6069945 2001-02-08 18:02 -0500 /224 rader/ BindView Security Advisory <advisory+ssh1crc@BOS.BINDVIEW.COM>
Sänt av: joel@lysator.liu.se
Importerad: 2001-02-09 02:18 av Brevbäraren (som är implementerad i) Python
Extern mottagare: BUGTRAQ@SECURITYFOCUS.COM
Externa svar till: advisory+ssh1crc@BOS.BINDVIEW.COM
Mottagare: Bugtraq (import) <15315>
Ärende: BindView advisory: sshd remote root (bug in deattack.c)
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From: BindView Security Advisory <advisory+ssh1crc@BOS.BINDVIEW.COM>
To: BUGTRAQ@SECURITYFOCUS.COM
Message-ID: <m3u264rd6e.fsf@jetcar.qnz.org>
Remote vulnerability in SSH daemon crc32 compensation attack detector
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Issue date: 8 February 2001
Author: Michal Zalewski <lcamtuf@razor.bindview.com>
Contact: Scott Blake <blake@razor.bindview.com>
CVE: CAN-2001-0144
Topic:
Remotely exploitable vulnerability condition exists in most ssh daemon
installations (F-SECURE, OpenSSH, SSH from ssh.com, OSSH).
Tested against:
** Vulnerable:
SSH 1.2.x (ssh.com) -- all recent releases
F-SECURE SSH 1.3.x -- all recent releases
OpenSSH prior to 2.3.0 (unless SSH protocol 1 support is
disabled)
OSSH 1.5.7 (by Bjoern Groenvall) and other ssh1/OpenSSH derived
daemons
** Not vulnerable:
SSH2 (ssh.com): all 2.x releases NOTE: SSH2 installations with
SSH1 fallback support are vulnerable
OpenSSH 2.3.0 (problem fixed)
SSH1 releases prior to 1.2.24 (vulnerable to crc attacks)
Cisco SSH (own implementation)
LSH (SSH protocol 1 not supported)
** Other SSH daemons: not tested
Overview:
An integer-overflow problem is present in common code of recent ssh
daemons, deattack.c, which was developed by CORE SDI to protect
against cryptographic attacks on SSH protocol.
Impact:
Insufficient range control calculations (16-bit unsigned variable is
used instead of 32-bit, which causes integer overflow) in the
detect_attack() function leads to table index overflow bug.
This effectively allows an attacker to overwrite arbitrary portions of
memory. The altered memory locations affect code that is executed by
the daemon with uid 0, and this can be leveraged to obtain general
root access to the system.
Details:
When the condition described above occurs, a 32-bit local variable,
which is set to 65536 for large input buffers, is assigned to a 16-bit
local variable, effectively causing it to be set to 0. Due to specific
malloc(0) behavior, memory allocation routine will be passed, creating
buffer of size (malloc_usable_size) 12. Next:
for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
We can see n-1 here, and n is equal to 0. Because i is an unsigned
32-bit integer, it would cause integer overflow. This code will be
equal to i = HASH(c) & 0xffffffff. Binary AND operator reduces this
to i = HASH(c). Pointer 'c' is referencing client-provided
cryptographic packet, and HASH function is simply responsible for
changing byte order in input stream.
Then, detect_attack() routine is trying to access h[i], causing
segmentation fault due to table index overflow bug.
To reproduce this condition, run your sshd server on localhost under
gdb with '-d' switch (to avoid forking). Then try (using OpenSSH
client - ssh.com client software crops the login name):
$ ssh -v -l `perl -e '{print "A"x88000}'` localhost
Program received signal SIGSEGV, Segmentation fault.
0x806cfbd in detect_attack ( ..., len=88016, IV=0x0) at deattack.c:138
136 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
We can inspect the table index (SEGV happened during h[i] !=
HASH_UNSIGNED comparsion):
(gdb) printf "%x\n",i
Results may vary with every connection, depending on the entropy seed
used by the client, crypto keys, etc. You can easily produce a wide
32-bit range of indexes by changing client parameters or simply
reconnecting. It is obvious there wouldn't be a problem to specify
very large index that would point outside our table, but will cause
address space wrap to point accessible memory (stack or another
segment). Then, few lines below, in the same loop, we can find
following statement:
h[i] = j;
...where j is a simple block counter.
