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AUSCERT External Security Bulletin Redistribution
OpenSSH Security Advisory: x11fwd.adv
17 March 2016
AusCERT Security Bulletin Summary
Operating System: Windows
UNIX variants (UNIX, Linux, OSX)
Impact/Access: Execute Arbitrary Code/Commands -- Existing Account
CVE Names: CVE-2016-3115
Revision History: March 17 2016: Added CVE reference
March 11 2016: Initial Release
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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
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
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.
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
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.
This issue was identified by github.com/tintinweb and
communicated to the OpenSSH developers on March 3rd, 2016.
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
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