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Ranjivost TCP protokola

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FreeBSD-SA-14:19.tcp Security Advisory
The FreeBSD Project

Topic: Denial of Service in TCP packet processing

Category: core
Module: inet
Announced: 2014-09-16
Credits: Jonathan Looney (Juniper SIRT)
Affects: All supported versions of FreeBSD.
Corrected: 2014-09-16 09:48:35UTC (stable/10, 10.1-PRERELEASE)
2014-09-16 09:48:35 UTC (stable/10, 10.1-BETA1-p1)
2014-09-16 09:50:19 UTC (releng/10.0, 10.0-RELEASE-p9)
2014-09-16 09:49:11 UTC (stable/9, 9.3-STABLE)
2014-09-16 09:50:19 UTC (releng/9.3, 9.3-RELEASE-p2)
2014-09-16 09:50:19 UTC (releng/9.2, 9.2-RELEASE-p12)
2014-09-16 09:50:19 UTC (releng/9.1, 9.1-RELEASE-p19)
2014-09-16 09:49:11 UTC (stable/8, 8.4-STABLE)
2014-09-16 09:50:19 UTC (releng/8.4, 8.4-RELEASE-p16)
CVE Name: CVE-2004-0230

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

I. Background

The Transmission Control Protocol (TCP) of the TCP/IP protocol suite
provides a connection-oriented, reliable, sequence-preserving data
stream service. New TCP connections are initiated using special SYN
flag in a datagram. Sequencing of data is controlled by 32-bit sequence
numbers, that start with a random value and are increased using modulo
2**32 arithmetic. TCP endpoints maintain a window of expected, and
thus allowed, sequence numbers for a connection.

II. Problem Description

When a segment with the SYN flag for an already existing connection arrives,
the TCP stack tears down the connection, bypassing a check that the
sequence number in the segment is in the expected window.

III. Impact

An attacker who has the ability to spoof IP traffic can tear down a
TCP connection by sending only 2 packets, if they know both TCP port
numbers. In case one of the two port numbers is unknown, a successful
attack requires less than 2**17 packets spoofed, which can be
generated within less than a second on a decent connection to the

IV. Workaround

It is possible to defend against these attacks with stateful traffic
inspection using a firewall. This can be done by enabling pf(4) on
the system and creating states for every connection. Even a default
ruleset to allow all traffic would be sufficient to mitigate this

V. Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

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

# fetch
# fetch
# gpg –verify tcp.patch.asc

b) Apply the patch. Execute the following commands as root:

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

c) Recompile your kernel as described in
<URL:> and reboot the

3) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

VI. Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path Revision
– ————————————————————————-
stable/8/ r271668
releng/8.4/ r271669
stable/9/ r271668
releng/9.1/ r271669
releng/9.2/ r271669
releng/9.3/ r271669
stable/10/ r271667
releng/10.0/ r271669
– ————————————————————————-

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN –summarize svn://

Or visit the following URL, replacing NNNNNN with the revision number:


VII. References


The latest revision of this advisory is available at

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AutorTomislav Protega
Cert idNCERT-REF-2014-09-0002-ADV
ID izvornikaFreeBSD-SA-14:19.tcp
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