OpenSSH >= 6.2 for support of AuthenticationMethods

ReadyNAS OS is based on Debian wheezy (current stable), which is shipping with v6.0p1 at the moment (https://packages.debian.org/wheezy/openssh-server).

root@tatooine:~# ssh -V
OpenSSH_6.0p1 Debian-4+deb7u1, OpenSSL 1.0.1e 11 Feb 2013

Our version is pulled from the ReadyNAS repository, however the current version is available from the main Debian repo:

root@tatooine:~# apt-cache policy openssh-server
openssh-server:
  Installed: 1:6.0p1-4+deb7u1
  Candidate: 1:6.0p1-4+deb7u1
  Version table:
     1:6.0p1-4+deb7u2 0
        500 http://mirrors.kernel.org/debian/ wheezy/main armel Packages
 *** 1:6.0p1-4+deb7u1 0
        900 http://apt.readynas.com/packages/readynasos/ 6.1.8/main armel Packages
        100 /var/lib/dpkg/status

Debian stable is not bleeding edge and never has been.
I would hope OpenBSD-5.5(current) is running latest, since it is OpenBSD Secure Shell ;)
Gentoo, rolling release, 'nough said.
You should be able to update select packages relatively easily from backports (http://backports.debian.org/Instructions/).
Current version of openssh-server in backports is v6.6p1 (https://packages.debian.org/wheezy-back ... ssh-server).

root@tatooine:~# echo "deb http://http.debian.net/debian wheezy-backports main" >> /etc/apt/sources.list
root@tatooine:~# apt-get update
[...]
Hit http://http.debian.net wheezy-backports Release.gpg
Hit http://http.debian.net wheezy-backports Release
Get:1 http://http.debian.net wheezy-backports/main armel Packages/DiffIndex [7,819 B]
Fetched 7,819 B in 8s (918 B/s)
Reading package lists... Done
root@tatooine:~# apt-cache policy openssh-server
openssh-server:
  Installed: 1:6.0p1-4+deb7u1
  Candidate: 1:6.0p1-4+deb7u1
  Version table:
     1:6.6p1-4~bpo70+1 0
        100 http://http.debian.net/debian/ wheezy-backports/main armel Packages
     1:6.0p1-4+deb7u2 0
        500 http://mirrors.kernel.org/debian/ wheezy/main armel Packages
 *** 1:6.0p1-4+deb7u1 0
        900 http://apt.readynas.com/packages/readynasos/ 6.1.8/main armel Packages
        100 /var/lib/dpkg/status
root@tatooine:~# apt-get -t wheezy-backports install openssh-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
  libswresample0
Use 'apt-get autoremove' to remove it.
The following extra packages will be installed:
  init-system-helpers libclass-isa-perl libgdbm3 libperl5.14 libswitch-perl openssh-client openssh-sftp-server perl perl-base
  perl-modules
Suggested packages:
  ssh-askpass libpam-ssh keychain monkeysphere rssh molly-guard ufw perl-doc libterm-readline-gnu-perl libterm-readline-perl-perl
  make libpod-plainer-perl
Recommended packages:
  xauth ncurses-term
The following NEW packages will be installed:
  init-system-helpers libclass-isa-perl libgdbm3 libswitch-perl openssh-sftp-server perl perl-modules
The following packages will be upgraded:
  libperl5.14 openssh-client openssh-server perl-base
4 upgraded, 7 newly installed, 0 to remove and 44 not upgraded.
Need to get 9,623 kB of archives.
After this operation, 28.0 MB of additional disk space will be used.
Do you want to continue [Y/n]?
[...]
root@tatooine:~# apt-cache policy openssh-server
openssh-server:
  Installed: 1:6.6p1-4~bpo70+1
  Candidate: 1:6.6p1-4~bpo70+1
  Version table:
 *** 1:6.6p1-4~bpo70+1 0
        100 http://http.debian.net/debian/ wheezy-backports/main armel Packages
        100 /var/lib/dpkg/status
     1:6.0p1-4+deb7u2 0
        500 http://mirrors.kernel.org/debian/ wheezy/main armel Packages
     1:6.0p1-4+deb7u1 0
        900 http://apt.readynas.com/packages/readynasos/ 6.1.8/main armel Packages
root@tatooine:~# ssh -V
OpenSSH_6.6.1p1 Debian-4~bpo70+1, OpenSSL 1.0.1e 11 Feb 2013

Don't have an x86 system to test with at the moment, but on my ARM system it upgrades a few dependencies and installs a few new packages, but otherwise seems OK. Your mileage may vary and this is definitely something our support team can void your warranty over (until you factory default) should you run into issues.