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From: FreeBSD Security Advisories (security-advisoriesfreebsd.org)
Date: Wed May 29 2002 - 11:36:31 CDT

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    FreeBSD-SA-02:26.accept Security Advisory
                                                              The FreeBSD Project

    Topic: Remote denial-of-service when using accept filters

    Category: core
    Module: kernel
    Announced: 2002-05-29
    Credits: Mike Silbersack <silbyFreeBSD.org>
    Affects: FreeBSD 4.5-RELEASE
                    FreeBSD 4-STABLE after 2001-11-22 and prior to the
                      correction date
    Corrected: 2002-05-21 18:03:16 UTC (RELENG_4)
                    2002-05-28 18:27:55 UTC (RELENG_4_5)
    FreeBSD only: YES

    I. Background

    FreeBSD features an accept_filter(9) mechanism which allows an
    application to request that the kernel pre-process incoming connections.
    For example, the accf_http(9) accept filter prevents accept(2) from
    returning until a full HTTP request has been buffered.

    No accept filters are enabled by default. A system administrator must
    either compile the FreeBSD kernel with a particular accept filter
    option (such as ACCEPT_FILTER_HTTP) or load the filter using
    kldload(8) in order to utilize accept filters.

    II. Problem Description

    In the process of adding a syncache to FreeBSD, mechanisms to remove
    entries from the incomplete listen queue were removed, as only sockets
    undergoing accept filtering now use the incomplete queue.

    III. Impact

    By simply connecting to a socket using accept filtering and holding a
    few hundred sockets open (~190 with the default backlog value), one
    may deny access to a service. In addition to malicious users, this
    affect has also been reported to be caused by worms such as Code Red
    which generate URLs that do not meet the http accept filter's

    Systems are not affected by this bug unless they have enabled accept
    filters in the kernel and are utilizing an application configured to
    take advantage of this feature. Apache (versions 1.3.14 and later) is
    the only application known to utilize accept filters by default.

    IV. Workaround

    Do not use accept filters. If you have enabled the ACCEPT_FILTER_DATA
    or ACCEPT_FILTER_HTTP options in your kernel, remove these options and
    recompile your kernel as described in
    <URL:http://www.freebsd.org/handbook/kernelconfig.html> and reboot the
    system. If you have loaded one of the kernel accept filters by using
    kldload(8), then you must modify your startup scripts not to load
    these modules and reboot your system. You may list loaded kernel
    modules by using kldstat(8). If loaded, the HTTP accept filter will
    be listed as `accf_http.ko', and the Data accept filter will be listed
    as `accf_data.ko'.

    For affected versions of Apache, accept filters may be disabled either
    by adding the directive ``AcceptFilter off'' to your configuration
    file, or via a compile-time option, depending upon the version.
    Please see the Apache documentation for details.

    V. Solution

    1) Upgrade your vulnerable system to 4.5-STABLE; or to the RELENG_4_5
    (4.5-RELEASE-p6) security branch dated after the respective correction

    2) To patch your present system:

    The following patch has been verified to apply to FreeBSD 4.5-RELEASE
    and 4.5-STABLE systems.

    a) Download the relevant patch from the location below, and verify the
    detached PGP signature using your PGP utility.

    # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:26/accept.patch
    # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:26/accept.patch.asc

    b) Execute the following commands as root:

    # cd /usr/src
    # patch < /path/to/patch

    c) Recompile your kernel as described in
    <URL:http://www.freebsd.org/handbook/kernelconfig.html> and reboot the

    VI. Correction details

    The following list contains the revision numbers of each file that was
    corrected in FreeBSD.

    Path Revision
    - -------------------------------------------------------------------------
    - -------------------------------------------------------------------------

    VII. References

    Version: GnuPG v1.0.7 (FreeBSD)

    -----END PGP SIGNATURE-----

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