*BASH User Commands Ubuntu 10.04.4 LTS Server coreutils
SSH-VULNKEY(1)            BSD General Commands Manual           SSH-VULNKEY(1)

     ssh-vulnkey -- check blacklist of compromised keys

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

     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 gener-
     ated 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 pri-
     vate 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 sys-
     tem'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 exam-

           $ 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:

             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 decid-
             ing 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

     -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.

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

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

           o   All the lines must be exactly the same length (20 characters
               followed by a newline) and must be in sorted order.
           o   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 finger-

     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.

             If present, contains the protocol version 2 RSA authentication
             identity of the user.

             If present, contains the protocol version 2 DSA authentication
             identity of the user.

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

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

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

             If present, contains the protocol version 2 RSA identity of the

             If present, contains the protocol version 2 DSA identity of the

             If present, contains the protocol version 1 RSA identity of the

             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.

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

     ssh-keygen(1), sshd(8)

     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.

BSD                            January 17, 2021                            BSD