On Fri, 26 Jun 2015 at 03:16 -0000, Alexandru Chiscan wrote:
On 06/25/2015 11:51 PM, Stuart Barkley wrote:
Then from your desktop (assuming Linux already running X) in a local xterm do something like:
ssh -Y remote-systemDo not use that because any user logged on the server can connect to your X server display and snoop what you are doing, open windows etc.
-Y disables all the X server authentication mechanisms (http://www.x.org/wiki/Development/Documentation/Security/)
I believe this is incorrect. The authentication protocols are still used (thus the need for the xorg-x11-xauth package on CentOS).
This is not the same as 'xhost +' which should never be used.
Both -X and -Y require read access to the ~/.Xauthority file on the remote system in order to connect back to the X server. You can see this by using the 'xauth remove' command to remove the authentication token for the display and clients can no longer connect.
Note about -X versus -Y with ssh:
-X enables basic X forwarding, It disables some X functionality making it "safer" to allow. -X also stops working after about 20 minutes (this is by design but not well documented). I only recently learned why it would stop working after pulling out the last of my hair.
I have been using ssh X forwarding for current work use (local betwork) for more than 15 years and never got into this kind of problem from RH 7 to Centos 7, AIX and Solaris.
Likewise, I've used ssh/X for 20+ years on a variety of systems. In most cases -X is sufficient, but some applications seem to require -Y and I have not dug into all of the reasons.
Maybe it is some other issue that is closing your ssh connection (maybe you should use the KeepAlive options on the ssh server/client); just guessing.
On Debian and FreeBSD 'man ssh_config' now shows:
ForwardX11Timeout Specify a timeout for untrusted X11 forwarding using the format described in the TIME FORMATS section of sshd_config(5). X11 connections received by ssh(1) after this time will be refused. The default is to disable untrusted X11 forwarding after twenty minutes has elapsed.
This option seems to have appeared in OpenSSH 6.0 [See more at the end of this email].
-Y allows the full X protocol which might be a security risk. Some applications will only work with -Y. With this, remote X applications can grab keyboard interactions, grab passwords, put windows on top of other windows (obscuring security messages), etc.
For my own choice I use -Y (although I only enable it occasionally to specific systems).
This is a recent behaviour change on my part with limited use to system I trust. Now that I know about the timeout I can probably just live with -X and will consider moving back to -X for my occasional use,
The documentation of the practical differences between -X and -Y is pretty obscure (mostly defering to the X Security extension documentation). I would like to see better clarification of the differences.
...some additional research looking at the source code and revision history...
The ForwardX11Timeout change was 5 years ago in OpenSSH 6.0. CentOS 6 still has OpenSSH 5.3 without this change (or if a patch was applied the documentation was not also patched). CentOS 7 has OpenSSH 6.6 which includes this change.
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/clientloop.c.diff?r...
Prior to that there was an intended hard coded 20 minute timeout since at least 2005:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/clientloop.c.diff?r...
Based upon the comments in the June 2010 revision it appears that the timeout was not working correctly and perhaps was falling back to trusted authentication after 20 minutes:
Add X11ForwardTimeout option to specify timeout for untrusted X11 authentication cookies to avoid fallback in X11 code to fully-trusted implicit authentication using SO_PEERCRED described at: http://lists.x.org/archives/xorg-devel/2010-May/008636.html
After the X11ForwardTimeout has expired the client will now refuse incoming X11 channel opens.
I will need to see it this is an unpatched security issue on CentOS/RedHat 6. If so, I claim credit for observing it as a possibility.
Stuart
I stand corrected.
Regards, Lec
On 06/26/2015 07:22 PM, Stuart Barkley wrote:
On Fri, 26 Jun 2015 at 03:16 -0000, Alexandru Chiscan wrote:
On 06/25/2015 11:51 PM, Stuart Barkley wrote:
Then from your desktop (assuming Linux already running X) in a local xterm do something like:
ssh -Y remote-systemDo not use that because any user logged on the server can connect to your X server display and snoop what you are doing, open windows etc.
-Y disables all the X server authentication mechanisms (http://www.x.org/wiki/Development/Documentation/Security/)
I believe this is incorrect. The authentication protocols are still used (thus the need for the xorg-x11-xauth package on CentOS).
This is not the same as 'xhost +' which should never be used.
Both -X and -Y require read access to the ~/.Xauthority file on the remote system in order to connect back to the X server. You can see this by using the 'xauth remove' command to remove the authentication token for the display and clients can no longer connect.
Note about -X versus -Y with ssh:
-X enables basic X forwarding, It disables some X functionality making it "safer" to allow. -X also stops working after about 20 minutes (this is by design but not well documented). I only recently learned why it would stop working after pulling out the last of my hair.
I have been using ssh X forwarding for current work use (local betwork) for more than 15 years and never got into this kind of problem from RH 7 to Centos 7, AIX and Solaris.
Likewise, I've used ssh/X for 20+ years on a variety of systems. In most cases -X is sufficient, but some applications seem to require -Y and I have not dug into all of the reasons.
Maybe it is some other issue that is closing your ssh connection (maybe you should use the KeepAlive options on the ssh server/client); just guessing.
On Debian and FreeBSD 'man ssh_config' now shows:
ForwardX11Timeout Specify a timeout for untrusted X11 forwarding using the format described in the TIME FORMATS section of sshd_config(5). X11 connections received by ssh(1) after this time will be refused. The default is to disable untrusted X11 forwarding after twenty minutes has elapsed.This option seems to have appeared in OpenSSH 6.0 [See more at the end of this email].
