In FreeBSD 12.0-RELEASE before 12.0-RELEASE-p13, a missing check in the ipsec packet processor allows reinjection of an old packet to be accepted by the ipsec endpoint. Depending on the higher-level protocol in use over ipsec, this could allow an action to be repeated.
https://security.FreeBSD.org/advisories/FreeBSD-SA-20:02.ipsec.asc