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  • KIP-86: Configurable SASL callback handlers
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Current state: "Under Discussion"

Discussion thread: here


Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).


Kafka currently supports SASL authentication using SASL/PLAIN mechanism and KIP-84 addresses the addition of SASL/SCRAM. Credential verification in SASL/PLAIN servers is currently based on hard-coded credentials in JAAS configuration similar to Digest-MD5 configuration in Zookeeper. This is useful as a sample, but not suitable for production use since clear passwords are stored on disk. KIP-84 added SCRAM mechanism with Zookeeper as the password store. In production installations where Zookeeper is not secure (e.g. Kafka on the cloud), an alternative password store may be required.

With the current Kafka implementation, the entire SaslServer implementation needs to be replaced to enable new credential providers. It will be useful to add an extension point for SASL that enables just the credential providers to be replaced with a custom implementation. This should be done in a consistent way for all SASL mechanisms.

This KIP proposes to enable customization of SASL server and clients using configurable callback handlers. Configurable callback handlers for SASL/PLAIN and SASL/SCRAM will enable credential providers to be replaced in a simple and consistent way. In addition to this, configurable callback handlers for both server and clients make it easier to configure new or custom SASL mechanisms that are not implemented in Kafka.

Public Interfaces

Configuration property

A new configuration property sasl.callback.handlers will be added to enable new callback handlers to be specified for brokers and clients. This will be a list of classes that implement the interface. A different handler may be provided for each enabled mechanism.

Callback Handler

The callback handler interface AuthenticateCallbackHandler will extend the standard interface, enabling the handler to be passed directly to SaslServer/SaslClient implementations. The callback handler configured for a mechanism must include the callbacks as described below:

  1. If using a SaslServer/SaslClient implementation from the JRE, the callbacks required for the mechanism are described in the Java SASL reference.
  2. When using the SaslServer/SaslClient implementation included in Kafka (PLAIN or SCRAM), the callback defined below for the SASL mechanism must be handled.
  3. Applications using custom implementations of SaslServer/SaslClient may define their own callbacks.

Callback handlers which require additional options at runtime (eg. URL of a credential server) may include arguments in the JAAS configuration using the config file or sasl.jaas.config property (KIP-85). This is similar to the way keytab location is configured for GSSAPI. Client callback handlers can retrieve Subject using Subject.getSubject(AccessController.getContext()) to obtain credentials populated by the login module.


SASL/PLAIN Server Callbacks

SASL/PLAIN servers using the SaslServer implementation included in Kafka must handle NameCallback and PlainAuthenticateCallback. The username for authentication is provided in NameCallback similar to other mechanisms in the JRE (eg. Digest-MD5). The password provided by the client during SASL authentication is provided in PlainAuthenticateCallback. The callback handler sets authenticated flag in the callback after verifying username and password.

SASL/PLAIN server callback handler is requested to handle NameCallback and PlainAuthenticateCallback in that order. See the sample SASL/PLAIN callback handler for an example.

SASL/SCRAM Server Callbacks

SASL/SCRAM servers using the SaslServer implementation included in Kafka must handle NameCallback and ScramCredentialCallback. The username for authentication is provided in NameCallback similar to other mechanisms in the JRE (eg. Digest-MD5). The callback handler must return SCRAM credential for the user if credentials are available for the username for the configured SCRAM mechanism.

SASL/SCRAM server callback handler is requested to handle NameCallback and ScramCredentialCallback in that order. See the sample SASL/SCRAM callback handler for an example.


SASL/PLAIN callbacks enable authentication of passwords using an external authentication server without requiring Kafka to have knowledge of actual passwords. The callback checks if the (user, password) combination provided during SASL/PLAIN exchange is valid. SASL/SCRAM callbacks do require the salted credentials to perform the SCRAM authentication and hence the credentials are requested in the callback.


Proposed Changes

ChannelBuilder  will create an instance of each configured callback handler using the default constructor. For mechanisms without a callback handler override, the existing default callback handlers (SaslServerCallbackHandler/SaslClientCallbackHandler) will be created. Callback handler instances will be created once for each enabled mechanism in ChannelBuilder, instead of per-connection. This enables callback handlers using external authentication servers to cache credentials or reuse connections if required. SaslClientCallbackHandler will be modified to obtain Subject using Subject.getSubject(AccessController.getContext()) to avoid the current per-connection state.


Use an external authentication server for SASL/PLAIN authentication using the SaslServer implementation for PLAIN included in Kafka 

 Define a new class that implements AuthenticateCallbackHandler  which handles NameCallback and PlainAuthenticateCallback and add the class to the broker's sasl.callback.handlers property. A single instance of this callback handler will be created for the broker. The configured callback handler is responsible for validating the password provided by clients and this may use an external authentication server.

Use custom credential store instead of Zookeeper for storing SCRAM credentials

Set broker callback handler to a class that implements AuthenticateCallbackHandler  which handles NameCallback and ScramCredentialCallback. SCRAM credentials from a custom store can be returned by the callback handler.

Use a custom SaslServer implementation for SCRAM

If a custom SaslServer implementation is used instead of the one included in Kafka, the custom implementation may require a different set of callbacks. A callback handler for these callbacks may be specified in sasl.callback.handlers.

Configure a new mechanism not included in Kafka using custom SaslServer/SaslClient

A handler that handles any callbacks required for these server/client implementations may be specified in sasl.callback.handlers for brokers and clients

Configure a new mechanism using the implementation provided by the JRE

Callbacks defined for the mechanism in the Java implementation must be handled by custom callback handlers if the behaviour differs from the default callbacks in Kafka.

Configure callbacks for different mechanisms on different listeners in the broker

KIP-103 introduced support for multiple listeners in the broker for the same security protocol. This allows brokers to configure different SASL mechanisms for internal and external traffic. The listener name prefix can be applied to sasl.callback.handlers to define different callback handlers for each of the listeners.


Sample Callback Handler for SASL/PLAIN

Sample SASL/PLAIN Callback Handler


For custom SASL/PLAIN authentication, override authenticate() with custom implementation that verifies the given password for username.

Sample Callback Handler for SASL/SCRAM

Sample SASL/SCRAM Callback Handler


For custom credential store for SCRAM, override credential() with alternative method that obtains credential from the custom store. If custom credential store supports a smaller subset of SCRAM mechanisms (eg. only SCRAM-SHA-256), override supportedSaslMechanisms() as well.

Compatibility, Deprecation, and Migration Plan

  • What impact (if any) will there be on existing users?


  • If we are changing behavior how will we phase out the older behavior?

Existing behaviour will be retained as default

Test Plan

Existing integration and system tests will test the default behaviour. Additional unit and integration tests will be added to test the new configuration:

  1. Test that PLAIN credential provider can be replaced
  2. Test that SCRAM password store can be replaced
  3. Test that new mechanisms not included in Kafka can be run with custom callback handlers

Rejected Alternatives

Define a new credential provider interface instead of using CallbackHandler

The format of credentials required for each mechanism is different. For SASL/PLAIN, the proposed callback handler can be used with external authentication servers which validate passwords without allowing the broker to retrieve password for a user. For SASL/SCRAM, the hashed, salted credentials required for the mechanism are provided to the broker. Different credential providers can be defined to capture these differences, which may make the interface slightly simpler in these two cases compared to callback handlers. But the use of standard Java CallbackHandler interface is more flexible and future-proof since it is the interface used by SaslServer/SaslClient implementations to obtain application-specific data.

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