UnixWare 7 uidadmin vulnerable to an exploitable buffer overflow
3 Dec. 1999
Summary
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 to any file, not just those traditionally writeable by gid-sys.
Credit:
The information has been provided by: Brock Tellier.
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 that 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. For example, ping won't work because it doesn't gain "driver" privileges anymore.
To get around this, make sure you login immediately after reboot, execute your root shell 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
#
(We have root privileges)
--- 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
#
###########################################################
