Review Board 1.6.19 and 1.7.15 fix a few issues in the API where users could access certain data they should not have been able to access, if using the Local Sites feature, invite-only groups, or private repositories. It also fixes cases with invite-only groups where the group name and list of private review requests would show up on some pages (though the review requests themselves were not accessible).
These issues do not affect most of the installations out there, but we strongly recommend upgrading anyway. There are no known cases of anyone exploiting these bugs, and in fact we discovered these internally while building new tools to test for security vulnerabilities in our codebase.
There are also some other bug fixes, and important changes needed for extensions that provide their own REST APIs.