Email was received stating
If you are currently using 4.6.7 and your site uses ACLs to segment access to contacts you are strongly encouraged to upgrade. The 4.6.7 release included a regression in the ACL system which caused certain contact access permissions to behave improperly. The 4.4 LTS branch was unaffected.
There is no additional information provided at https://civicrm.org/advisory, and I don't see any issues in the Jira queue marked with a 4.6.8 fix version that would give more context for site administrators to judge whether an immediate upgrade is necessary or if it could be delayed.