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USN-612-2: OpenSSH vulnerability

13 May 2008

OpenSSH vulnerability

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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.

Reduce your security exposure

Ubuntu Pro provides ten-year security coverage to 25,000+ packages in Main and Universe repositories, and it is free for up to five machines.

Learn more about Ubuntu Pro

Update instructions

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

Ubuntu 8.04
Ubuntu 7.10
Ubuntu 7.04

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.

  2. 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:

    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

    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.

  3. 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

    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.

  4. 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

  5. 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.

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