Seems that the OSSEC HIDS packages (for Debian/Ubuntu at least) have somewhere an error in updating the Linux user rights. I had the error only in German and instead of trying to translate it 100% I cut most of the German words simply away:
Starting upgrade from ossec-hids-agent 2.8.1-1wheezy (to .../ossec-hids-agent_2.8.2-1wheezy_amd64.deb) ... Replacement for ossec-hids-agent unpacking ... Removing user »ossec« ... Warning: The group »ossec« has no members anymore. userdel: Group ossec is the primary group of another user and will not be removed. Ready. /usr/sbin/delgroup: »ossecm« still has »ossec« as primary group! dpkg: Warning: Subprozess old post-removal-script returned error code 7 ... dpkg: Error ... /var/cache/apt/archives/ossec-hids-agent_2.8.2-1wheezy_amd64.deb (--unpack): ... E: Sub-process /usr/bin/dpkg returned an error code (1)
How to fix? My solution is perhaps not the best, but working: I've removed manually the OSSEC-generated users, added a new one (which gets removed during the update then):
# userdel ossecm # userdel ossecr # useradd ossec -g ossec
After these 3 lines retry the upgrade, should work :)