The GNU Tar program provides the ability to create tar archives, as well as various other kinds of manipulation.
Dmitry V. Levin discovered a directory traversal vulnerability in the contains_dot_dot() function in file src/names.c.
By enticing a user to extract a specially crafted tar archive, a remote attacker could extract files to arbitrary locations outside of the specified directory with the permissions of the user running GNU Tar.
There is no known workaround at this time.
All GNU Tar users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=app-arch/tar-1.18-r2"