Kpathsea is a library to do path searching. It is used by TeX Live and others TeX related software.
It was discovered that the mpost program from the shell_escape_commands list is capable of executing arbitrary external programs during the conversion of .tex files. The responsible function is runpopen() (texmfmp.c).
A remote attacker, by enticing a user to open a specially crafted .tex file, could possibly execute arbitrary code with the privileges of the process.
There is no known workaround at this time.
All Kpathsea users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose
">=dev-libs/kpathsea-6.2.2_p20160523"
Packages which depend on this library may need to be recompiled. Tools such as revdep-rebuild may assist in identifying some of these packages.