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DCSLIB :serious security problem in XKB




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 ]