Conclusion:
By carefully preparing encrypted data, an attacker can point used,
accessible memory (that would pass check in line 136 without SEGV),
and then, he will able to alter dword at chosen address, replacing it
with value of j. The attacker can alter stack variables, alter malloc
structures, etc, and attack later due to improper execution of daemon
code. This condition is relatively difficult to exploit, but there
are no technical reasons that would make this impossible.
Currently, we are not aware of working exploits for this
vulnerability.
Note that the attacker needs to make a TCP connection from an IP
address for which sshd will enter into a key-exchange dialogue. If the
attacker's packets have a source IP address that is disallowed by
(for example) DenyHosts in the sshd configuration file, the key
exchange will not happen and the attacker will not have an opportunity
to compose the required exploit data.
Solution:
Included are a few patches for various versions/implementations of
SSH. This is not meant to be an all-inclusive list, as there are a
number of implementers of SSH daemons that are not open source. If
you *do* have the source code for SSH, it should be fairly simply to
study the patches below, see what has been done, and patch your own
code. Note that this is a fix for the one issue that we've found, and
should not be construed as the results of a complete audit of the
code.
SSH1 software:
8<---------------------patch for ssh-1.2.31---------------------------
--- deattack.c.orig Wed Feb 7 13:53:47 2001
+++ deattack.c Wed Feb 7 13:54:24 2001
@@ -79,7 +79,7 @@
detect_attack(unsigned char *buf, word32 len, unsigned char *IV)
{
static word16 *h = (word16 *) NULL;
- static word16 n = HASH_MINSIZE / HASH_ENTRYSIZE;
+ static word32 n = HASH_MINSIZE / HASH_ENTRYSIZE;
register word32 i, j;
word32 l;
register unsigned char *c;
8<---------------------patch for ssh-1.2.31---------------------------
Bjoern Groenvall's ossh (ftp://ftp.pdc.kth.se/pub/krypto/ossh/):
8<---------------------patch for ossh-1.5.7---------------------------
--- deattack.c.orig Wed Feb 7 14:11:23 2001
+++ deattack.c Wed Feb 7 14:11:46 2001
@@ -91,7 +91,7 @@
detect_attack(const unsigned char *buf, word32 len)
{
static u_int16_t *h = (u_int16_t *) NULL;
- static u_int16_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
+ static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
register word32 i, j;
word32 l;
const unsigned char *c, *d;
8<---------------------patch for ossh-1.5.7---------------------------
OpenSSH:
Upgrade to 2.3.0 or above. If you have 2.2.0:
8<-------------------patch for openssh-2.2.0--------------------------
--- deattack.c.orig Wed Feb 7 14:18:23 2001
+++ deattack.c Wed Feb 7 14:19:33 2001
@@ -84,7 +84,7 @@
detect_attack(unsigned char *buf, u_int32_t len, unsigned char *IV)
{
static u_int16_t *h = (u_int16_t *) NULL;
- static u_int16_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
+ static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
register u_int32_t i, j;
u_int32_t l;
register unsigned char *c;
8<-------------------patch for openssh-2.2.0--------------------------
Vendor Response:
CORE SDI has issued their own advisory detailing fix information and
has also pointed out that SSH1 clients are also vulnerable.
Bjorn Gronvall - OSSH
Fixed in version ossh-1.5.8
AppGate
The default configuration of the AppGate server is not
vulnerable since it has SSH-1 support disabled. However
customers who need ssh1-support can contact
support@appgate.com to get patches.
Mindbright
The MindTerm client does not have this vulnerability.
SSH
Current release 2.4.0 is not vulnerable. Previous versions
of SSH1 are not supported but a fix has been commited to the
source tree. SSH recommends customers upgrade to SSH2.
F-Secure
Unfortunately, after many attempts to contact F-Secure via
email and telephone no response has been received.
Thanks:
Special thanks to Mark Loveless for his significant contributions
to the Fix section. Thanks to RAZOR team members Todd Sabin, Scott
Blake, and Steve Manzuik for their assistance with this issue.
Thanks also to Ivan Arce of CORE SDI for his patience with us.
(6069945) --------------------------------(Ombruten)