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Category:   OS (UNIX)  >   FreeBSD Kernel Vendors:   FreeBSD
FreeBSD LAST_ACK State Transition Bug Lets Remote Users Consume Excessive CPU Resources on the Target System
SecurityTracker Alert ID:  1033007
SecurityTracker URL:
CVE Reference:   CVE-2015-5358   (Links to External Site)
Date:  Jul 22 2015
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 8.4, 9.3, 10.1
Description:   A vulnerability was reported in the FreeBSD Kernel. A remote user can consume excessive resources on the target system.

A remote user can establish a TCP connection with the target system and then cause the connection to transition to the LAST_ACK state while the target daemon is not finished sending data to cause the socket to get stuck in the LAST_ACK state. As a result, the remote user can consume mbufs and connections.

Lawrence Stewart (Netflix, Inc.) and Jonathan Looney (Juniper SIRT) reported this vulnerability.

Impact:   A remote user can consume excessive resources on the target system.
Solution:   The vendor has issued a fix.

The vendor's advisory is available at:

Vendor URL: (Links to External Site)
Cause:   State error

Message History:   This archive entry has one or more follow-up message(s) listed below.
Aug 27 2015 (McAfee Issues Fix for McAfee Firewall Enterprise) FreeBSD LAST_ACK State Transition Bug Lets Remote Users Consume Excessive CPU Resources on the Target System
McAfee has issued a fix for McAfee Firewall Enterprise.

 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-15:13.tcp

Hash: SHA512

FreeBSD-SA-15:13.tcp                                        Security Advisory
                                                          The FreeBSD Project

Topic:          Resource exhaustion due to sessions stuck in LAST_ACK state

Category:       core
Module:         inet
Announced:      2015-07-21
Credits:        Lawrence Stewart (Netflix, Inc.),
                Jonathan Looney (Juniper SIRT)
Affects:        All supported versions of FreeBSD.
Corrected:      2015-07-21 23:42:17 UTC (stable/10, 10.2-PRERELEASE)
                2015-07-21 23:42:17 UTC (stable/10, 10.2-BETA1-p1)
                2015-07-21 23:42:17 UTC (stable/10, 10.2-BETA2-p1)
                2015-07-21 23:42:56 UTC (releng/10.1, 10.1-RELEASE-p15)
                2015-07-21 23:42:20 UTC (stable/9, 9.3-STABLE)
                2015-07-21 23:42:56 UTC (releng/9.3, 9.3-RELEASE-p20)
                2015-07-21 23:42:20 UTC (stable/8, 8.4-STABLE)
                2015-07-21 23:42:56 UTC (releng/8.4, 8.4-RELEASE-p34)
CVE Name:       CVE-2015-5358

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.

A socket enters the LAST_ACK state when the local process closes its socket
after a FIN has already been received from the remote peer.  The socket
will remain in the LAST_ACK state until the kernel has transmitted a FIN to
the remote peer and the kernel has received an acknowledgement of that FIN
from the remote peer, or all retransmits of the FIN have failed and the
connection times out.

II.  Problem Description

TCP connections transitioning to the LAST_ACK state can become permanently
stuck due to mishandling of protocol state in certain situations, which in
turn can lead to accumulated consumption and eventual exhaustion of system
resources, such as mbufs and sockets.

III. Impact

An attacker who can repeatedly establish TCP connections to a victim system
(for instance, a Web server) could create many TCP connections that are
stuck in LAST_ACK state and cause resource exhaustion, resulting in a
denial of service condition.  This may also happen in normal operation
where no intentional attack is conducted, but an attacker who can send
specifically crafted packets can trigger this more reliably.

IV.  Workaround

No workaround is available, but systems that do not provide TCP based
service to untrusted networks are not vulnerable.

Note that the tcpdrop(8) utility can be used to purge connections which
have become wedged.  For example, the following command can be used to
generate commands that would drop all connections whose last rcvtime is
more than 100s:

	netstat -nxp tcp | \
	awk '{ if (int($NF) > 100) print "tcpdrop " $4 " " $5 }'

The system administrator can then run the generated script as a temporary
measure.  Please refer to the tcpdump(8) manual page for additional

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 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

3) 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.

[FreeBSD 10.1]
# fetch
# fetch
# gpg --verify tcp.patch.asc

[FreeBSD 9.x and 8.x]
# fetch
# fetch
# gpg --verify tcp-9.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

VI.  Correction details

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

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/8/                                                         r285779
releng/8.4/                                                       r285780
stable/9/                                                         r285779
releng/9.3/                                                       r285780
stable/10/                                                        r285778
releng/10.1/                                                      r285780
- -------------------------------------------------------------------------

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
Version: GnuPG v2.1.6 (FreeBSD)

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