OpenBSD Journal

[OpenSSH] Clarification of SSH1 Vulnerabilites

Contributed by Dengue on from the markus dept.

Markus Friedl (the not often enough credited primary developer of OpenSSH), has posted to misc@ and BUGTRAQ, et al, a Special Security Note about OpenSSH and the recent SSH Protocol version 1 implementation weaknesses.

Read more for the full report:

Subject: OpenSSH is _not_ vulnerable the several known problems in SSH-1
   Date: Thu, 15 Feb 2001 09:13:41 +0100
   From: Markus Friedl


                  Special OpenBSD Security Note
                        February 14, 2001
 OpenSSH is _not_ vulnerable the several known problems in SSH-1


The CERT Coordination Center has published the following notes about
weaknesses in various SSH protocol version 1 implementations.

Since many people using OpenSSH are worried about these issues,
we decided to publish these notes.

   "Passwords sent via SSH encrypted with RC4 can be easily cracked"

   "SSH connections using RC4 and password authentication can be

   "Weak CRC allows RC4 encrypted SSH packets to be modified without

   "SSH allows client authentication to be forwarded if encryption
   is disabled"

   "Last block of IDEA-encrypted SSH packet can be changed without

   "SSH host key authentication can be bypassed when DNS is used
   to resolve localhost"

   "Older SSH clients do not allow users to disable X11 forwarding"

OpenSSH is _not_ vulnerable to #1, #2 and #3 since OpenSSH does not
allow RC4 in its SSH protocol 1 implementation.

OpenSSH is _not_ vulnerable to #4 since OpenSSH does not allow
encryption to be disabled.

OpenSSH is _not_ vulnerable to #5 since OpenSSH does not support

OpenSSH is _not_ vulnerable to #6 since OpenSSH does not resolve
"localhost".  OpenSSH uses the resolved IP-address and disables the
host key authentication for only.

OpenSSH is _not_ vulnerable to #7 since OpenSSH permits users to
disable X11 forwarding, and this is the default configuration in
the OpenSSH client.

The SSH protocol version 2 (a.k.a. SecSH) is not affected by problems
#1, #2, #3, #4 and #5.

The OpenSSH client currenly defaults to preferring SSH-1 protocol
over SSH-2 protocol, but in a future release the default will soon
change, since the SSH-2 protocol support has improved considerably.


(Comments are closed)

  1. By brendan () on

    Same link issue as last posting.

    Are you doing a cut and paste?


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