Further 2.3.x releases will be made for bug fixes. New features will go into the 2.4.x series, which is already under development. This was primarily a security and bug-fix release over 2.2.2. Among other things, this fixed the widely-reported security hole in verifycancels. Anyone running INN 2.0 or later was strongly encouraged to upgrade to INN 2.2.3.
More information is available in the original announcement .
Here is a longer description of the vulnerability and the recommended actions , courtesy of Russ Allbery. 1999 Version 2.2 has some new features as well as many bug fixes from version 2.1. New cnfsstat program to show stats of cnfs buffers. Note the absense of a version 1.6 in the release history. 1.5.1sec2 did, plus a lot of extra functionality.