CORE-SDI-04: SSH insertion attack

Ivan Arce
Thu, 11 Jun 1998 22:25:14 -0300 (ART)


                              CORE SDI S.A.
                          Buenos Aires, Argentina

                           Security Advisory
                            June 11th, 1998                       

                          SSH insertion attack

  This advisory addresses a vulnerability present in the SSH software
  package that allows an attacker to execute arbitrary commands on the 
  SSH server or otherwise subvert an encrypted SSH channel with
  arbitrary data.

Problem Description
  SSH (Secure Shell) is a program that provides strong authentication and 
  secure communications over insecure channels.
  Its widely used for logging in to remote computers, file transfers and
  tunneling of other protocols over the encrypted comunications channel.
  All communications are automatically and transparently encrypted.
  Encryption is also used for integrity checking purposes although 
  current implementations rely on a 32 bit Cyclic Redundancy Check
  to perform integrity checks after the decryption of an incoming packet.
  Encryption is done using one of a list of supported algorithms that
  is exchanged between client and server.
  Upon conection establishment client and server perform a protocol
  negotiation that includes mutual authentication, selection
  of a cipher supported by both ends for subsequent communications and
  of a session key to be used with the cipher. Encryption is then
  turned on using the selected cipher and session key, all further 
  communications are encrypted.

  Currently supported ciphers are:

  - Blowfish
    Bruce Schneier's block cipher using a 128 bit key 
  - IDEA
    A 128 bit block cipher 
  - DES
    The Data Encryption Standard 56-bit block cipher
  - Triple DES (3DES)
    A three-key triple-DES algorithm with an effective key lenght
    of 112 bits.
    An RC4 compatible stream cipher using a 128 bit key

   The use of these algorithms in CBC (Cipher Block Chaining) or
   CFB (Cipher Feedback 64 bits) modes with the CRC-32 integrity check
   allows to perform a known plaintext attack (with as few as
   16 bytes of known plaintext) that permits the insertion of encrypted
   packets with any choosen plaintext in the client to server stream
   that will subvert the integrity checks on the server and decrypt to
   the given plaintext, thus allowing an attacker to execute arbitrary
   commands on the server.
   The attack is equally feasible on the server to client stream ,
   although it just gives the ability to send arbitrary data the
   user's terminal. The implications of such an attack are probably not
   as severe as an attack to the server side of the connection but
   must be taken in consideration in the process of applying fixes.

Technical details
   After the protocol identification phase, where the server
   sends a plaintext string specifiying its the protocol and software 
   versions, all communication is done encapsulating data in a
   packet format described as 'The Binary Packet Protocol' [1]
   The packet layout is as follows:

              32         24        16         8         0
               |        data length (bytes)             |
               |       1 to 8 bytes of padding          |
               =                                        =
               |   type   |                             |
               +----------+                             +
               |                                        |
               =                 data                   =
               |                                        |
               |                CRC-32                  |

   Data length: Length in bytes of the given packet, not including 
                the length field and padding 
   Padding    : 8 - (length mod 8) bytes of random data, putting
                random data at the beginning of the packet is an 
                effort to make known plaintext attacks more difficult.
   Packet type: An 8-bit unsigned byte.
   Data       : length - 5 data bytes
   CRC-32     : the four 8-bit check bytes, MSB first. The CRC is
                computed before any encryption
  Encryption is done on the padding+type+data+CRC fields, the length
  field is never encrypted. The encrypted portion of the packet has
  a length that is always a multiple of 8 bytes.

  Knowning certain characteristics of the cipher modes  being used,
  i.e. CBC, with a known plaintext an attacker is able to build a
  custom SSH packet (i.e. a type SSH_CMSG_STDIN_DATA packet) with
  the padding bytes computed in a way such that the next 8-bytes
  of the encrypted data will decrypt to arbitrary plaintext. In
  this particular case, the decrypted data will correspond to the
  type field and 7 data bytes. 
  After the 16 bytes (padding+type+7 data bytes) the attacker would
  include a variable length of data bytes specifically crafted to
  produce a valid CRC-32 field for the whole packet once it is
  This attack and several variations using the same technique can
  be performed due to the usage of weak integrity check schemes, in
  particular CRC-32 has certain properties that allows the attacker
  to forge a valid CRC for her corrupted packet.

  However, for the attack to succeed the attacker must be able to
  perform an active network attack, by either intercepting the
  legit SSH connection at any point between the client and server and
  injecting a forged packet or by performing a TCP session hijack attack.
  Such an attack is described in [6] and for SSH the two methods of
  TCP desynchronization can be used. In particular the method described
  as "Null data desynchronization" can be carried out using packets of

  Note that the new revision for the SSH protocol, proposed and
  published as Internet Drafts [2],[3],[4] [5] makes use of
  cryptographycally strong message authentication codes for
  integrity checks that wont fail to these attacks. 
  Its is important to mention that despise the vulnerabilities
  found in the SSH protocol, it still remains to be a much more
  secure alternative to telnet, rsh and rlogin applications.

