USN-612-2: OpenSSH vulnerability

13 May 2008

openssh vulnerability

A security issue affects these releases of Ubuntu and its derivatives:

  • Ubuntu 8.04 LTS
  • Ubuntu 7.10
  • Ubuntu 7.04

Software Description

  • openssh


A weakness has been discovered in the random number generator used by OpenSSL on Debian and Ubuntu systems. As a result of this weakness, certain encryption keys are much more common than they should be, such that an attacker could guess the key through a brute-force attack given minimal knowledge of the system. This particularly affects the use of encryption keys in OpenSSH.

This vulnerability only affects operating systems which (like Ubuntu) are based on Debian. However, other systems can be indirectly affected if weak keys are imported into them.

We consider this an extremely serious vulnerability, and urge all users to act immediately to secure their systems.

Update instructions

The problem can be corrected by updating your system to the following package versions:

Ubuntu 8.04 LTS
openssh-client - 1:4.7p1-8ubuntu1.1
openssh-server - 1:4.7p1-8ubuntu1.1
Ubuntu 7.10
openssh-client - 1:4.6p1-5ubuntu0.3
openssh-server - 1:4.6p1-5ubuntu0.3
Ubuntu 7.04
openssh-client - 1:4.3p2-8ubuntu1.3
openssh-server - 1:4.3p2-8ubuntu1.3

To update your system, please follow these instructions:

Updating your system:

  1. Install the security updates

Once the update is applied, weak user keys will be automatically rejected where possible (though they cannot be detected in all cases). If you are using such keys for user authentication, they will immediately stop working and will need to be replaced (see step 3).

OpenSSH host keys can be automatically regenerated when the OpenSSH security update is applied. The update will prompt for confirmation before taking this step.

  1. Update OpenSSH known_hosts files

The regeneration of host keys will cause a warning to be displayed when connecting to the system using SSH until the host key is updated in the known_hosts file. The warning will look like this:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY! Someone could be eavesdropping on you right now (man-in-the-middle attack)! It is also possible that the RSA host key has just been changed.

In this case, the host key has simply been changed, and you should update the relevant known_hosts file as indicated in the error message.

  1. Check all OpenSSH user keys

The safest course of action is to regenerate all OpenSSH user keys, except where it can be established to a high degree of certainty that the key was generated on an unaffected system.

Check whether your key is affected by running the ssh-vulnkey tool, included in the security update. By default, ssh-vulnkey will check the standard location for user keys (~/.ssh/id_rsa, ~/.ssh/id_dsa and ~/.ssh/identity), your authorized_keys file (~/.ssh/authorized_keys and ~/.ssh/authorized_keys2), and the system’s host keys (/etc/ssh/ssh_host_dsa_key and /etc/ssh/ssh_host_rsa_key).

To check all your own keys, assuming they are in the standard locations (~/.ssh/id_rsa, ~/.ssh/id_dsa, or ~/.ssh/identity):

$ ssh-vulnkey

To check all keys on your system:

$ sudo ssh-vulnkey -a

To check a key in a non-standard location:

$ ssh-vulnkey /path/to/key

If ssh-vulnkey says “Unknown (no blacklist information)”, then it has no information about whether that key is affected. If in doubt, destroy the key and generate a new one.

  1. Regenerate any affected user keys

OpenSSH keys used for user authentication must be manually regenerated, including those which may have since been transferred to a different system after being generated.

New keys can be generated using ssh-keygen, e.g.:

$ ssh-keygen Generating public/private rsa key pair. Enter file in which to save the key (/home/user/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/user/.ssh/id_rsa. Your public key has been saved in /home/user/.ssh/ The key fingerprint is: 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 user@host

  1. Update authorized_keys files (if necessary)

Once the user keys have been regenerated, the relevant public keys must be propagated to any authorized_keys files on remote systems. Be sure to delete the affected key.