Apache HDFS ACLs
Also available as:

Configuring ACLs on HDFS

How to configure ACLs on HDFS.

Only one property needs to be specified in the hdfs-site.xml file in order to enable ACLs on HDFS:
dfs.namenode.acls.enabled = true

Set this property to "true" to enable support for ACLs. ACLs are disabled by default. When ACLs are disabled, the NameNode rejects all attempts to set an ACL.