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OpenSSH protocol version 2 challenge-response authentication - NetBSD Security Advisory 2002-005


		 NetBSD Security Advisory 2002-005

Topic:		OpenSSH protocol version 2 challenge-response authentication

Version:	NetBSD-current:	prior to May 14, 2002
		NetBSD-1.6_BETAx: affected
		NetBSD-1.5.2:	affected
		NetBSD-1.5.1:	affected
		NetBSD-1.5:	affected
		NetBSD-1.4.*:	not affected (does not ship with OpenSSH)
		pkgsrc:		packages prior to openssh-

Severity:	high, remote root compromise

Workaround:	NetBSD-current:		May 14, 2002
		NetBSD-1.6 branch:	partial by default (priv sep)
		NetBSD-1.5 branch:	instructions below, OpenSSH 3 and later
		pkgsrc:			June 25, 2002 (with openssh-

Fixed:		NetBSD-current:		June 26, 2002 (OpenSSH 3.4)
		NetBSD-1.6 branch:	June 26, 2002 (OpenSSH 3.4)
		NetBSD-1.5 branch:	June 26, 2002 (patch on advisory)
		pkgsrc:			June 26, 2002 (with openssh-

		Version string "NetBSD_Secure_Shell-20020626" will identify
		that the fix is in place.


OpenSSH has a vulnerability in protocol version 2 challenge-response
authentication.  OpenSSH 3.4 must be installed to completely overcome the

Technical Details

Vulnerability itself:

CERT CA-2002-18

Solutions and Workarounds

Some workarounds are available, which may somewhat mitigate the risk:

 - Turn off challenge-response authentication by having the following
   in sshd_config:
	ChallengeResponseAuthentication no

   On some systems, the following option is also required together
   with the above. It is not relevant for NetBSD.
	PAMAuthenticationViaKbdInt no

   Note that turning these features off will disable SSH logins via
   S/Key (OTP) authentication.  Compiling OpenSSH without support for
   S/Key and PAM authentication will also eliminate this

 - If you do not require SSH Protocol version 2 support, disabling it
   will eliminate the vulnerable codepath. Remember that version 1 is
   considered significantly less secure than version 2, and this
   workaround is not recommended for long term use. Additionally, if
   your users use version 2 authentication methods, they will be unable
   to connect.

 - The new Privilege Separation feature (available since OpenSSH
   3.2.x) has been promoted as a potential mitigation of this issue.
   This feature is available in NetBSD-current as of May 14th, and is
   enabled by default.

   Privilege Separation might provide a benefit, potentially
   preventing this or future vulnerabilities from being root exploits,
   and limiting their nature to a denial of service.  Although a
   useful defensive feature, this is not guaranteed, expecially given
   the implementation has not yet met the test of time.

   Do not avoid patching this issue simply because you have enabled
   Privilege Separation.

   Effect of privilege separation:

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

Releases of NetBSD 1.5.3 and NetBSD 1.6 are imminent. This is a reminder
to consider upgrading when they are available, if you are running
anything older than NetBSD 1.5.3.  Many security-related improvements
have been made.

To check if your system has a vulnerable version of sshd, run "sshd -V"
(it is an invalid argument, but it will present the version number).
Any version dated "NetBSD_Secure_Shell-20020626" or later will identify
that the fix is in place.

Although workarounds were provided above, update your binaries to make
very sure that you don't have vulnerable binaries around.  

* NetBSD-current:

	Systems running NetBSD-current dated from before 2002-05-13
	should be upgraded to NetBSD-current dated 2002-05-14 or later
	if you wish to use privilege separation support as a stopgap measure.
	It is recommended to update to source dated 2002-06-26 for a
	complete fix (with OpenSSH 3.4).

	The following directories need to be updated from the
	netbsd-current CVS branch (aka HEAD):

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

		# make cleandir dependall
		# make install

	You also need to have an sshd UID and GID, as well as /var/chroot/sshd
	directory (chroot jail), as below:

		Create a group, with /usr/sbin/groupadd, or vi /etc/group

		Create a user, with vipw, or /usr/sbin/useradd

		sshd:*:16:16::0:0:sshd privsep:/var/chroot/sshd:/sbin/nologin

		Create the directory /var/chroot/sshd
	Make sure you have "UsePrivilegeSeparation yes" in your
	/etc/ssh/sshd_config (or it can be commented out, as the default value
	is "yes"), to mitigate future issues.

* NetBSD 1.6 and beta:

	Systems running NetBSD 1.6 beta systems have OpenSSH privilege
	separation turned on by default, follow the workaround section
	as approriate for your environment, and upgrade to source dated
	2002-06-26 (with OpenSSH 3.4) to close this vulnerability.

	NetBSD 1.6 will ship with OpenSSH 3.4, which has a complete fix.

	The following directories need to be updated from the
	netbsd-1-6 CVS branch:

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

		# make cleandir dependall
		# make install

	The sshd user, group, and chroot jail directories should already exist
	in a 1.6 installation.

* NetBSD 1.5, 1.5.1, 1.5.2:

	Systems running NetBSD 1.5.* releases dated from before
	2002-06-26 should be upgraded to sources dated 2002-06-26
	or later.  Sources on the branch after that date include changes
	presented in the following advisory:

	NOTE: the upgrade process will pull in changes presented in this
	advisory.  Therefore, (1) it won't get you OpenSSH 3.4, (2) It
	won't make your sshd support privilege separation.  If you need
	to enable privilege separation, install OpenSSH from pkgsrc

	The following directories need to be updated from the
	netbsd-1-5 CVS branch:

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

		# make cleandir dependall
		# make install

* pkgsrc: (All systems, including NetBSD 1.4.*)

	For a complete fix, openssh- or later should be
	installed from pkgsrc/security/openssh.  See above, as well as
	console messages during pkgsrc build, for instructions to enable
	privilege separation functionality.

Thanks To

Markus Friedl and Jun-ichiro itojun Hagino for patches, and initial
advisory text.

Revision History

	2002-06-26	Initial release

More Information

An up-to-date PGP signed copy of this release will be maintained at

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

Copyright 2002, The NetBSD Foundation, Inc.  All Rights Reserved.

$NetBSD: NetBSD-SA2002-005.txt,v 1.26 2002/06/27 17:37:36 david Exp $

Version: GnuPG v1.0.6 (NetBSD)
Comment: For info see http://www.gnupg.org


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