-Y allows the full X protocol which might be a security risk. Some applications will only work with -Y. With this, remote X applications can grab keyboard interactions, grab passwords, put windows on top of other windows (obscuring security messages), etc.
For my own choice I use -Y (although I only enable it occasionally to specific systems).
This is a recent behaviour change on my part with limited use to system I trust. Now that I know about the timeout I can probably just live with -X and will consider moving back to -X for my occasional use,
The documentation of the practical differences between -X and -Y is pretty obscure (mostly defering to the X Security extension documentation). I would like to see better clarification of the differences.
...some additional research looking at the source code and revision history...
The ForwardX11Timeout change was 5 years ago in OpenSSH 6.0. CentOS 6 still has OpenSSH 5.3 without this change (or if a patch was applied the documentation was not also patched). CentOS 7 has OpenSSH 6.6 which includes this change.
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/clientloop.c.diff?r1=1.220&r2=1.221Prior to that there was an intended hard coded 20 minute timeout since at least 2005:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/clientloop.c.diff?r1=1.137&r2=1.138Based upon the comments in the June 2010 revision it appears that the timeout was not working correctly and perhaps was falling back to trusted authentication after 20 minutes:
Add X11ForwardTimeout option to specify timeout for untrusted X11 authentication cookies to avoid fallback in X11 code to fully-trusted implicit authentication using SO_PEERCRED described at: http://lists.x.org/archives/xorg-devel/2010-May/008636.html After the X11ForwardTimeout has expired the client will now refuse incoming X11 channel opens.I will need to see it this is an unpatched security issue on CentOS/RedHat 6. If so, I claim credit for observing it as a possibility.
Stuart
On 2015-06-26, Stuart Barkley stuartb@4gh.net wrote:
[...]
The documentation of the practical differences between -X and -Y is pretty obscure (mostly defering to the X Security extension documentation). I would like to see better clarification of the differences.
One practical difference I have seen is the improved performance of -Y over -X. I have long attributed that to the relaxation of security controls in the former case.
On 07/05/2015 04:51 AM, Liam O'Toole wrote:
One practical difference I have seen is the improved performance of -Y over -X. I have long attributed that to the relaxation of security controls in the former case.
When and how did you measure that?
The -Y change was introduced in Fedora Core 3, in November 2004. The default was changed to ForwardX11Trusted=yes just a month or two later. I'm not sure -X and -Y ever behaved differently on Enterprise Linux or CentOS.
At this point, I don't think it's even possible to set ForwardX11Trusted=no any more. The X SECURITY extension was replaced with "X Access Control Extension" several years ago.
On 2015-07-05, Gordon Messmer gordon.messmer@gmail.com wrote:
On 07/05/2015 04:51 AM, Liam O'Toole wrote:
One practical difference I have seen is the improved performance of -Y over -X. I have long attributed that to the relaxation of security controls in the former case.
When and how did you measure that?
The -Y change was introduced in Fedora Core 3, in November 2004. The default was changed to ForwardX11Trusted=yes just a month or two later. I'm not sure -X and -Y ever behaved differently on Enterprise Linux or CentOS.
At this point, I don't think it's even possible to set ForwardX11Trusted=no any more. The X SECURITY extension was replaced with "X Access Control Extension" several years ago.
The perceived difference was a general impression on my part, and not measured scientifically. Moreover, it was formed years ago, and on a variety of Linux systems. I concede that it may well be obsolete.
On Mon, 6 Jul 2015, Liam O'Toole wrote:
On 2015-07-05, Gordon Messmer > gordon.messmer@gmail.com wrote:
On 07/05/2015 04:51 AM, Liam O'Toole wrote:
At this point, I don't think it's even possible to set ForwardX11Trusted=no any more. The X SECURITY extension was replaced with "X Access Control Extension" several years ago.
The perceived difference was a general impression on my part, and not measured scientifically. Moreover, it was formed years ago, and on a variety of Linux systems. I concede that it may well be obsolete.
EL6:
ssh -X -o ForwardX11Trusted=no somehost xterm <select some text in the window>
X Error of failed request: BadAccess (attempt to access private resource denied)
ssh -Y -o ForwardX11Trusted=no somehost xterm <select some text in the window>
All well.
ssh -X -o ForwardX11Trusted=yes somehost xterm <select some text in the window>
All well (unsurprising really, seeing as it means the same thing).
-X/-Y/ForwardX11Trusted still do exactly what they've always done, no?
You're trusting the remote host to not misbehave if you use -Y or ForwardX11Trusted=yes since at the very least you're opening up a fairly large information leakage to the remote host. That's fine if you do trust it, but it really isn't if you don't, surely?
jh
On 07/06/2015 04:31 AM, John Hodrien wrote:
EL6:
ssh -X -o ForwardX11Trusted=no somehost xterm
<select some text in the window>
X Error of failed request: BadAccess (attempt to access private resource denied)
Interesting. On Fedora 22, "-o ForwardX11Trusted=no" seems to have no effect. Copy and paste work normally with gnome-terminal, so I'm certain that X SECURITY isn't available and doesn't affect the application.
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Alexandru Chiscan -
Gordon Messmer -
John Hodrien -
Liam O'Toole -
Stuart Barkley