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FreeBSD-SA-20:24.ipv6 Security Advisory
The FreeBSD Project

Topic: IPv6 Hop-by-Hop options use-after-free bug

Category: core
Module: kernel
Announced: 2020-09-02
Affects: FreeBSD 11.3
Corrected: 2020-05-07 01:28:59 UTC (stable/11, 11.4-PRERELEASE)
2020-09-02 16:23:15 UTC (releng/11.3, 11.3-RELEASE-p13)
CVE Name: CVE-2020-7462

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I. Background

IPv6 is a network layer supporting Hop-by-Hop options, which can be sent by
applications via the socket API. The memory management for packet handling
is done using mbufs.

II. Problem Description

Due to improper mbuf handling in the kernel, a use-after-free bug might be
triggered by sending IPv6 Hop-by-Hop options over the loopback interface.

III. Impact

Triggering the use-after-free situation may result in unintended kernel
behaviour including a kernel panic.

IV. Workaround

No workaround is available.

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 i386 or amd64
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.

# fetch https://security.FreeBSD.org/patches/SA-20:24/ipv6.patch
# fetch https://security.FreeBSD.org/patches/SA-20:24/ipv6.patch.asc
# gpg –verify ipv6.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
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI. Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path Revision
– ————————————————————————-
stable/11/ r360733
releng/11.3/ r365255
– ————————————————————————-

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN –summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-7462>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:24.ipv6.asc>
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