mirrorselect is a tool to help select distfiles mirrors for Gentoo.
Ervin Nemeth discovered that mirrorselect creates temporary files in world-writable directories with predictable names.
A local attacker could create symbolic links in the temporary files directory, pointing to a valid file somewhere on the filesystem. When mirrorselect is executed, this would result in the file being overwritten with the rights of the user running the utility, which could be the root user.
There is no known workaround at this time.
All mirrorselect users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=app-portage/mirrorselect-0.89"