...

// this test is to clarify that the issue exists for the consumer group command line only, and not the API
@Test
def testDescribeGroupApiWithGroupDescribe() {
  addAndVerifyAcls(Set(new Acl(KafkaPrincipal.ANONYMOUS, Allow, Acl.WildCardHost, Describe)), groupResource)
  addAndVerifyAcls(Set(new Acl(KafkaPrincipal.ANONYMOUS, Allow, Acl.WildCardHost, Describe)), topicResource)
  AdminClient.createSimplePlaintext(brokerList).describeConsumerGroup(group)
}

// this test highlights the issue with command line, where the supposedly sufficient 'Describe' access is not enough to run the command
@Test(expected = classOf[GroupAuthorizationException])
def testDescribeGroupCliWithGroupDescribe() {
  addAndVerifyAcls(Set(new Acl(KafkaPrincipal.ANONYMOUS, Allow, Acl.WildCardHost, Describe)), groupResource)
  addAndVerifyAcls(Set(new Acl(KafkaPrincipal.ANONYMOUS, Allow, Acl.WildCardHost, Describe)), topicResource)

  val cgcArgs = Array("--bootstrap-server", brokerList, "--describe", "--group", group)
  val opts = new ConsumerGroupCommandOptions(cgcArgs)
  val consumerGroupService = new KafkaConsumerGroupService(opts)
  consumerGroupService.describeGroup()
}

// this test confirms that a minimum of 'Read' access is required to successfully run the command
@Test
def testDescribeGroupCliWithGroupRead() {
  addAndVerifyAcls(Set(new Acl(KafkaPrincipal.ANONYMOUS, Allow, Acl.WildCardHost, Read)), groupResource)
  addAndVerifyAcls(Set(new Acl(KafkaPrincipal.ANONYMOUS, Allow, Acl.WildCardHost, Describe)), topicResource)

  val cgcArgs = Array("--bootstrap-server", brokerList, "--describe", "--group", group)
  val opts = new ConsumerGroupCommandOptions(cgcArgs)
  val consumerGroupService = new KafkaConsumerGroupService(opts)
  consumerGroupService.describeGroup()
}

Proposed Changes

The change proposed by this KIP is very simple: to lower the minimum required permission of the OffsetFetch API from Read to Describe. These minimum required permissions are hard-coded in kafka.server.KafkaApis.scala inside each API handler method. For example, the part that enforces the minimum required permission for the OffsetFetch API currently looks like this:

...

if (!authorize(request.session, Describe, new Resource(Group, offsetFetchRequest.groupId))) 
          offsetFetchRequest.getErrorResponse(requestThrottleMs, Errors.GROUP_AUTHORIZATION_FAILED)

Additional Food for Thought

As I was experimenting with the ACLs for this KIP I made some observations that made me wonder about the rationale behind their current ACL settings; and whether they need adjustments too: 

Users can see all groups in the cluster (using consumer group’s --list option) provided that they have Describe access to the cluster. Would it make sense to modify that experience and limit what is listed in the output to only those groups they have Describe access to? The reason is, almost anything else is accessible by a user only if the access is specifically granted (through ACL --add); and this scenario should not be an exception. The potential change would be updating the minimum required permission of ListGroup from Describe (Cluster) to Describe (Group), which means the implementation of KafkaApis.handleListGroupsRequest would change from

...

...

_AUTHORIZATION_FAILED

...

to something like this:

def handleListGroupsRequest(request: RequestChannel.Request) {
  val (error, groups) = groupCoordinator.handleListGroups()
  val allGroups = groups.filter { group => authorize(request.session, Describe, new Resource(Group, group.groupId)) }
                        .map { group => new ListGroupsResponse.Group(group.groupId, group.protocolType) }
  sendResponseMaybeThrottle(request, requestThrottleMs =>
    new ListGroupsResponse(requestThrottleMs, error, allGroups.asJava))
}

...

)

...

Compatibility, Deprecation, and Migration Plan

• A user that already has Read permission to a consumer group, with this change, would still be able to query the group like before (Read implies Describe). For this user the change is backward compatible.
• Consider a user with Describe access. The group Describe access implies access to DescribeGroup and FindCoordinator APIs; even though this user cannot make use of DescribeGroup, as explained above. Giving this user access to OffsetFetch API means fixing that broken experience.

In general, As a result of this change, Kafka admins may need to revisit the relevant ACLs and update them if necessary. 

Rejected Alternatives