TUCoPS :: BSD :: ciacl075.txt

FreeBSD ipfilter vulnerability



                       The U.S. Department of Energy
                     Computer Incident Advisory Center
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

                 FreeBSD IPFilter May Incorrectly Pass Packets
                 [FreeBSD Bulletin FreeBSD-SA-01:32 [Revised]]

April 20, 2001 00:00 GMT                                          Number L-075
PROBLEM:       The IPFilter package is used to implement the FreeBSD firewall 
               function. A vulnerability may allow packets to bypass the 
PLATFORM:      FreeBSD 3.x (all releases), FreeBSD 4.x (all releases), FreeBSD 
               3.5-STABLE, and 4.2-STABLE prior to the correction date that 
               use the IPFilter function. 
DAMAGE:        A malicious user could create packets that would bypass the 
SOLUTION:      If you use the IPFilter function, install the patches as shown 
               in the FreeBSD bulletin FreeBSD-SA-01:32 
VULNERABILITY  The risk is LOW. An intruder would have to know the state 
ASSESSMENT:    information of existing packet streams in order to bypass the 
               firewall function. If you do not use the IPFilter function, you 
               are not impacted. 

   [***** Start FreeBSD Bulletin *****]

FreeBSD-SA-01:32                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:          IPFilter may incorrectly pass packets [REVISED]

Category:       core
Module:         IPFilter
Announced:      2001-04-16
Revised:        2001-04-19
Credits:        Thomas Lopatic <thomas@lopatic.de>
Affects:        FreeBSD 3.x (all releases), FreeBSD 4.x (all releases),
                FreeBSD 3.5-STABLE, and 4.2-STABLE prior to the
                correction date.
Corrected:      2001-04-07 (FreeBSD 4.2-STABLE)
Vendor status:  Corrected
FreeBSD only:   NO

0.   Revision History

v1.0  2001-04-16  Initial release
v1.1  2001-04-19  Corrected patch location

I.   Background

IPFilter is a multi-platform packet filtering package.

II.  Problem Description

When matching a packet fragment, insufficient checks were performed
to ensure the fragment is valid.  In addition, the fragment cache is
checked before any rules are checked.  Even if all fragments are
blocked with a rule, fragment cache entries can be created by
packets that match currently held state information.  Because of
these discrepancies, certain packets may bypass filtering rules.

All versions of FreeBSD prior to the correction date, including
FreeBSD 3.5.1 and 4.2, contain this problem.  The base system that
will ship with FreeBSD 4.3 does not contain this problem since it
was corrected during the beta cycle before the release.

III. Impact

Malicious remote users may be able to bypass filtering rules, allowing
them to potentially circumvent the firewall.

IPFilter is not enabled by default.  If you have not enabled IPFilter,
your system is not vulnerable to this problem.

IV.  Workaround

Since fragment cache matching occurs before filtering rules checking,
it is not possible to work around this problem using IPFilter rules.

V.   Solution

[FreeBSD 3.x]

Due to the age of the IPFilter package shipped with FreeBSD 3.x, it
is recommended that FreeBSD 3.x systems update to IPFilter 3.4.17
using the package available from the authors website:

[FreeBSD 4.x]

One of the following:

1) Upgrade to FreeBSD 4.2-STABLE after the correction date.

2) Download the patch and detached PGP signature from the following

The following patch applies to FreeBSD 4.1-RELEASE through 4.2-STABLE.

# fetch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-01:32/ipfilter.patch
# fetch

Verify the detached signature using your PGP utility.

Issue the following commands as root:

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

If the system is using ipfilter as a kernel module, the module may be
rebuilt and installed and ipfilter rules reloaded with the following

# cd /usr/src/sys/modules/ipfilter
# make all install
# kldunload ipl && kldload ipf && ipf -Fa -f /etc/ipf.rules

Otherwise, if ipfilter is compiled into the kernel, a new kernel will
need to be compiled and installed and the system will have to be
rebooted for the changes to take effect.

  [*****End FreeBSD Bulletin*****]

CIAC wishes to acknowledge the contributions of FreeBSD for the 
information contained in this bulletin.

CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC
can be contacted at:
    Voice:    +1 925-422-8193 (7x24)
    FAX:      +1 925-423-8002
    STU-III:  +1 925-423-2604
    E-mail:   ciac@ciac.org

Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.

   World Wide Web:      http://www.ciac.org/
                        (or http://ciac.llnl.gov -- they're the same machine)
   Anonymous FTP:       ftp.ciac.org
                        (or ciac.llnl.gov -- they're the same machine)

PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins.  If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at http://www.first.org/.

This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial products,
process, or service by trade name, trademark, manufacturer, or
otherwise, does not necessarily constitute or imply its endorsement,
recommendation or favoring by the United States Government or the
University of California. The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government or the University of California, and shall not be used for
advertising or product endorsement purposes.

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Version: PGP for Business Security 5.5.2


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