backdoor by authorized_keys2 leftovers

Iain Morgan imorgan at
Fri May 13 08:47:19 EST 2011

On Thu, May 12, 2011 at 14:14:02 -0500, Dan Kaminsky wrote:
> On Thu, May 12, 2011 at 11:49 AM, Markus Friedl <mfriedl at> wrote:
> > looks like we've been waiting too long :)
> >
> >
> >
> > 2) The files
> > /etc/ssh_known_hosts2
> > ~/.ssh/known_hosts2
> > ~/.ssh/authorized_keys2
> >  are now obsolete, you can use
> > /etc/ssh_known_hosts
> > ~/.ssh/known_hosts
> > ~/.ssh/authorized_keys
> >  For backward compatibility ~/.ssh/authorized_keys2 will still used for
> >  authentication and hostkeys are still read from the known_hosts2.
> >  However, those deprecated files are considered 'readonly'. Future
> >  releases are likely not to read these files.
> >
> In no uncertain terms, removal of authorized_keys2 support will cause
> outages, up to and including requiring physical access for administrators to
> resolve.  Documentation is not an excuse to make this change.
> It's completely reasonable, desirable even, to allow a new configuration
> option to explicitly define the set of files that can contain authorized
> keys.  It'd even be convenient to have an AuthorizationCommand option, that
> sent properly escaped strings to a command for external testing and
> validation.
Provided that sysadmins are aware of the change ahead of time, proactive
measures can easily be taken. On a per-system basis, it's obviously a
simple matter to find and rename such deprecated files. However, we can
expect that some will be caught off guard.

In hindsight, what probably should have been done is have sshd return a
warning to the client about the deprecated file after successful login.
And, for good measure, an appropriate message should be logged on the
server, if that's not already the case.

Given that support for authorized_keys2 hasn't been documented for a
number of years, I have to wonder just how widespread its use is.

Iain morgan

More information about the openssh-unix-dev mailing list