X11forwarding yes: how to debug/setup after xauth fix

[Michael Felt]

On 13/10/2017 13:40, Michael Felt wrote:
>>
>>> debug3: send packet: type 4
>> sshd sends a SSH2_MSG_DEBUG back to client, probably indicating
>> why the request failed
>>
>>> debug3: send packet: type 100
>> sshd sends SSH2_MSG_CHANNEL_FAILURE.
>>
>> The debug message would probably give you the reason it fails. You could
>> try to wheedle it out of PuTTY,
> I'll try an iptrace trace - to see what putty is masking with XXXXX.
>> apply the patch below to have it shown
>> at LogLevel=debug3 or try to guess which of one of likely ones it is
>> from session.c:session_setup_x11fwd()
>>
>>> packet_send_debug("X11 forwarding disabled in user configuration 
>>> file.");
>>> packet_send_debug("X11 forwarding disabled in server configuration 
>>> file.");
>>> packet_send_debug("No xauth program; cannot forward with spoofing.");
>>> packet_send_debug("Can't get IP address for X11 DISPLAY.");
>
> My 'quess' is that it somehow related to 'auth' - as there was a 
> security-fix for auth that was introduced in version 7.2 (as I 
> mentioned before: https://www.openssh.com/security.html, March 9, 
> 2016) and https://www.openssh.com/txt/release-7.2p2
>
>  * sshd(8): sanitise X11 authentication credentials to avoid xauth
>    command injection when X11Forwarding is enabled.
>
> My guess is that AIX is still sending either one of '\n', '\r', or 
> even both.
>
>> -d
>>
>> diff --git a/packet.c b/packet.c
>> index f114ea52..5dda4243 100644
>> --- a/packet.c
>> +++ b/packet.c
>> @@ -1774,6 +1774,8 @@ ssh_packet_send_debug(struct ssh *ssh, const 
>> char *fmt,...)
>>       vsnprintf(buf, sizeof(buf), fmt, args);
>>       va_end(args);
>>   +    debug3("sending debug message: %s", buf);
>> +
> Will also try this!

This verifies it is xauth related:

debug3: sending debug message: No xauth program; cannot forward with 
spoofing.

so, added an extra debug - and this is what I see:

debug1: session_input_channel_req: session 0 req x11-req
debug3: setup_x11fwd: xauth_location == /usr/X11R6/bin/xauth
debug3: sending debug message: No xauth program; cannot forward with 
spoofing.

but the xauth_location should be:
# which xauth
/usr/bin/X11/xauth

and fyi:

# ls -l /usr/bin/X11
lrwxrwxrwx    1 bin      bin              16 Aug 31 17:40 /usr/bin/X11 
-> /usr/lpp/X11/bin

# ls -ld /usr/X11R6
drwxr-xr-x    2 bin      bin             256 Aug 29 12:16 /usr/X11R6
# ls -l /usr/X11R6
total 0
lrwxrwxrwx    1 bin      bin              19 Aug 31 17:40 lib -> 
/usr/lpp/X11/lib/R6

so, obviously - the stat() calls for /usr/X11R6/bin/xauth return -1.
# ls -l /usr/X11R6/bin/xauth
ls: 0653-341 The file /usr/X11R6/bin/xauth does not exist.

And, I expect noone from IBM has ever said anything - as they still use 
OpenSSH-6.0 as a base, although they are also using OpenSSH-7.1, both of 
which are not affected by the fix for OpenSSH-7.2.

Studying further - but maybe you already know what needs to be done.


>>       if ((r = sshpkt_start(ssh, SSH2_MSG_DEBUG)) != 0 ||
>>           (r = sshpkt_put_u8(ssh, 0)) != 0 || /* always display */
>>           (r = sshpkt_put_cstring(ssh, buf)) != 0 ||