ssh-vulnkey

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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.

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.

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.

exit status

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

see also
~/.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.