Hash: SHA1

             AUSCERT External Security Bulletin Redistribution

                   OpenSSH Security Advisory: x11fwd.adv
                               17 March 2016


        AusCERT Security Bulletin Summary

Product:           OpenSSH
Publisher:         OpenSSH
Operating System:  Windows
                   UNIX variants (UNIX, Linux, OSX)
Impact/Access:     Execute Arbitrary Code/Commands -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2016-3115  

Original Bulletin: 

Revision History:  March 17 2016: Added CVE reference
                   March 11 2016: Initial Release

- --------------------------BEGIN INCLUDED TEXT--------------------

OpenSSH Security Advisory: x11fwd.adv

This document may be found at: http://www.openssh.com/txt/x11fwd.adv

1. Affected configurations

        All versions of OpenSSH prior to 7.2p2 with X11Forwarding

2. Vulnerability

	Missing sanitisation of untrusted input allows an
	authenticated user who is able to request X11 forwarding
	to inject commands to xauth(1).

	Injection of xauth commands grants the ability to read
	arbitrary files under the authenticated user's privilege,
	Other xauth commands allow limited information leakage,
	file overwrite, port probing and generally expose xauth(1),
	which was not written with a hostile user in mind, as an
	attack surface.

	xauth(1) is run under the user's privilege, so this
	vulnerability offers no additional access to unrestricted
	accounts, but could circumvent key or account restrictions
	such as sshd_config ForceCommand, authorized_keys
	command="..." or restricted shells.

3. Mitigation

        Set X11Forwarding=no in sshd_config. This is the default.

	For authorized_keys that specify a "command" restriction,
	also set the "restrict" (available in OpenSSH >=7.2) or
	"no-x11-forwarding" restrictions.

4. Details

        As part of establishing an X11 forwarding session, sshd(8)
	accepts an X11 authentication credential from the client.
	This credential is supplied to the xauth(1) utility to
	establish it for X11 applications that the user subsequently

	The contents of the credential's components (authentication
	scheme and credential data) were not sanitised to exclude
	meta-characters such as newlines. An attacker could
	therefore supply a credential that injected commands to
	xauth(1). The attacker could then use a number of xauth
	commands to read or overwrite arbitrary files subject to
	file permissions, connect to local ports or perform attacks
	on xauth(1) itself.

	OpenSSH 7.2p2 implements a whitelist of characters that
	are permitted to appear in X11 authentication credentials.

5. Credit

        This issue was identified by github.com/tintinweb and
	communicated to the OpenSSH developers on March 3rd, 2016.

6. Fix

        Portable OpenSSH 7.2p2 contains a fix for this vulnerability.

	Patches for supported OpenBSD releases (5.7, 5.8 and 5.9) have
	been committed to the -STABLE branches and are available on the
	errata pages:


- --------------------------END INCLUDED TEXT--------------------

You have received this e-mail bulletin as a result of your organisation's
registration with AusCERT. The mailing list you are subscribed to is
maintained within your organisation, so if you do not wish to continue
receiving these bulletins you should contact your local IT manager. If
you do not know who that is, please send an email to auscert@auscert.org.au
and we will forward your request to the appropriate person.

NOTE: Third Party Rights
This security bulletin is provided as a service to AusCERT's members.  As
AusCERT did not write the document quoted above, AusCERT has had no control
over its content. The decision to follow or act on information or advice
contained in this security bulletin is the responsibility of each user or
organisation, and should be considered in accordance with your organisation's
site policies and procedures. AusCERT takes no responsibility for consequences
which may arise from following or acting on information or advice contained in
this security bulletin.

NOTE: This is only the original release of the security bulletin.  It may
not be updated when updates to the original are made.  If downloading at
a later date, it is recommended that the bulletin is retrieved directly
from the author's website to ensure that the information is still current.

Contact information for the authors of the original document is included
in the Security Bulletin above.  If you have any questions or need further
information, please contact them directly.

Previous advisories and external security bulletins can be retrieved from:


Australian Computer Emergency Response Team
The University of Queensland
Qld 4072

Internet Email: auscert@auscert.org.au
Facsimile:      (07) 3365 7031
Telephone:      (07) 3365 4417 (International: +61 7 3365 4417)
                AusCERT personnel answer during Queensland business hours
                which are GMT+10:00 (AEST).
                On call after hours for member emergencies only.
Comment: http://www.auscert.org.au/render.html?it=1967