SSH-VULNKEY(1) MidnightBSD General Commands Manual SSH-VULNKEY(1)

NAME

ssh-vulnkey — check blacklist of compromised keys

SYNOPSIS

ssh-vulnkey [−q] 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 -

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

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

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.

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.

/etc/ssh/blacklist.TYPE-LENGTH

If present, lists the blacklisted keys of type TYPE (‘‘RSA1’’, ‘‘RSA’’, or ‘‘DSA’’) and bit length LENGTH. The format of this file is described above.

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.

MidnightBSD 0.3 April 11, 2009 MidnightBSD 0.3