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KB-7691: SSH -X fails, X11Forwarding

Authentication Service ,  

14 November,16 at 05:33 PM

Applies to: 

Centrify-enabled OpenSSH 7.2p2 on all supported platforms 


In Centrify-enabled OpenSSH 7.2p2 (Suite2016.1), SSH X11 'untrusted-mode' forwarding, aka, 'ssh -X', fails. 


For 'ssh -X', OpenSSH calls 'xauth' on the client machine, which utilizes the XC-Security extension in xorg-server. However, this extension is disabled as default as of xorg-server version 1.7.5. Because of this, system vendors may enable or disable it in their own xorg-server spin. This includes Centrify-enabled OpenSSH. In the case of the XC-Security extension being disabled, 'xauth' will fail. For Centrify-enabled OpenSSH versions prior to 7.2p2, if 'xauth' fails in 'untrusted-mode', it will use the fallback setting and still succeed the forwarding, however starting in version 7.2p2, it will directly fail. This is a security enhancement adopted per Xorg and OpenSSH best practices. 

More details on this can be found from the official Xorg documentation. 


Centrify no longer supports untrusted X11 forwarding.

Going forward, Centrify-enabled OpenSSH will support only 'ssh -Y' in future, but '-X' will be kept for backward compatibility. For 'ssh -X', OpenSSH will call 'xauth' program in client machine, which will use XC-Security extension in xorg-server, yet that extension was disabled as default since xorg-server version 1.7.5, and system vendors may enabled or disabled it in their own xorg-server building, so, for disabled ones, 'xauth' will fail, please check the doc of Xorg for details.