In pf packet processing with a 'scrub fragment reassemble' rule, a packet containing multiple IPv6 fragment headers would be reassembled, and then immediately processed. That is, a packet with multiple fragment extension headers would not be recognized as the correct ultimate payload. Instead a packet with multiple IPv6 fragment headers would unexpectedly be interpreted as a fragmented packet, rather than as whatever the real payload is.

As a result, IPv6 fragments may bypass pf firewall rules written on the assumption all fragments have been reassembled and, as a result, be forwarded or processed by the host.

References

• https://nvd.nist.gov/vuln/detail/CVE-2023-4809
• https://security.FreeBSD.org/advisories/FreeBSD-SA-23:10.pf.asc
• http://www.openwall.com/lists/oss-security/2023/09/08/5
• http://www.openwall.com/lists/oss-security/2023/09/08/6
• http://www.openwall.com/lists/oss-security/2023/09/08/7
• https://security.netapp.com/advisory/ntap-20231221-0009/