MAN DB is a man replacement that utilizes BerkelyDB instead of flat files.
The /var/cache/man directory as part of the MAN DB package has group permissions set to root.
A local user who does not belong to the root group, but has the ability to modify the /var/cache/man directory can escalate privileges to the group root.
There is no known workaround at this time.
All MAN DB users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=sys-apps/man-db-2.7.6.1-r2:0"