Remote vulnerability in SSH daemon crc32 compensation attack detector
-----------------------------------------------------------------------

Issue date: 8 February 2001

Author: Michal Zalewski <lcamtufrazor.bindview.com>
Contact: Scott Blake <blakerazor.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
  supportappgate.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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