Remote vulnerability in SSH daemon crc32 compensation attack detector

Issue Date: February 8, 2001
Author: Michal Zalewski
Contact: Scott Blake
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.

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