The recently announced vulnerability in Debian's openssl package (DSA-1571-1, CVE-2008-0166) indirectly affects OpenSSH. As a result, all user and host keys generated using broken versions of the openssl package must be considered untrustworthy, even after the openssl update has been applied.
1. Install the security updates
This update contains a dependency on the openssl update and will automatically install a corrected version of the libssl0.9.8 package, and a new package openssh-blacklist.
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:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ @ 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. It is recommended that you use a trustworthy channel to exchange the server key. It is found in the file /etc/ssh/ssh_host_rsa_key.pub on the server; it's fingerprint can be printed using the command:
ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key.pub
In addition to user-specific known_hosts files, there may be a system-wide known hosts file /etc/ssh/ssh_known_hosts. This is file is used both by the ssh client and by sshd for the hosts.equiv functionality. This file needs to be updated as well.
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 /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):
To check all keys on your system:
sudo ssh-vulnkey -a
To check a key in a non-standard location:
If ssh-vulnkey says "Unknown (no blacklist information)", then it has no information about whether that key is affected. In this case, you can examine the modification time (mtime) of the file using "ls -l". Keys generated before September 2006 are not affected. Keep in mind that, although unlikely, backup procedures may have changed the file date back in time (or the system clock may have been incorrectly set). If in doubt, generate a new key and remove the old one from any servers.
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/id_rsa.pub. 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 (and authorized_keys2 files, if applicable) on remote systems. Be sure to delete the lines containing old keys from those files.
In addition to countermeasures to mitigate the randomness vulnerability, this OpenSSH update fixes several other vulnerabilities:
CVE-2008-1483: Timo Juhani Lindfors discovered that, when using X11 forwarding, the SSH client selects an X11 forwarding port without ensuring that it can be bound on all address families. If the system is configured with IPv6 (even if it does not have working IPv6 connectivity), this could allow a local attacker on the remote server to hijack X11 forwarding.
CVE-2007-4752: Jan Pechanec discovered that ssh falls back to creating a trusted X11 cookie if creating an untrusted cookie fails, potentially exposing the local display to a malicious remote server when using X11 forwarding.
For the stable distribution (etch), these problems have been fixed in version 4.3p2-9etch1. Currently, only a subset of all supported architectures have been built; further updates will be provided when they become available.
For the unstable distribution (sid) and the testing distribution (lenny), these problems have been fixed in version 4.7p1-9.
We recommend that you upgrade your openssh packages and take the measures indicated above.