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Category:   OS (UNIX)  >   NetBSD Vendors:   NetBSD
NetBSD LAST_ACK State Transition Bug Lets Remote Users Deny Service
SecurityTracker Alert ID:  1033915
SecurityTracker URL:
CVE Reference:   CVE-2015-5358   (Links to External Site)
Date:  Oct 22 2015
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 5.1 - 5.1.5, 5.2 - 5.2.3, 6.0 - 6.0.6, 6.1 - 6.1.5
Description:   A vulnerability was reported in NetBSD. A remote user can consume excessive resources on the target system.

A remote user can establish a TCP connection with the target device 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 memory on the target system.

Impact:   A remote user can consume excessive memory 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:   None.

 Source Message Contents

Subject:  NetBSD Security Advisory 2015-009: TCP LAST_ACK state memory exhaustion

Hash: SHA256

		 NetBSD Security Advisory 2015-009

Topic:		TCP LAST_ACK state memory exhaustion

Version:	NetBSD-current:		source prior to Mon, Jul 24th 2015
		NetBSD 7.0: 		not affected
		NetBSD 6.1 - 6.1.5:	affected
		NetBSD 6.0 - 6.0.6: 	affected
		NetBSD 5.2 - 5.2.3: 	affected
		NetBSD 5.1 - 5.1.5: 	affected

Severity:	Potential remote denial of service

Fixed:		NetBSD-current:		Jul 24th, 2015 
		NetBSD-7 branch:	Jul 24th, 2015
		NetBSD-6 branch:	Jul 24th, 2015
		NetBSD-6-1 branch:	Jul 24th, 2015
		NetBSD-6-0 branch:	Jul 24th, 2015
		NetBSD-5 branch:	Jul 24th, 2015
		NetBSD-5-2 branch:	Jul 24th, 2015
		NetBSD-5-1 branch	Jul 24th, 2015

Please note that NetBSD releases prior to 5.1 are no longer supported.
It is recommended that all users upgrade to a supported release.


TCP sockets that remain in the LAST_ACK state may hold resources
for an unspecified amount of time, which may lead to denial of
service due to memory exhaustion. This vulnerability has been
assigned CVE-2015-5358.

Technical Details

When closing a connection the TCP socket is entering the LAST_ACK
state in which kernel waits for acknowledgement that FIN was
delivered to the peer or failure of all packet retransmission. In
certain circumstances a socket in this state may hold a significant
amount of memory (mbufs) which can be held for indefinite time,
because the "persist" timer responsible for cleaning up that memory
was previously deactivated. If an attacker is able to make the
attacked systems sockets enter that state, then remote denial of
service is possible due to memory exhaustion.

Solutions and Workarounds

+ Fix from NetBSD autobuild

The fastest way to upgrade to an unaffected kernel, if you are
running or can run a standard kernel built as part of the NetBSD
release process, is to obtain the corresponding kernel from the
daily NetBSD autobuild output and install it on your system.

You can obtain such kernels from
where they are sorted by NetBSD branch, date, and architecture. To
fix a system running e.g. NetBSD 6.0 or the stable NetBSD 6.0
branch, the most appropriate kernel will be the "netbsd-6-0" kernel.

To fix a system running NetBSD-current, the "HEAD" kernel should
be used.  In all cases, a kernel from an autobuild dated newer than
the fix date for the branch you are using must be used to fix the

+ Fix from source

For all NetBSD versions, if you want to upgrade to a safe kernel
from source, you need to obtain fixed kernel sources, rebuild
and install the new kernel, and reboot the system.
The fixed source may be obtained from the NetBSD CVS repository.        
The following instructions briefly summarise how to upgrade your        
kernel.  In these instructions, replace:

  ARCH     with your architecture (from uname -m), and                  
  KERNCONF with the name of your kernel configuration file.    
  NEWVERSION  with the CVS version of the fix

File versions containing the fix:

FILE                         HEAD   netbsd-7  netbsd-6  netbsd-6-1 netbsd-6-0
+--------------------------- -----  --------- --------- ---------- ----------
src/sys/netinet/tcp_input.c  1.179  1.334.2.2 1.321.2.1 1.321.8.1  1.321.6.1
src/sys/netinet/tcp_output.c 1.184

FILE                         netbsd-5   netbsd-5-2     netbsd-5-1
+--------------------------- ---------- -------------- -------------

To update from CVS, re-build, and re-install the kernel:

	# cd src
	# cvs update -d -P -r NEWVERSION src/sys/netinet/tcp_input.c
	# cvs update -d -P -r NEWVERSION src/sys/netinet/tcp_output.c
	# ./ kernel=KERNCONF
	# mv /netbsd /netbsd.old
	# cp sys/arch/ARCH/compile/obj/KERNCONF/netbsd /netbsd 
	# shutdown -r now

For more information on how to do this, see:

Thanks To

Matt Thomas for fixing this issue.
Lawrence Stewart (Netflix, Inc.) and Jonathan Looney (Juniper SIRT) for
reporting this issue.

Revision History

	2015-10-22	Initial release

More Information

Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at

Information about NetBSD and NetBSD security can be found at and .

Copyright 2015, The NetBSD Foundation, Inc.  All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$Id: NetBSD-SA2015-009.txt,v 1.2 2015/10/22 00:02:31 tonnerre Exp $

Version: GnuPG v2


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