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The following instructions relate to CVE-2008-0166. They were prepared by
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Matt Zimmerman, assisted by Colin Watson.
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A weakness has been discovered in the random number generator used by OpenSSL
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on Debian and Ubuntu systems. As a result of this weakness, certain encryption
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keys are much more common than they should be, such that an attacker could
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guess the key through a brute-force attack given minimal knowledge of the
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system. This particularly affects the use of encryption keys in OpenSSH,
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OpenVPN and SSL certificates.
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This vulnerability only affects operating systems which (like Ubuntu) are based
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on Debian. However, other systems can be indirectly affected if weak keys are
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We consider this an extremely serious vulnerability, and urge all users to act
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immediately to secure their systems.
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Systems which are running any of the following releases:
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* Ubuntu 7.04 (Feisty)
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* Ubuntu 8.04 LTS (Hardy)
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* Ubuntu "Intrepid Ibex" (development): libssl <= 0.9.8g-8
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and have openssh-server installed or have been used to create an OpenSSH key or
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X.509 (SSL) certificate.
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All OpenSSH and X.509 keys generated on such systems must be considered
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untrustworthy, regardless of the system on which they are used, even after the
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update has been applied.
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This includes the automatically generated host keys used by OpenSSH, which are
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the basis for its server spoofing and man-in-the-middle protection.
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The specific package versions affected are:
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* Debian 4.0: libssl <= 0.9.8c-4etch3
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* Ubuntu 7.04: libssl <= 0.9.8c-4ubuntu0.2
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* Ubuntu 7.10: libssl <= 0.9.8e-5ubuntu3.1
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* Ubuntu 8.04: libssl <= 0.9.8g-4ubuntu3
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== What to do if you are affected ==
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1. Install the security updates
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Once the update is applied, weak user keys will be automatically rejected
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where possible (though they cannot be detected in all cases). If you are
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using such keys for user authentication, they will immediately stop working
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and will need to be replaced (see step 3).
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OpenSSH host keys can be automatically regenerated when the OpenSSH security
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update is applied. The update will prompt for confirmation before taking
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2. Update OpenSSH known_hosts files
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The regeneration of host keys will cause a warning to be displayed when
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connecting to the system using SSH until the host key is updated in the
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known_hosts file. The warning will look like this:
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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
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@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
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IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
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Someone could be eavesdropping on you right now (man-in-the-middle attack)!
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It is also possible that the RSA host key has just been changed.
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In this case, the host key has simply been changed, and you should update
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the relevant known_hosts file as indicated in the error message.
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3. Check all OpenSSH user keys
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The safest course of action is to regenerate all OpenSSH user keys,
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except where it can be established to a high degree of certainty that the
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key was generated on an unaffected system.
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Check whether your key is affected by running the ssh-vulnkey tool, included
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in the security update. By default, ssh-vulnkey will check the standard
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location for user keys (~/.ssh/id_rsa, ~/.ssh/id_dsa and ~/.ssh/identity),
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your authorized_keys file (~/.ssh/authorized_keys and
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~/.ssh/authorized_keys2), and the system's host keys
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(/etc/ssh/ssh_host_dsa_key and /etc/ssh/ssh_host_rsa_key).
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To check all your own keys, assuming they are in the standard
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locations (~/.ssh/id_rsa, ~/.ssh/id_dsa, or ~/.ssh/identity):
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To check all keys on your system:
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To check a key in a non-standard location:
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ssh-vulnkey /path/to/key
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If ssh-vulnkey says "Unknown (no blacklist information)", then it has no
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information about whether that key is affected. If in doubt, destroy the
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key and generate a new one.
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4. Regenerate any affected user keys
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OpenSSH keys used for user authentication must be manually regenerated,
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including those which may have since been transferred to a different system
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after being generated.
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New keys can be generated using ssh-keygen, e.g.:
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Generating public/private rsa key pair.
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Enter file in which to save the key (/home/user/.ssh/id_rsa):
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Enter passphrase (empty for no passphrase):
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Enter same passphrase again:
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Your identification has been saved in /home/user/.ssh/id_rsa.
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Your public key has been saved in /home/user/.ssh/id_rsa.pub.
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The key fingerprint is:
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00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 user@host
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5. Update authorized_keys files (if necessary)
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Once the user keys have been regenerated, the relevant public keys must
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be propagated to any authorized_keys files on remote systems. Be sure to
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delete the affected key.
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1. Install the security update
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2. Create new certificates to replace any server or client certificates in use
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3. If certificates have been generated for use on other systems, they must be
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found and replaced as well.
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== Removing openssh-blacklist ==
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For the moment, the openssh-server package depends on openssh-blacklist, in
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order that the blacklist is deployed to the maximum possible number of
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systems to reduce the potential spread of worms exploiting this
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vulnerability. We acknowledge that this may be inconvenient for some small
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systems, but nevertheless feel that this was the best course of action.
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If you absolutely need to remove the blacklist from your system, then you
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can run the following commands to substitute a fake package for
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sudo apt-get install equivs
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equivs-control openssh-blacklist.ctl
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sed -i 's/^Package:.*/Package: openssh-blacklist/' openssh-blacklist.ctl
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sed -i 's/^# Version:.*/Version: 9:1.0/' openssh-blacklist.ctl
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equivs-build openssh-blacklist.ctl
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sudo dpkg -i openssh-blacklist_1.0_all.deb
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Be warned: this circumvents a security measure for the sake of disk space.
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You should only do this if you have no other option, and if you are certain
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that no compromised keys will ever be generated on or copied onto this
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Once a sufficient amount of time and number of releases have passed, the
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openssh-blacklist package will be phased out.
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removed
debian/README.compromised-keys
