In FreeBSD 11.3-PRERELEASE before r345378, 12.0-STABLE before r345377, 11.2-RELEASE before 11.2-RELEASE-p10, and 12.0-RELEASE before 12.0-RELEASE-p4, a bug in pf does not check if the outer ICMP or ICMP6 packet has the same destination IP as the source IP of the inner protocol packet allowing a maliciously crafted ICMP/ICMP6 packet could bypass the packet filter rules and be passed to a host that would otherwise be unavailable.
https://www.synacktiv.com/posts/systems/icmp-reachable.html
https://www.oracle.com/technetwork/security-advisory/cpujul2019-5072835.html
https://security.netapp.com/advisory/ntap-20190611-0001/
https://security.FreeBSD.org/advisories/FreeBSD-SA-19:06.pf.asc
http://www.securityfocus.com/bid/108395
http://packetstormsecurity.com/files/152934/FreeBSD-Security-Advisory-FreeBSD-SA-19-06.pf.html