SSH-VULNKEY(1) BSD General Commands Manual SSH-VULNKEY(1)NAMEssh-vulnkey — check blacklist of compromised keys
SYNOPSISssh-vulnkey [-q] file ...
ssh-vulnkey-a
DESCRIPTIONssh-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‐
ple:
$ ssh-keyscan -t rsa remote.example.org | ssh-vulnkey-
ssh-vulnkey will exit zero if any of the given keys were in the compro‐
mised 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 finger‐
print).
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 ALSOssh-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.
BSD April 25, 2024 BSD
