a kid that isn’t yet in the cache, the JWKS endpoint will be queried again on demand. However, we prefer polling via a background thread to hopefully pre-populate the cache with any forthcoming keys before any JWT requests that include them are received.

  2. Providing a JWKS file. On startup, the broker will load the JWKS file from the configured location. Any updates to the file will require a restart of the broker. The means by which the JWKS file is updated is left to the cluster administrator. In the event that an unknown JWT key is encountered, this implementation will simply issue an error and validation will fail.


Warning

If the the URL or file that is specified cannot be read, the broker will fail to start up. In the case of an HTTP(S)-based URL, the configured sasl.login.retry.backoff.ms and sasl.login.retry.backoff.max.ms values will be used to make attempts to connect to the remote OAuth provider.

It is also important that the JWKS is retrieved before the broker's ports are opened. Otherwise clients that connect to the broker before the JWKS is retrieved will experience spurious authentication failures (e.g. during broker restarts).


It is expected that an OAuth provider will publish a new JWKS well in advance of issuing any JWTs that contain those keys. However, in the case that a broker receives a JWT with a key ID that it doesn't have stored in its cached JWKS, the broker will need to remediate the issue:

  1. If the JWKS URL is HTTP(S)-based and if the broker hasn't already attempted to resolve the key ID, enqueue a background thread to reload the JWKS from the HTTP(S) endpoint. The broker will keep track of key ID resolution failures so it doesn't repeatedly attempt to do so. If the JWKS URL is file://-based, no remediation processing will occur.
  2. Send an authentication failure to the client. Since it is unknown at this point in processing of the key ID is valid-but-missing or if the key ID is just invalid, the broker will always issue an authentication error. Between the authentication failure delay mechanism and any client retry, there may be sufficient time for the broker to update the JWKS.

The common case will be that the key ID is one that has been published and accessed recently. The broker shouldn’t need to reach out to the JWKS endpoint in the on-demand fashion described above normally.


Warning

The broker will not perform any loading of the JWKS in the network thread. This needs to be performed either prior to opening the ports during startup (as described above) or performed in a background thread. 


The code will need to have a means to expunge old JWKS that are no longer needed. If there is a tangible benefit, perhaps the broker can keep track of recently removed/aged out key IDs so as to provide a more helpful message to the user.

Broker-to-broker Support

The use of OAuth credentials for broker-to-broker communication will continue to be supported. As with the existing implementation, users can specify the protocols and implementations to use for broker-based communication. This would require providing the appropriate configuration for both client login and broker validation.

Testing

In addition to unit and integration tests, there will be a standalone tool in the tools directory/module named org.apache.kafka.tools.OAuthCompatibilityTool. This test can be run via the existing bin/kafka-run-class.sh script thusly:


./bin/kafka-run-class.sh org.apache.kafka.tools.OAuthCompatibilityTool \
  --client-id foo \
  --client-secret bar \
  --token-endpoint-url https://example.com/oauth2/v1/token \
  --jwks-endpoint-url https://example.com/oauth2/v1/keys


As seen in the example invocation above, the various configuration options for the client and broker properties are passed in via command line options. The test will connect to remote systems as needed to authenticate, retrieve tokens, retrieve JWKS, and perform validation (nothing is "mocked"). The command line options can be listed using the standard --help command line option. 

This tool serves three basic purposes:

  1. Allows validation that a given OAuth/OIDC provider is compliant with our implementation of the specification
  2. Provides users a means to reproduce and diagnose issues for support needs
  3. Provides a means to perform system tests against live providers for tracking regressions

The output of the test is easily consumable:


PASSED 1/5: client configuration
PASSED 2/5: client JWT retrieval
PASSED 3/5: client JWT validation
PASSED 4/5: broker configuration
PASSED 5/5: broker JWT validation


Additionally debugging can be selectively enabled using the standard tools-log4j.properties file if errors are detected by the tool. All configuration options supported by the client and server are also supported by the tool; it attempts to be as faithful as possible to the runtime logic to minimize discrepancies between the tool and the client and broker

In the above configuration the broker points to the appropriate OAuth provider jwksEndpointUri to retrieve a the set of JWKs for validation. In this example, a non-default value for scopeClaimName has ben provided because the provider uses scp for the name of the scope claim in the JWT it produces.

JWKS Management Logic

The JWKS will be kept up-to-date in two main ways:

  1. Providing a JWKS URL. In this mode, the JWKS data will be retrieved from the OAuth provider via the configured URL on broker startup. All then-current keys will be cached on the broker (per the ‘max age’; the jose4j library has a means to keep these-up-to-date when they age out) for incoming requests. If an authentication request is received for a JWT that includes a kid that isn’t yet in the cache, the JWKS endpoint will be queried again on demand. However, we prefer polling via a background thread to hopefully pre-populate the cache with any forthcoming keys before any JWT requests that include them are received.

  2. Providing a JWKS file. On startup, the broker will load the JWKS file from a configured location and will watch the file for updates, allowing for dynamic configuration updates. The means by which the JWKS file is updated is left to the cluster administrator. In the event that an unknown JWT key is encountered, this implementation will simply issue an error and validation will fail.

Broker-to-broker Support

The use of OAuth credentials for broker-to-broker communication will continue to be supported. As with the existing implementation, users can specify the protocols and implementations to use for broker-based communication.

Compatibility, Deprecation, and Migration Plan

Users can continue to use the OAuthBearerUnsecuredLoginCallbackHandler and/or other AuthenticateCallbackHandler implementations. Users will need to update both clients and brokers in order to use the new functionality.

Rejected Alternatives

Removing OAuthBearerUnsecuredLoginCallbackHandler

Although this change will provide an out-of-the-box implementation of an AuthenticateCallbackHandler that can be used to communicate with OAuth/OIDC, the exist unsecured implementation is still usable for development and testing. Given that its non-secure status is in its name and the documentation, it shouldn’t need to be removed or deprecated at this time.

...

Technically the new implementation could be developed and shipped as a plugin separate from the main Apache Kafka project. Community adoption would be improved by an in-tree solution. Including this inside Apache Kafka doesn’t preclude alternative alternative AuthenticateCallbackHandler implementations from use by clients, if desired.