The rtsol(8) and rtsold(8) programs do not validate the domain search list options provided in router advertisement messages; the option body is passed to resolvconf(8) unmodified. resolvconf(8) is a shell script which does not validate its input. A lack of quoting meant that shell commands pass as input to resolvconf(8) may be executed.
https://security.freebsd.org/advisories/FreeBSD-SA-25:12.rtsold.asc