Provided by: openssh-client_5.9p1-5ubuntu1_amd64 bug

NAME

     ssh-vulnkey — check blacklist of compromised keys

SYNOPSIS

     ssh-vulnkey [-q | -v] file ...
     ssh-vulnkey -a

DESCRIPTION

     ssh-vulnkey checks a key against a blacklist of compromised keys.

     A substantial number of keys are known to have been generated using a broken version of
     OpenSSL distributed by Debian which failed to seed its random number generator correctly.
     Keys generated using these OpenSSL versions should be assumed to be compromised.  This tool
     may be useful in checking for such keys.

     Keys that are compromised cannot be repaired; replacements must be generated using
     ssh-keygen(1).  Make sure to update authorized_keys files on all systems where compromised
     keys were permitted to authenticate.

     The argument list will be interpreted as a list of paths to public key files or
     authorized_keys files.  If no suitable file is found at a given path, ssh-vulnkey will
     append .pub and retry, in case it was given a private key file.  If no files are given as
     arguments, ssh-vulnkey will check ~/.ssh/id_rsa, ~/.ssh/id_dsa, ~/.ssh/identity,
     ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2, as well as the system's host keys if
     readable.

     If “-” is given as an argument, ssh-vulnkey will read from standard input.  This can be used
     to process output from ssh-keyscan(1), for example:

           $ ssh-keyscan -t rsa remote.example.org | ssh-vulnkey -

     Unless the PermitBlacklistedKeys option is used, sshd(8) will reject attempts to
     authenticate with keys in the compromised list.

     The output from ssh-vulnkey looks like this:

           /etc/ssh/ssh_host_key:1: COMPROMISED: RSA1 2048 xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx root@host
           /home/user/.ssh/id_dsa:1: Not blacklisted: DSA 1024 xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx /home/user/.ssh/id_dsa.pub
           /home/user/.ssh/authorized_keys:3: Unknown (blacklist file not installed): RSA 1024 xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx user@host

     Each line is of the following format (any lines beginning with “#” should be ignored by
     scripts):

           filename:line: status: type size fingerprint comment

     It is important to distinguish between the possible values of status:

     COMPROMISED
             These keys are listed in a blacklist file, normally because their corresponding
             private keys are well-known.  Replacements must be generated using ssh-keygen(1).

     Not blacklisted
             A blacklist file exists for this key type and size, but this key is not listed in
             it.  Unless there is some particular reason to believe otherwise, this key may be
             used safely.  (Note that DSA keys used with the broken version of OpenSSL
             distributed by Debian may be compromised in the event that anyone captured a network
             trace, even if they were generated with a secure version of OpenSSL.)

     Unknown (blacklist file not installed)
             No blacklist file exists for this key type and size.  You should find a suitable
             published blacklist and install it before deciding whether this key is safe to use.

     The options are as follows:

     -a      Check keys of all users on the system.  You will typically need to run ssh-vulnkey
             as root to use this option.  For each user, ssh-vulnkey will check ~/.ssh/id_rsa,
             ~/.ssh/id_dsa, ~/.ssh/identity, ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2.
             It will also check the system's host keys.

     -q      Quiet mode.  Normally, ssh-vulnkey outputs the fingerprint of each key scanned, with
             a description of its status.  This option suppresses that output.

     -v      Verbose mode.  Normally, ssh-vulnkey does not output anything for keys that are not
             listed in their corresponding blacklist file (although it still produces output for
             keys for which there is no blacklist file, since their status is unknown).  This
             option causes ssh-vulnkey to produce output for all keys.

EXIT STATUS

     ssh-vulnkey will exit zero if any of the given keys were in the compromised list, otherwise
     non-zero.

BLACKLIST FILE FORMAT

     The blacklist file may start with comments, on lines starting with “#”.  After these initial
     comments, it must follow a strict format:

           ·   All the lines must be exactly the same length (20 characters followed by a
               newline) and must be in sorted order.
           ·   Each line must consist of the lower-case hexadecimal MD5 key fingerprint, without
               colons, and with the first 12 characters removed (that is, the least significant
               80 bits of the fingerprint).

     The key fingerprint may be generated using ssh-keygen(1):

           $ ssh-keygen -l -f /path/to/key

     This strict format is necessary to allow the blacklist file to be checked quickly, using a
     binary-search algorithm.

FILES

     ~/.ssh/id_rsa
             If present, contains the protocol version 2 RSA authentication identity of the user.

     ~/.ssh/id_dsa
             If present, contains the protocol version 2 DSA authentication identity of the user.

     ~/.ssh/identity
             If present, contains the protocol version 1 RSA authentication identity of the user.

     ~/.ssh/authorized_keys
             If present, lists the public keys (RSA/DSA) that can be used for logging in as this
             user.

     ~/.ssh/authorized_keys2
             Obsolete name for ~/.ssh/authorized_keys.  This file may still be present on some
             old systems, but should not be created if it is missing.

     /etc/ssh/ssh_host_rsa_key
             If present, contains the protocol version 2 RSA identity of the system.

     /etc/ssh/ssh_host_dsa_key
             If present, contains the protocol version 2 DSA identity of the system.

     /etc/ssh/ssh_host_key
             If present, contains the protocol version 1 RSA identity of the system.

     /usr/share/ssh/blacklist.TYPE-LENGTH
             If present, lists the blacklisted keys of type TYPE (“RSA” or “DSA”) and bit length
             LENGTH.  The format of this file is described above.  RSA1 keys are converted to RSA
             before being checked in the blacklist.  Note that the fingerprints of RSA1 keys are
             computed differently, so you will not be able to find them in the blacklist by hand.

     /etc/ssh/blacklist.TYPE-LENGTH
             Same as /usr/share/ssh/blacklist.TYPE-LENGTH, but may be edited by the system
             administrator to add new blacklist entries.

SEE ALSO

     ssh-keygen(1), sshd(8)

AUTHORS

     Colin Watson <cjwatson@ubuntu.com>

     Florian Weimer suggested the option to check keys of all users, and the idea of processing
     ssh-keyscan(1) output.