Upgrading Solaris 11.3 to 11.4 – SSH

When you upgrade from Solaris 11.3 to Solaris 11.4 there are a lot of changes where SSH is one of them. Where Solaris 11.3 defaults to use SunSSH, Solaris 11.4 changed this to OpenSSH.

In the environment I work with, some hardening was configured to disallow certain unsafe Ciphers and MACs. Therefor the directives were configured as:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr

MACs hmac-sha1,hmac-sha1-96,hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96

Those values reflect the manual page for sshd_config on FIPS-140:

Ciphers

           Specifies the ciphers allowed for protocol version 2. Cipher order-
           ing  on  the  server side is not relevant. Multiple ciphers must be
           comma separated.

           Valid ciphers are: aes128-ctr, aes192-ctr, aes256-ctr,  aes128-cbc,
           aes192-cbc,  aes256-cbc, arcfour, arcfour128, arcfour256, 3des-cbc,
           and blowfish-cbc.

           The default cipher list is:

             aes128-ctr,aes192-ctr,aes256-ctr,arcfour128,
             arcfour256,arcfour

           Using CBC modes on the server side is not recommended due to poten-
           tial security issues in connection with the SSH protocol version 2.

           When  configuring sshd to run OpenSSL in FIPS-140 mode, the default
           cipher list is: aes128-cbc, aes192-cbc,  aes256-cbc.  Specifying  a
           non-FIPS approved cipher will return an error.

           For  the  case  of  ssh  with  FIPS-140 enabled logging into a non-
           FIPS-140 sshd, the supported and  approved  FIPS  ciphers  must  be
           explicitly  specified  in sshd_config using "Ciphers" for this sce-
           nario.


MACs

       Specifies  the  available  MAC  (message authentication code) algo-
       rithms. The MAC algorithm is used in protocol version  2  for  data
       integrity  protection. Multiple algorithms must be comma-separated.
       The default is  hmac-md5,hmac-sha1,hmac-sha1-96,hmac-md5-96,  hmac-
       sha2-256, hmac-sha2-256-96, hmac-sha2-512, and hmac-sha2-512-96.

       When  configuring sshd to run OpenSSL in FIPS-140 mode, the default
       is hmac-sha1, hmac-sha1-96, hmac-sha2-256, hmac-sha2-256-96,  hmac-
       sha2-512,  and hmac-sha2-512-96. Specifying a non-FIPS approved MAC
       algorithm will return an error.

As said earlier Solaris 11.4 uses OpenSSH and the Ciphers and MACs are just a bit different in this version. The supported Ciphers and MACs are:

The supported ciphers are:

    3des-cbc
    aes128-cbc
    aes192-cbc
    aes256-cbc
    aes128-ctr
    aes192-ctr
    aes256-ctr
    aes128-gcm@openssh.com
    aes256-gcm@openssh.com
    blowfish-cbc
    chacha20-poly1305@openssh.com

The default is:

    chacha20-poly1305@openssh.com,
    aes128-ctr,aes192-ctr,aes256-ctr,
    aes128-gcm@openssh.com,aes256-gcm@openssh.com

The following ciphers are FIPS-140 approved and are supported in
FIPS-140 mode:

    aes128-ctr,aes192-ctr,aes256-ctr,
    aes128-gcm@openssh.com,aes256-gcm@openssh.com,
    aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc

The supported MACs are:

    hmac-ripemd160
    hmac-sha1
    hmac-sha1-96
    hmac-sha2-256
    hmac-sha2-512
    umac-64@openssh.com
    umac-128@openssh.com
    hmac-ripemd160-etm@openssh.com
    hmac-sha1-etm@openssh.com
    hmac-sha1-96-etm@openssh.com
    hmac-sha2-256-etm@openssh.com
    hmac-sha2-512-etm@openssh.com
    umac-64-etm@openssh.com
    umac-128-etm@openssh.com

The default is:

    umac-64-etm@openssh.com,umac-128-etm@openssh.com,
    hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
    hmac-sha1-etm@openssh.com,
    umac-64@openssh.com,umac-128@openssh.com,
    hmac-sha2-256,hmac-sha2-512,hmac-sha1

The following MACs are FIPS-140 approved and are supported in
FIPS-140 mode:

    hmac-sha2-256-etm@openssh.com,
    hmac-sha2-512-etm@openssh.com,
    hmac-sha2-256,hmac-sha2-512,
    hmac-sha1-etm@openssh.com,
    hmac-sha1-96-etm@openssh.com
    hmac-sha1,hmac-sha1-96

It seems that OpenSSH does not knows about the two MACs:

hmac-sha2-256-96
hmac-sha2-512-96

If you use one of those MACs specifically /etc/ssh/sshd_config and you upgrade from Solaris 11.3 to Solaris 11.4 you will be unable to login using SSH because the SSH service will be in maintenance mode:

tail /var/svc/log/network-ssh:default.log
[ 2018 Oct  1 14:53:18 Executing start method ("/lib/svc/method/sshd start"). ]
/etc/ssh/sshd_config line 15: Bad SSH2 mac spec 'hmac-sha1,hmac-sha1-96,hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96'.
[ 2018 Oct  1 14:53:19 Method "start" exited with status 255. ]

The resolution is to login to the console, remove both MACs (or comment out the whole MACs directive) and clear the service:

svcadm clear ssh