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Category:   OS (UNIX)  >   TCP/IP Stack Implementation Vendors:   FreeBSD
FreeBSD TCP SACK Processing May Let Remote Users Deny Service
SecurityTracker Alert ID:  1015566
SecurityTracker URL:  http://securitytracker.com/id/1015566
CVE Reference:   CVE-2006-0433   (Links to External Site)
Date:  Feb 2 2006
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 5.3, 5.4
Description:   A vulnerability was reported in FreeBSD in the TCP/IP stack. A remote user can cause denial of service conditions.

If there is insufficient memory available on the target system to process an incoming selective acknowledgement (SACK), the TCP/IP stack may enter an infinite loop. A remote user can open a TCP connection to the target system and send a specially crafted series of packets to cause denial of service conditions.

Scott Wood is credited with reporting this vulnerability.

Impact:   A remote user can cause a denial of service conditions on the target system.
Solution:   The vendor has issued the following solution instructions [quoted]:

Perform one of the following:

1) Upgrade your vulnerable system to 5-STABLE or to the RELENG_5_4 or
RELENG_5_3 security branch dated after the correction date.

2) To patch your present system:

The following patch have been verified to apply to FreeBSD 5.3 and
5.4 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-06:08/sack.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:08/sack.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.

The vendor's advisory is available at:

ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-06:08.sack.asc

Vendor URL:  www.freebsd.org/ (Links to External Site)
Cause:   State error

Message History:   None.


 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-06:08.sack

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

=============================================================================
FreeBSD-SA-06:08.sack                                       Security Advisory
                                                          The FreeBSD Project

Topic:          Infinite loop in SACK handling

Category:       core
Module:         netinet
Announced:      2006-02-01
Credits:        Scott Wood
Affects:        FreeBSD 5.3 and 5.4
Corrected:      2006-01-24 01:16:18 UTC (RELENG_5, 5.4-STABLE)
                2006-02-01 19:43:10 UTC (RELENG_5_4, 5.4-RELEASE-p11)
                2006-02-01 19:43:36 UTC (RELENG_5_3, 5.3-RELEASE-p26)
CVE Name:       CVE-2006-0433

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit
<URL:http://www.freebsd.org/security/>.

I.   Background

SACK (Selective Acknowledgement) is an extension to the TCP/IP protocol
that allows hosts to acknowledge the receipt of some, but not all, of
the packets sent, thereby reducing the cost of retransmissions.

II.  Problem Description

When insufficient memory is available to handle an incoming selective
acknowledgement, the TCP/IP stack may enter an infinite loop.

III. Impact

By opening a TCP connection and sending a carefully crafted series of
packets, an attacker may be able to cause a denial of service.

IV.  Workaround

On FreeBSD 5.4, the net.inet.tcp.sack.enable sysctl can be used to
disable the use of SACK:

# sysctl net.inet.tcp.sack.enable=0

No workaround is available for FreeBSD 5.3.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to 5-STABLE or to the RELENG_5_4 or
RELENG_5_3 security branch dated after the correction date.

2) To patch your present system:

The following patch have been verified to apply to FreeBSD 5.3 and
5.4 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-06:08/sack.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-06:08/sack.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_5
  src/sys/netinet/tcp_sack.c                                     1.3.2.10
RELENG_5_4
  src/UPDATING                                            1.342.2.24.2.20
  src/sys/conf/newvers.sh                                  1.62.2.18.2.16
  src/sys/netinet/tcp_sack.c                                  1.3.2.5.2.1
RELENG_5_3
  src/UPDATING                                            1.342.2.13.2.29
  src/sys/conf/newvers.sh                                  1.62.2.15.2.31
  src/sys/netinet/tcp_sack.c                                      1.3.4.1
- -------------------------------------------------------------------------

VII. References

http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-0433

The latest revision of this advisory is available at
ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-06:08.sack.asc
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Version: GnuPG v1.4.2 (FreeBSD)

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