• [FreeBSD-Announce] FreeBSD Security Advisory FreeBSD-SA-20:04.tcp

    From FreeBSD Security Advisories@21:1/5 to All on Thu Mar 19 18:00:01 2020
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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> -----BEGIN PGP SIGNATURE-----

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