Headline
CVE-2023-4809
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.
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 ============================================================================= FreeBSD-SA-23:10.pf Security Advisory The FreeBSD Project Topic: pf incorrectly handles multiple IPv6 fragment headers Category: core Module: pf Announced: 2023-09-06 Credits: Enrico Bassetti [email protected] (NetSecurityLab @ Sapienza University of Rome) Affects: All supported versions of FreeBSD. Corrected: 2023-08-04 14:08:05 UTC (stable/13, 13.2-STABLE) 2023-09-06 16:58:39 UTC (releng/13.2, 13.2-RELEASE-p3) 2023-08-04 14:14:08 UTC (stable/12, 12.4-STABLE) 2023-09-06 17:38:31 UTC (releng/12.4, 12.4-RELEASE-p5) CVE Name: CVE-2023-4809 For general information regarding FreeBSD Security Advisories, including descriptions of the fields above, security branches, and the following sections, please visit . I. Background pf is an Internet Protocol packet filter originally written for OpenBSD. pf can reassemble fragmented IPv6 packets in order to apply rules on the reassembled packet. This allows pf to filter based on the upper layer protocol (e.g. TCP, UDP) information. IPv6 packets may be fragmented by the originating node, and will then contain a fragment extension header. An IPv6 packet will normally contain only one fragment extension header. II. Problem Description 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. III. Impact IPv6 fragments may bypass firewall rules written on the assumption all fragments have been reassembled and, as a result, be forwarded or processed by the host. IV. Workaround No workaround is available but systems not using the pf firewall are not affected. V. Solution Upgrade your vulnerable system to a supported FreeBSD stable or release / security branch (releng) dated after the correction date and reboot. Perform one of the following: 1) To update your vulnerable system via a binary patch: Systems running a RELEASE version of FreeBSD on the amd64, i386, or (on FreeBSD 13 and later) arm64 platforms can be updated via the freebsd-update(8) utility: # freebsd-update fetch # freebsd-update install # shutdown -r +10min “Rebooting for a security update” 2) To update your vulnerable system via a source code patch: The following patches have been verified to apply to the applicable FreeBSD release branches. a) Download the relevant patch from the location below, and verify the detached PGP signature using your PGP utility. [FreeBSD 13.2] # fetch https://security.FreeBSD.org/patches/SA-23:10/pf.13.patch # fetch https://security.FreeBSD.org/patches/SA-23:10/pf.13.patch.asc # gpg --verify pf.13.patch.asc [FreeBSD 12.4] # fetch https://security.FreeBSD.org/patches/SA-23:10/pf.12.patch # fetch https://security.FreeBSD.org/patches/SA-23:10/pf.12.patch.asc # gpg --verify pf.12.patch.asc b) Apply the patch. Execute the following commands as root: # cd /usr/src # patch < /path/to/patch c) Recompile your kernel as described in and reboot the system. VI. Correction details This issue is corrected by the corresponding Git commit hash or Subversion revision number in the following stable and release branches: Branch/path Hash Revision - ------------------------------------------------------------------------- stable/13/ 3a0461f23a4f stable/13-n255953 releng/13.2/ 41b7760991ef releng/13.2-n254631 stable/12/ r373157 releng/12.4/ r373186 - ------------------------------------------------------------------------- For FreeBSD 13 and later: Run the following command to see which files were modified by a particular commit: # git show --stat Or visit the following URL, replacing NNNNNN with the hash: To determine the commit count in a working tree (for comparison against nNNNNNN in the table above), run: # git rev-list --count --first-parent HEAD For FreeBSD 12 and earlier: Run the following command to see which files were modified by a particular revision, replacing NNNNNN with the revision number: # svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base Or visit the following URL, replacing NNNNNN with the revision number: VII. References The latest revision of this advisory is available at -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEthUnfoEIffdcgYM7bljekB8AGu8FAmT4vykACgkQbljekB8A Gu9Mow//ZodkaAf0AGC2T+CSDco592Mq7+T8V5YyqIZxGXRn55sFuVKS8cQ8a0cT DJ98QV0ht0WITYrBPGbllzVvT4w3bos1U0SB2z3tPjrbfNL8vaXgVl/Du3KZaPAs 0h4fNR/R3b6XzHgFhqYKG8Q7/u21fLmwu9HpYHQ7nplWg2mS2uQeuTMtr+uoOBS2 XPc/FpYtL2VXO2aEY3K1A/QCY6lBRxqKTTEi01j9gnyuK4L3QoLWqDdrAKM3RoDc wmstnn/KQAJkeMnmIOmDh0GdnAVdVyPBdI0KM86pz5L0AT0uQib0sal0yj72kCsg oi6flocqESDNzYPgh/nZEjCHzcRhGWxcsjhTzjBQSTW/HSarQ+wbZuIpUlUQG3A6 oEhRBj201t4+FUSwCQfr5QdivxwtMHHJYSXqo4nyD3AsRQ2HTnFNcqq26h+bgjhR HmdBvffQ5lQUrtDKDb4XXr8RLFbk2RmjeD/zZfb1zhezSmJi4cD6LrClxer5aRFo djoqVwjzKsg/9gLaDqr/UDObF6Ke6hs03yTs1Hjrp/DV29wWjJ8NKShezIEJOPTm lgK+jhcEbs5vR4woG3vll7Jfaz7W8vniM9cOz/7bvWOp924cHMmwWFod4DMVf9ry USB3v/ClFl5caJnoYYwKiIfc/EyYrprTvMLcO6yzDkhWUlaws88= =bpCy -----END PGP SIGNATURE-----