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

Topic:          many out-of-sequence TCP packets denial-of-service

Category:       core
Module:         kernel
Announced:      2004-03-02
Credits:        iDEFENSE
Affects:        All FreeBSD releases
Corrected:      2004-03-02 17:19:18 UTC (RELENG_4)
                2004-03-02 17:24:46 UTC (RELENG_5_2, 5.2.1-RELEASE-p1)
                2004-03-02 17:26:33 UTC (RELENG_4_9, 4.9-RELEASE-p3)
                2004-03-02 17:27:47 UTC (RELENG_4_8, 4.8-RELEASE-p16)
CVE Name:       CAN-2004-0171
FreeBSD only:   NO

I.   Background

The Transmission Control Protocol (TCP) of the TCP/IP protocol suite
provides a connection-oriented, reliable, sequence-preserving data
stream service.  When network packets making up a TCP stream (``TCP
segments'') are received out-of-sequence, they are maintained in a
reassembly queue by the destination system until they can be re-ordered
and re-assembled.

II.  Problem Description

FreeBSD does not limit the number of TCP segments that may be held in a
reassembly queue.

III. Impact

A remote attacker may conduct a low-bandwidth denial-of-service attack
against a machine providing services based on TCP (there are many such
services, including HTTP, SMTP, and FTP).  By sending many
out-of-sequence TCP segments, the attacker can cause the target machine
to consume all available memory buffers (``mbufs''), likely leading to
a system crash.

IV.  Workaround

It may be possible to mitigate some denial-of-service attacks by
implementing timeouts at the application level.

V.   Solution

Do one of the following:

1) Upgrade your vulnerable system to 4-STABLE, or to the RELENG_5_2,
RELENG_4_9, or RELENG_4_8 security branch dated after the correction
date.

OR

2) Patch your present system:

The following patch has been verified to apply to FreeBSD 4.x and 5.x
systems.

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

[FreeBSD 5.2]
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-04:04/tcp52.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-04:04/tcp52.patch.asc

[FreeBSD 4.8, 4.9]
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-04:04/tcp47.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-04:04/tcp47.patch.asc

b) Apply the patch.

# 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
system.

VI.  Correction details

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

Branch                                                           Revision
  Path
- -------------------------------------------------------------------------
RELENG_4
  src/UPDATING                                                  1.73.2.90
  src/sys/conf/newvers.sh                                       1.44.2.33
  src/sys/netinet/tcp_input.c                                  1.107.2.40
  src/sys/netinet/tcp_subr.c                                    1.73.2.33
  src/sys/netinet/tcp_var.h                                     1.56.2.15
RELENG_5_2
  src/UPDATING                                                  1.282.2.9
  src/sys/conf/newvers.sh                                        1.56.2.8
  src/sys/netinet/tcp_input.c                                   1.217.2.2
  src/sys/netinet/tcp_subr.c                                    1.169.2.4
  src/sys/netinet/tcp_var.h                                      1.93.2.2
RELENG_4_9
  src/UPDATING                                              1.73.2.89.2.4
  src/sys/conf/newvers.sh                                   1.44.2.32.2.4
  src/sys/netinet/tcp_input.c                              1.107.2.38.2.1
  src/sys/netinet/tcp_subr.c                                1.73.2.31.4.1
  src/sys/netinet/tcp_var.h                                 1.56.2.13.4.1
RELENG_4_8
  src/UPDATING                                             1.73.2.80.2.19
  src/sys/conf/newvers.sh                                  1.44.2.29.2.17
  src/sys/netinet/tcp_input.c                              1.107.2.37.2.1
  src/sys/netinet/tcp_subr.c                                1.73.2.31.2.1
  src/sys/netinet/tcp_var.h                                 1.56.2.13.2.1
- -------------------------------------------------------------------------

VII. References

<URL:http://www.idefense.com/application/poi/display?id=78&type=vulnerabilities>
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