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FreeBSD-SA-20:04.tcp Security Advisory
The FreeBSD Project

Topic: TCP IPv6 SYN cache kernel information disclosure

Category: core
Module: tcp
Announced: 2020-03-19
Credits: Michael Tuexen (Netflix, contractor)
Affects: All supported versions of FreeBSD.
Corrected: 2020-03-08 14:48:21 UTC (stable/12, 12.1-STABLE)
2020-03-19 16:46:01 UTC (releng/12.1, 12.1-RELEASE-p3)
2020-03-08 14:48:32 UTC (stable/11, 11.3-STABLE)
2020-03-19 16:46:01 UTC (releng/11.3, 11.3-RELEASE-p7)
CVE Name: CVE-2020-7451

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

The Internet Protocol version 6 (IPv6) header contains a one byte field
called Traffic Class. Two bits of this field are used for Explicit
Congestion Notification (ECN), the other six bits are used as Differentiated
Services Field Codepoints (DSCP).

The Transmission Control Protocol (TCP) is a connection oriented transport
protocol, which can be used as an upper layer of IPv6. A TCP endpoint is
either acting as a client (sending initially a SYN segment) or as a server
(initially waiting to receive a SYN segment and then responding with a
SYN-ACK segment).

To mitigate the impact of some attacks against TCP servers (like
SYN-flooding), FreeBSD uses specific code to handle the TCP connection setup
for servers. This includes the transmission and retransmission of SYN-ACK
segments or responding with a challenge ACK segment to a received RST
segment.

II. Problem Description

When a TCP server transmits or retransmits a TCP SYN-ACK segment over IPv6,
the Traffic Class field is not initialized. This also applies to challenge ACK
segments, which are sent in response to received RST segments during the TCP
connection setup phase.

III. Impact

For each TCP SYN-ACK (or challenge TCP-ACK) segment sent over IPv6, one byte
of kernel memory is transmitted over the network.

IV. Workaround

No workaround is available. Systems not using IPv6 are unaffected.

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-20:04/tcp.patch
# fetch https://security.FreeBSD.org/patches/SA-20:04/tcp.patch.asc
# gpg –verify tcp.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/12/ r358739
releng/12.1/ r359138
stable/11/ r358740
releng/11.3/ r359138
– ————————————————————————-

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-2020-7451>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:04.tcp.asc>
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