NetBSD Security Advisory 2009-005: Plaintext Recovery Attack Against SSH

To: NetBSD Security Announcements <security-announce%NetBSD.org@localhost>
Subject: NetBSD Security Advisory 2009-005: Plaintext Recovery Attack Against SSH
From: NetBSD Security Officer <security-officer%NetBSD.org@localhost>
Date: Tue, 14 Jul 2009 21:33:52 +0000

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                 NetBSD Security Advisory 2009-005
                 =================================

Topic:          Plaintext Recovery Attack Against SSH

Version:        NetBSD-current: source prior to June 8, 2009
                NetBSD 5.0:     source prior to June 30, 2009
                NetBSD 4.0.1:   source prior to June 30, 2009
                NetBSD 4.0:     source prior to June 30, 2009
                pkgsrc:         openssh packages prior to 5.2

Severity:       Information leakage from SSH sessions

Fixed:          NetBSD-current:    June 8, 2009
                NetBSD-5 branch:   June 30, 2009 (5.0.1 will include the fix)
                NetBSD-4 branch:   June 30, 2009 (4.1 will include the fix)
                NetBSD-4-0 branch: June 30, 2009 (4.0.2 will include the fix)
                pkgsrc 2009Q1:     openssh-5.2 corrects this issue

Please note that NetBSD releases prior to 4.0 are no longer supported.
It is recommended that all users upgrade to a supported release.


Abstract
========

A defect exists in SSH protocol that allows active attackers to
recover plaintext from an SSH session if a CBC mode cipher is in
use. Updated versions of OpenSSH mitigate this problem.

http://secunia.com/advisories/32760/
http://www.cpni.gov.uk/Docs/Vulnerability_Advisory_SSH.txt
http://www.kb.cert.org/vuls/id/958563


Technical Details
=================

The CBC cipher mode of the SSH protocol allows a remote attacker
to recover up to 32 bits of plaintext data from an existing SSH
session by sending specially crafted packets. This can be mitigated
either through changes of the sshd_config and ssh_config files or
by updating to the latest version of OpenSSH.

http://secunia.com/advisories/32760/
http://www.cpni.gov.uk/Docs/Vulnerability_Advisory_SSH.txt
http://www.kb.cert.org/vuls/id/958563


Solutions and Workarounds
=========================


The problem can be mitigated by disabling the CBC mode ciphers
using the following directive in sshd_config and ssh_config:

    Ciphers aes128-ctr,aes256-ctr,arcfour256,arcfour,aes128-cbc,aes256-cbc

This makes the CBC ciphers most unlikely to be selected, reducing
the likelyhood of exposure.

In order to fix the underlying problem, users should upgrade their
SSH clients and servers. The information leak vulnerability requires
an active attack and is not easy to exploit, but is potentially
dangerous.

The following instructions describe how to upgrade your SSH
binaries by updating your source tree and rebuilding and
installing a new version of SSH.

* NetBSD-current:

        Systems running NetBSD-current dated from before 2009-06-08
        should be upgraded to NetBSD-current dated 2009-06-09 or later.

        The following directories need to be updated from the
        netbsd-current CVS branch (aka HEAD):
                crypto/external/bsd/openssh/dist

        To update from CVS, re-build, and re-install SSH:
                # cd src
                # cvs update -d -P crypto/external/bsd/openssh/dist
                # cd usr.bin/ssh

                # make USETOOLS=no cleandir dependall
                # make USETOOLS=no install


* NetBSD 5.0:

        The binary distribution of NetBSD 5.0 is vulnerable.

        Systems running NetBSD 5.0 sources dated from before
        2009-06-29 23:00 UTC should be upgraded from NetBSD 5.0
        sources dated 2009-06-30 or later.

        NetBSD 5.0.1 and 5.1 will include the fix.

        The following directories need to be updated from the
        netbsd-5-0 CVS branch:
                crypto/dist/ssh

        To update from CVS, re-build, and re-install SSH:

                # cd src
                # cvs update -d -P -r netbsd-5-0 crypto/dist/ssh
                # cd usr.bin/ssh

                # make USETOOLS=no cleandir dependall
                # make USETOOLS=no install


        Alternatively, apply the following patch (with potential offset
        differences):
                
ftp://ftp.NetBSD.org/pub/NetBSD/security/patches/SA2009-005-openssh-nb5.patch

        To patch, re-build and re-install SSH:

                # cd src/crypto/dist/ssh
                # patch < /path/to/SA2009-005-openssh-nb5.patch

                # cd ../../../usr.bin/ssh
                # make cleandir dependall
                # make install


* NetBSD 4.0, 4.0.1:

        The binary distributions of NetBSD 4.0 and 4.0.1 are vulnerable.

        Systems running NetBSD 4.0 sources dated from before
        2009-06-30 01:00 UTC should be upgraded from NetBSD 4.0
        sources dated 2009-06-30 or later.

        NetBSD 4.1 and 4.0.2 will include the fix.

        The following directories need to be updated from the
        netbsd-4-0 CVS branch:
                crypto/dist/ssh

        To update from CVS, re-build, and re-install SSH:

                # cd src
                # cvs update -d -P -r netbsd-4-0 crypto/dist/ssh
                # cd usr.bin/ssh

                # make USETOOLS=no cleandir dependall
                # make USETOOLS=no install


        Alternatively, apply the following patch (with potential offset
        differences):
                
ftp://ftp.NetBSD.org/pub/NetBSD/security/patches/SA2009-005-openssh-nb4.patch

        To patch, re-build and re-install SSH:

                # cd src/dist/ssh
                # patch < /path/to/SA2009-005-openssh-nb4.patch

                # cd ../../../usr.bin/ssh
                # make cleandir dependall
                # make install


Thanks To
=========

Martin Albrecht, Kenny Paterson and Gaven Watson from the
Information Security Group at Royal Holloway, University of
London for finding and reporting the issue.


Revision History
================

        2009-06-30      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 
  http://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2009-005.txt.asc

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.org/ and http://www.NetBSD.org/Security/.


Copyright 2009, The NetBSD Foundation, Inc.  All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$NetBSD: NetBSD-SA2009-005.txt,v 1.1 2009/06/30 00:32:25 tonnerre Exp $

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