klibc is a minimalistic libc used for making an initramfs.
The ipconfig utility in klibc writes DHCP options to /tmp/net-$DEVICE.conf, and this file is later sourced by other scripts to get defined variables. The options written to this file are not properly escaped.
A remote attacker could send a specially crafted DHCP reply, which could execute arbitrary shell code with the privileges of any process which sources DHCP options.
There is no known workaround at this time.
All klibc users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=dev-libs/klibc-1.5.25"