FreeBSD Security Advisory FreeBSD-SA-01:32.ipfilter [REVISED]

FreeBSD Security Advisories security-advisories at
Thu Apr 19 12:24:11 PDT 2001


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 at>
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
# 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.

Version: GnuPG v1.0.4 (FreeBSD)
Comment: For info see


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