  [1] "The SSH (Secure Shell) Remote Login Protocol", T. Ylonen
       Helsinki University of Technology. November 15th 1995
	   (draft expired on May 15th, 1996)

	   Included as the file ./RFC in the ssh distribution

  [2] "SSH Protocol Architecture", draft-ietf-secsh-architecture-01.txt.gz
       T. Ylonen, T. Kivinen, M. Saarinen. SSH. November 7th, 1997 

  [3] "SSH Connection Protocol", draft-ietf-secsh-connect-03.txt.gz
       T. Ylonen, T. Kivinen, M. Saarinen. SSH. November 7th, 1997 

  [4] "SSH Authentication Protocol", draft-ietf-secsh-userauth-03.txt.gz
       T. Ylonen, T. Kivinen, M. Saarinen. SSH. November 7th, 1997 

  [5] "SSH Transport Layer Protocol",draft-ietf-secsh-transport-03.txt.gz
       T. Ylonen, T. Kivinen, M. Saarinen. SSH. November 7th, 1997 
	   (drafts expired on May 7th, 1998)

  All Internet drafts are available at <>

  [6] "Simple Active Attack Against TCP", Laurent Joncheray, 
      Merit Networks Inc., 5th USENIX Security Simposium. 1995.

   An attacker with access to the encrypted SSH stream may insert
   encrypted blocks in the stream that will decrypt to 
   arbitrary commands to be executed on the SSH server.

Fix Information:
   Upgrade to the upcoming SSH protocol version 2.

   Commercial F-Secure SSH users contact Data Fellows Inc. for
   information on how to upgrade to F-Secure 2.0

   Notice that version 2 of the SSH protocol is not 
   compatible with the previous version, thus you
   will need to upgrade all the SSH clients as well.
   In the meantime, upgrade to version 1.2.25 of SSH, which
   fixes the problem. The SSH 1.2.25 distribution can be 
   obtained from:

   F-Secure SSH version 1.3.5 fixes this security problem.
   If you are using the commercial Data Fellows SSH package and you
   have a support contract, you can obtain the 1.3.5 from your local

   Users without a support contract can obtain a patch which fixes 
   this problem from:


   A patch for the free SSH 1.2.23 distribution and the complete
   SSH 1.2.23 package, with the patch applied, can be obtained at:


  Below  are the MD5 hashes for the provided files

   MD5 (ssh-1.2.23.patch) = 6bdb63d57f893907191986c5ced557ab
   MD5 (ssh-1.2.23-core.tar.Z) = fffb52122aae26c1f212c051a305a310
   MD5 (ssh-1.2.23-core.tar.gz) = f9509ba0f0715637805c6b116adc0869

Vulnerable Systems:

   All systems running implementations of SSH using protocol version 1.x
   are vulnerable.
   This includes SSH software versions up to 1.2.23 and F-Secure SSH 1.3.4
   To obtain the version of the SSH server that is running
   on a given host you can issue the following commands:

   $ telnet <IP address> 22
   Trying <IPaddress>...
   Connected to <IPaddress>.
   Escape character is '^]'.
       \ / \--------- software version
        |------------ protocol version

   telnet> close
   Connection closed.
   $ exit

Additional Information:
   These vulnerabilities were discovered by Ariel Futoransky
   <> and Emiliano Kargieman <>

   CORE SDI wishes to thank the SSH maintainers Tatu Ylonen <>
   and Tero Kivinen <> for their quick response to the
   issues rised by this advisory.
   Olli Voima <> of Data Fellows Inc. 
   provided the fix information for the F-Secure products.
   Comments and questions regarding this advisory should be sent to:

	   Ariel Futoransky <> 
	   Emiliano Kargieman <>

   For more information about CORE SDI S.A.  contact <>
   or visit <>
   You can contact CORE SDI S.A. at <> using the
   the following PGP key:

Type Bits/KeyID    Date       User ID
pub  1024/CF4E0CF5 1998/05/18 CORELABS <>

Version: 2.6.3ia
Comment: Requires PGP version 2.6 or later.


Copyright Notice:
The contents of this advisory are Copyright (C) 1998 CORE SDI S.A.,
and may be distributed freely provided that no fee is charged for
this distribution, and proper credit is given.

$Id: ssh-advisory.txt,v 1.8 1998/06/11 22:05:03 iarce Exp $
==============================[ CORE Seguridad de la Informacion S.A. ]=======
Ivan Arce
Gerencia de Tecnologia                          Email     :
Av. Santa Fe 2861 5to C                         TE        : +54-1-821-1030
CP 1425                                         FAX       : +54-1-821-1030
Buenos Aires, Argentina                         Mensajeria: +54-1-317-4157